XFA: Rename libpng to libpng16.chromium/2625chromium/2624

Otherwise this conflicts with Chromium's copy if the include paths just
happens to be in the wrong order. Since the two copies of libpng are of
different versions, we cannot use Chromium's copy.

TBR=tsepez@chromium.org,ochang@chromium.org

Review URL: https://codereview.chromium.org/1599513004 .

1 files changed, 4136 insertions, 0 deletions

diff --git a/third_party/libpng16/pngread.c b/third_party/libpng16/pngread.c
new file mode 100644
index 0000000000..9cb4d2e41d
--- /dev/null
+++ b/third_party/libpng16/pngread.c
@@ -0,0 +1,4136 @@
+
+/* pngread.c - read a PNG file
+ *
+ * Last changed in libpng 1.6.17 [March 26, 2015]
+ * Copyright (c) 1998-2015 Glenn Randers-Pehrson
+ * (Version 0.96 Copyright (c) 1996, 1997 Andreas Dilger)
+ * (Version 0.88 Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.)
+ *
+ * This code is released under the libpng license.
+ * For conditions of distribution and use, see the disclaimer
+ * and license in png.h
+ *
+ * This file contains routines that an application calls directly to
+ * read a PNG file or stream.
+ */
+
+#include "pngpriv.h"
+#if defined(PNG_SIMPLIFIED_READ_SUPPORTED) && defined(PNG_STDIO_SUPPORTED)
+#  include <errno.h>
+#endif
+
+#ifdef PNG_READ_SUPPORTED
+
+/* Create a PNG structure for reading, and allocate any memory needed. */
+PNG_FUNCTION(png_structp,PNGAPI
+png_create_read_struct,(png_const_charp user_png_ver, png_voidp error_ptr,
+    png_error_ptr error_fn, png_error_ptr warn_fn),PNG_ALLOCATED)
+{
+#ifndef PNG_USER_MEM_SUPPORTED
+   png_structp png_ptr = png_create_png_struct(user_png_ver, error_ptr,
+      error_fn, warn_fn, NULL, NULL, NULL);
+#else
+   return png_create_read_struct_2(user_png_ver, error_ptr, error_fn,
+       warn_fn, NULL, NULL, NULL);
+}
+
+/* Alternate create PNG structure for reading, and allocate any memory
+ * needed.
+ */
+PNG_FUNCTION(png_structp,PNGAPI
+png_create_read_struct_2,(png_const_charp user_png_ver, png_voidp error_ptr,
+    png_error_ptr error_fn, png_error_ptr warn_fn, png_voidp mem_ptr,
+    png_malloc_ptr malloc_fn, png_free_ptr free_fn),PNG_ALLOCATED)
+{
+   png_structp png_ptr = png_create_png_struct(user_png_ver, error_ptr,
+      error_fn, warn_fn, mem_ptr, malloc_fn, free_fn);
+#endif /* USER_MEM */
+
+   if (png_ptr != NULL)
+   {
+      png_ptr->mode = PNG_IS_READ_STRUCT;
+
+      /* Added in libpng-1.6.0; this can be used to detect a read structure if
+       * required (it will be zero in a write structure.)
+       */
+#     ifdef PNG_SEQUENTIAL_READ_SUPPORTED
+         png_ptr->IDAT_read_size = PNG_IDAT_READ_SIZE;
+#     endif
+
+#     ifdef PNG_BENIGN_READ_ERRORS_SUPPORTED
+         png_ptr->flags |= PNG_FLAG_BENIGN_ERRORS_WARN;
+
+         /* In stable builds only warn if an application error can be completely
+          * handled.
+          */
+#        if PNG_RELEASE_BUILD
+            png_ptr->flags |= PNG_FLAG_APP_WARNINGS_WARN;
+#        endif
+#     endif
+
+      /* TODO: delay this, it can be done in png_init_io (if the app doesn't
+       * do it itself) avoiding setting the default function if it is not
+       * required.
+       */
+      png_set_read_fn(png_ptr, NULL, NULL);
+   }
+
+   return png_ptr;
+}
+
+
+#ifdef PNG_SEQUENTIAL_READ_SUPPORTED
+/* Read the information before the actual image data.  This has been
+ * changed in v0.90 to allow reading a file that already has the magic
+ * bytes read from the stream.  You can tell libpng how many bytes have
+ * been read from the beginning of the stream (up to the maximum of 8)
+ * via png_set_sig_bytes(), and we will only check the remaining bytes
+ * here.  The application can then have access to the signature bytes we
+ * read if it is determined that this isn't a valid PNG file.
+ */
+void PNGAPI
+png_read_info(png_structrp png_ptr, png_inforp info_ptr)
+{
+#ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED
+   int keep;
+#endif
+
+   png_debug(1, "in png_read_info");
+
+   if (png_ptr == NULL || info_ptr == NULL)
+      return;
+
+   /* Read and check the PNG file signature. */
+   png_read_sig(png_ptr, info_ptr);
+
+   for (;;)
+   {
+      png_uint_32 length = png_read_chunk_header(png_ptr);
+      png_uint_32 chunk_name = png_ptr->chunk_name;
+
+      /* IDAT logic needs to happen here to simplify getting the two flags
+       * right.
+       */
+      if (chunk_name == png_IDAT)
+      {
+         if ((png_ptr->mode & PNG_HAVE_IHDR) == 0)
+            png_chunk_error(png_ptr, "Missing IHDR before IDAT");
+
+         else if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE &&
+             (png_ptr->mode & PNG_HAVE_PLTE) == 0)
+            png_chunk_error(png_ptr, "Missing PLTE before IDAT");
+
+         else if ((png_ptr->mode & PNG_AFTER_IDAT) != 0)
+            png_chunk_benign_error(png_ptr, "Too many IDATs found");
+
+         png_ptr->mode |= PNG_HAVE_IDAT;
+      }
+
+      else if ((png_ptr->mode & PNG_HAVE_IDAT) != 0)
+         png_ptr->mode |= PNG_AFTER_IDAT;
+
+      /* This should be a binary subdivision search or a hash for
+       * matching the chunk name rather than a linear search.
+       */
+      if (chunk_name == png_IHDR)
+         png_handle_IHDR(png_ptr, info_ptr, length);
+
+      else if (chunk_name == png_IEND)
+         png_handle_IEND(png_ptr, info_ptr, length);
+
+#ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED
+      else if ((keep = png_chunk_unknown_handling(png_ptr, chunk_name)) != 0)
+      {
+         png_handle_unknown(png_ptr, info_ptr, length, keep);
+
+         if (chunk_name == png_PLTE)
+            png_ptr->mode |= PNG_HAVE_PLTE;
+
+         else if (chunk_name == png_IDAT)
+         {
+            png_ptr->idat_size = 0; /* It has been consumed */
+            break;
+         }
+      }
+#endif
+      else if (chunk_name == png_PLTE)
+         png_handle_PLTE(png_ptr, info_ptr, length);
+
+      else if (chunk_name == png_IDAT)
+      {
+         png_ptr->idat_size = length;
+         break;
+      }
+
+#ifdef PNG_READ_bKGD_SUPPORTED
+      else if (chunk_name == png_bKGD)
+         png_handle_bKGD(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_cHRM_SUPPORTED
+      else if (chunk_name == png_cHRM)
+         png_handle_cHRM(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_gAMA_SUPPORTED
+      else if (chunk_name == png_gAMA)
+         png_handle_gAMA(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_hIST_SUPPORTED
+      else if (chunk_name == png_hIST)
+         png_handle_hIST(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_oFFs_SUPPORTED
+      else if (chunk_name == png_oFFs)
+         png_handle_oFFs(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_pCAL_SUPPORTED
+      else if (chunk_name == png_pCAL)
+         png_handle_pCAL(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_sCAL_SUPPORTED
+      else if (chunk_name == png_sCAL)
+         png_handle_sCAL(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_pHYs_SUPPORTED
+      else if (chunk_name == png_pHYs)
+         png_handle_pHYs(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_sBIT_SUPPORTED
+      else if (chunk_name == png_sBIT)
+         png_handle_sBIT(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_sRGB_SUPPORTED
+      else if (chunk_name == png_sRGB)
+         png_handle_sRGB(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_iCCP_SUPPORTED
+      else if (chunk_name == png_iCCP)
+         png_handle_iCCP(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_sPLT_SUPPORTED
+      else if (chunk_name == png_sPLT)
+         png_handle_sPLT(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_tEXt_SUPPORTED
+      else if (chunk_name == png_tEXt)
+         png_handle_tEXt(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_tIME_SUPPORTED
+      else if (chunk_name == png_tIME)
+         png_handle_tIME(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_tRNS_SUPPORTED
+      else if (chunk_name == png_tRNS)
+         png_handle_tRNS(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_zTXt_SUPPORTED
+      else if (chunk_name == png_zTXt)
+         png_handle_zTXt(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_iTXt_SUPPORTED
+      else if (chunk_name == png_iTXt)
+         png_handle_iTXt(png_ptr, info_ptr, length);
+#endif
+
+      else
+         png_handle_unknown(png_ptr, info_ptr, length,
+            PNG_HANDLE_CHUNK_AS_DEFAULT);
+   }
+}
+#endif /* SEQUENTIAL_READ */
+
+/* Optional call to update the users info_ptr structure */
+void PNGAPI
+png_read_update_info(png_structrp png_ptr, png_inforp info_ptr)
+{
+   png_debug(1, "in png_read_update_info");
+
+   if (png_ptr != NULL)
+   {
+      if ((png_ptr->flags & PNG_FLAG_ROW_INIT) == 0)
+      {
+         png_read_start_row(png_ptr);
+
+#        ifdef PNG_READ_TRANSFORMS_SUPPORTED
+            png_read_transform_info(png_ptr, info_ptr);
+#        else
+            PNG_UNUSED(info_ptr)
+#        endif
+      }
+
+      /* New in 1.6.0 this avoids the bug of doing the initializations twice */
+      else
+         png_app_error(png_ptr,
+            "png_read_update_info/png_start_read_image: duplicate call");
+   }
+}
+
+#ifdef PNG_SEQUENTIAL_READ_SUPPORTED
+/* Initialize palette, background, etc, after transformations
+ * are set, but before any reading takes place.  This allows
+ * the user to obtain a gamma-corrected palette, for example.
+ * If the user doesn't call this, we will do it ourselves.
+ */
+void PNGAPI
+png_start_read_image(png_structrp png_ptr)
+{
+   png_debug(1, "in png_start_read_image");
+
+   if (png_ptr != NULL)
+   {
+      if ((png_ptr->flags & PNG_FLAG_ROW_INIT) == 0)
+         png_read_start_row(png_ptr);
+
+      /* New in 1.6.0 this avoids the bug of doing the initializations twice */
+      else
+         png_app_error(png_ptr,
+            "png_start_read_image/png_read_update_info: duplicate call");
+   }
+}
+#endif /* SEQUENTIAL_READ */
+
+#ifdef PNG_SEQUENTIAL_READ_SUPPORTED
+#ifdef PNG_MNG_FEATURES_SUPPORTED
+/* Undoes intrapixel differencing,
+ * NOTE: this is apparently only supported in the 'sequential' reader.
+ */
+static void
+png_do_read_intrapixel(png_row_infop row_info, png_bytep row)
+{
+   png_debug(1, "in png_do_read_intrapixel");
+
+   if (
+       (row_info->color_type & PNG_COLOR_MASK_COLOR) != 0)
+   {
+      int bytes_per_pixel;
+      png_uint_32 row_width = row_info->width;
+
+      if (row_info->bit_depth == 8)
+      {
+         png_bytep rp;
+         png_uint_32 i;
+
+         if (row_info->color_type == PNG_COLOR_TYPE_RGB)
+            bytes_per_pixel = 3;
+
+         else if (row_info->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
+            bytes_per_pixel = 4;
+
+         else
+            return;
+
+         for (i = 0, rp = row; i < row_width; i++, rp += bytes_per_pixel)
+         {
+            *(rp) = (png_byte)((256 + *rp + *(rp + 1)) & 0xff);
+            *(rp+2) = (png_byte)((256 + *(rp + 2) + *(rp + 1)) & 0xff);
+         }
+      }
+      else if (row_info->bit_depth == 16)
+      {
+         png_bytep rp;
+         png_uint_32 i;
+
+         if (row_info->color_type == PNG_COLOR_TYPE_RGB)
+            bytes_per_pixel = 6;
+
+         else if (row_info->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
+            bytes_per_pixel = 8;
+
+         else
+            return;
+
+         for (i = 0, rp = row; i < row_width; i++, rp += bytes_per_pixel)
+         {
+            png_uint_32 s0   = (*(rp    ) << 8) | *(rp + 1);
+            png_uint_32 s1   = (*(rp + 2) << 8) | *(rp + 3);
+            png_uint_32 s2   = (*(rp + 4) << 8) | *(rp + 5);
+            png_uint_32 red  = (s0 + s1 + 65536) & 0xffff;
+            png_uint_32 blue = (s2 + s1 + 65536) & 0xffff;
+            *(rp    ) = (png_byte)((red >> 8) & 0xff);
+            *(rp + 1) = (png_byte)(red & 0xff);
+            *(rp + 4) = (png_byte)((blue >> 8) & 0xff);
+            *(rp + 5) = (png_byte)(blue & 0xff);
+         }
+      }
+   }
+}
+#endif /* MNG_FEATURES */
+
+void PNGAPI
+png_read_row(png_structrp png_ptr, png_bytep row, png_bytep dsp_row)
+{
+   png_row_info row_info;
+
+   if (png_ptr == NULL)
+      return;
+
+   png_debug2(1, "in png_read_row (row %lu, pass %d)",
+       (unsigned long)png_ptr->row_number, png_ptr->pass);
+
+   /* png_read_start_row sets the information (in particular iwidth) for this
+    * interlace pass.
+    */
+   if ((png_ptr->flags & PNG_FLAG_ROW_INIT) == 0)
+      png_read_start_row(png_ptr);
+
+   /* 1.5.6: row_info moved out of png_struct to a local here. */
+   row_info.width = png_ptr->iwidth; /* NOTE: width of current interlaced row */
+   row_info.color_type = png_ptr->color_type;
+   row_info.bit_depth = png_ptr->bit_depth;
+   row_info.channels = png_ptr->channels;
+   row_info.pixel_depth = png_ptr->pixel_depth;
+   row_info.rowbytes = PNG_ROWBYTES(row_info.pixel_depth, row_info.width);
+
+#ifdef PNG_WARNINGS_SUPPORTED
+   if (png_ptr->row_number == 0 && png_ptr->pass == 0)
+   {
+   /* Check for transforms that have been set but were defined out */
+#if defined(PNG_WRITE_INVERT_SUPPORTED) && !defined(PNG_READ_INVERT_SUPPORTED)
+   if ((png_ptr->transformations & PNG_INVERT_MONO) != 0)
+      png_warning(png_ptr, "PNG_READ_INVERT_SUPPORTED is not defined");
+#endif
+
+#if defined(PNG_WRITE_FILLER_SUPPORTED) && !defined(PNG_READ_FILLER_SUPPORTED)
+   if ((png_ptr->transformations & PNG_FILLER) != 0)
+      png_warning(png_ptr, "PNG_READ_FILLER_SUPPORTED is not defined");
+#endif
+
+#if defined(PNG_WRITE_PACKSWAP_SUPPORTED) && \
+    !defined(PNG_READ_PACKSWAP_SUPPORTED)
+   if ((png_ptr->transformations & PNG_PACKSWAP) != 0)
+      png_warning(png_ptr, "PNG_READ_PACKSWAP_SUPPORTED is not defined");
+#endif
+
+#if defined(PNG_WRITE_PACK_SUPPORTED) && !defined(PNG_READ_PACK_SUPPORTED)
+   if ((png_ptr->transformations & PNG_PACK) != 0)
+      png_warning(png_ptr, "PNG_READ_PACK_SUPPORTED is not defined");
+#endif
+
+#if defined(PNG_WRITE_SHIFT_SUPPORTED) && !defined(PNG_READ_SHIFT_SUPPORTED)
+   if ((png_ptr->transformations & PNG_SHIFT) != 0)
+      png_warning(png_ptr, "PNG_READ_SHIFT_SUPPORTED is not defined");
+#endif
+
+#if defined(PNG_WRITE_BGR_SUPPORTED) && !defined(PNG_READ_BGR_SUPPORTED)
+   if ((png_ptr->transformations & PNG_BGR) != 0)
+      png_warning(png_ptr, "PNG_READ_BGR_SUPPORTED is not defined");
+#endif
+
+#if defined(PNG_WRITE_SWAP_SUPPORTED) && !defined(PNG_READ_SWAP_SUPPORTED)
+   if ((png_ptr->transformations & PNG_SWAP_BYTES) != 0)
+      png_warning(png_ptr, "PNG_READ_SWAP_SUPPORTED is not defined");
+#endif
+   }
+#endif /* WARNINGS */
+
+#ifdef PNG_READ_INTERLACING_SUPPORTED
+   /* If interlaced and we do not need a new row, combine row and return.
+    * Notice that the pixels we have from previous rows have been transformed
+    * already; we can only combine like with like (transformed or
+    * untransformed) and, because of the libpng API for interlaced images, this
+    * means we must transform before de-interlacing.
+    */
+   if (png_ptr->interlaced != 0 &&
+       (png_ptr->transformations & PNG_INTERLACE) != 0)
+   {
+      switch (png_ptr->pass)
+      {
+         case 0:
+            if (png_ptr->row_number & 0x07)
+            {
+               if (dsp_row != NULL)
+                  png_combine_row(png_ptr, dsp_row, 1/*display*/);
+               png_read_finish_row(png_ptr);
+               return;
+            }
+            break;
+
+         case 1:
+            if ((png_ptr->row_number & 0x07) || png_ptr->width < 5)
+            {
+               if (dsp_row != NULL)
+                  png_combine_row(png_ptr, dsp_row, 1/*display*/);
+
+               png_read_finish_row(png_ptr);
+               return;
+            }
+            break;
+
+         case 2:
+            if ((png_ptr->row_number & 0x07) != 4)
+            {
+               if (dsp_row != NULL && (png_ptr->row_number & 4))
+                  png_combine_row(png_ptr, dsp_row, 1/*display*/);
+
+               png_read_finish_row(png_ptr);
+               return;
+            }
+            break;
+
+         case 3:
+            if ((png_ptr->row_number & 3) || png_ptr->width < 3)
+            {
+               if (dsp_row != NULL)
+                  png_combine_row(png_ptr, dsp_row, 1/*display*/);
+
+               png_read_finish_row(png_ptr);
+               return;
+            }
+            break;
+
+         case 4:
+            if ((png_ptr->row_number & 3) != 2)
+            {
+               if (dsp_row != NULL && (png_ptr->row_number & 2))
+                  png_combine_row(png_ptr, dsp_row, 1/*display*/);
+
+               png_read_finish_row(png_ptr);
+               return;
+            }
+            break;
+
+         case 5:
+            if ((png_ptr->row_number & 1) || png_ptr->width < 2)
+            {
+               if (dsp_row != NULL)
+                  png_combine_row(png_ptr, dsp_row, 1/*display*/);
+
+               png_read_finish_row(png_ptr);
+               return;
+            }
+            break;
+
+         default:
+         case 6:
+            if ((png_ptr->row_number & 1) == 0)
+            {
+               png_read_finish_row(png_ptr);
+               return;
+            }
+            break;
+      }
+   }
+#endif
+
+   if ((png_ptr->mode & PNG_HAVE_IDAT) == 0)
+      png_error(png_ptr, "Invalid attempt to read row data");
+
+   /* Fill the row with IDAT data: */
+   png_read_IDAT_data(png_ptr, png_ptr->row_buf, row_info.rowbytes + 1);
+
+   if (png_ptr->row_buf[0] > PNG_FILTER_VALUE_NONE)
+   {
+      if (png_ptr->row_buf[0] < PNG_FILTER_VALUE_LAST)
+         png_read_filter_row(png_ptr, &row_info, png_ptr->row_buf + 1,
+            png_ptr->prev_row + 1, png_ptr->row_buf[0]);
+      else
+         png_error(png_ptr, "bad adaptive filter value");
+   }
+
+   /* libpng 1.5.6: the following line was copying png_ptr->rowbytes before
+    * 1.5.6, while the buffer really is this big in current versions of libpng
+    * it may not be in the future, so this was changed just to copy the
+    * interlaced count:
+    */
+   memcpy(png_ptr->prev_row, png_ptr->row_buf, row_info.rowbytes + 1);
+
+#ifdef PNG_MNG_FEATURES_SUPPORTED
+   if ((png_ptr->mng_features_permitted & PNG_FLAG_MNG_FILTER_64) != 0 &&
+       (png_ptr->filter_type == PNG_INTRAPIXEL_DIFFERENCING))
+   {
+      /* Intrapixel differencing */
+      png_do_read_intrapixel(&row_info, png_ptr->row_buf + 1);
+   }
+#endif
+
+#ifdef PNG_READ_TRANSFORMS_SUPPORTED
+   if (png_ptr->transformations)
+      png_do_read_transformations(png_ptr, &row_info);
+#endif
+
+   /* The transformed pixel depth should match the depth now in row_info. */
+   if (png_ptr->transformed_pixel_depth == 0)
+   {
+      png_ptr->transformed_pixel_depth = row_info.pixel_depth;
+      if (row_info.pixel_depth > png_ptr->maximum_pixel_depth)
+         png_error(png_ptr, "sequential row overflow");
+   }
+
+   else if (png_ptr->transformed_pixel_depth != row_info.pixel_depth)
+      png_error(png_ptr, "internal sequential row size calculation error");
+
+#ifdef PNG_READ_INTERLACING_SUPPORTED
+   /* Expand interlaced rows to full size */
+   if (png_ptr->interlaced != 0 &&
+      (png_ptr->transformations & PNG_INTERLACE) != 0)
+   {
+      if (png_ptr->pass < 6)
+         png_do_read_interlace(&row_info, png_ptr->row_buf + 1, png_ptr->pass,
+            png_ptr->transformations);
+
+      if (dsp_row != NULL)
+         png_combine_row(png_ptr, dsp_row, 1/*display*/);
+
+      if (row != NULL)
+         png_combine_row(png_ptr, row, 0/*row*/);
+   }
+
+   else
+#endif
+   {
+      if (row != NULL)
+         png_combine_row(png_ptr, row, -1/*ignored*/);
+
+      if (dsp_row != NULL)
+         png_combine_row(png_ptr, dsp_row, -1/*ignored*/);
+   }
+   png_read_finish_row(png_ptr);
+
+   if (png_ptr->read_row_fn != NULL)
+      (*(png_ptr->read_row_fn))(png_ptr, png_ptr->row_number, png_ptr->pass);
+
+}
+#endif /* SEQUENTIAL_READ */
+
+#ifdef PNG_SEQUENTIAL_READ_SUPPORTED
+/* Read one or more rows of image data.  If the image is interlaced,
+ * and png_set_interlace_handling() has been called, the rows need to
+ * contain the contents of the rows from the previous pass.  If the
+ * image has alpha or transparency, and png_handle_alpha()[*] has been
+ * called, the rows contents must be initialized to the contents of the
+ * screen.
+ *
+ * "row" holds the actual image, and pixels are placed in it
+ * as they arrive.  If the image is displayed after each pass, it will
+ * appear to "sparkle" in.  "display_row" can be used to display a
+ * "chunky" progressive image, with finer detail added as it becomes
+ * available.  If you do not want this "chunky" display, you may pass
+ * NULL for display_row.  If you do not want the sparkle display, and
+ * you have not called png_handle_alpha(), you may pass NULL for rows.
+ * If you have called png_handle_alpha(), and the image has either an
+ * alpha channel or a transparency chunk, you must provide a buffer for
+ * rows.  In this case, you do not have to provide a display_row buffer
+ * also, but you may.  If the image is not interlaced, or if you have
+ * not called png_set_interlace_handling(), the display_row buffer will
+ * be ignored, so pass NULL to it.
+ *
+ * [*] png_handle_alpha() does not exist yet, as of this version of libpng
+ */
+
+void PNGAPI
+png_read_rows(png_structrp png_ptr, png_bytepp row,
+    png_bytepp display_row, png_uint_32 num_rows)
+{
+   png_uint_32 i;
+   png_bytepp rp;
+   png_bytepp dp;
+
+   png_debug(1, "in png_read_rows");
+
+   if (png_ptr == NULL)
+      return;
+
+   rp = row;
+   dp = display_row;
+   if (rp != NULL && dp != NULL)
+      for (i = 0; i < num_rows; i++)
+      {
+         png_bytep rptr = *rp++;
+         png_bytep dptr = *dp++;
+
+         png_read_row(png_ptr, rptr, dptr);
+      }
+
+   else if (rp != NULL)
+      for (i = 0; i < num_rows; i++)
+      {
+         png_bytep rptr = *rp;
+         png_read_row(png_ptr, rptr, NULL);
+         rp++;
+      }
+
+   else if (dp != NULL)
+      for (i = 0; i < num_rows; i++)
+      {
+         png_bytep dptr = *dp;
+         png_read_row(png_ptr, NULL, dptr);
+         dp++;
+      }
+}
+#endif /* SEQUENTIAL_READ */
+
+#ifdef PNG_SEQUENTIAL_READ_SUPPORTED
+/* Read the entire image.  If the image has an alpha channel or a tRNS
+ * chunk, and you have called png_handle_alpha()[*], you will need to
+ * initialize the image to the current image that PNG will be overlaying.
+ * We set the num_rows again here, in case it was incorrectly set in
+ * png_read_start_row() by a call to png_read_update_info() or
+ * png_start_read_image() if png_set_interlace_handling() wasn't called
+ * prior to either of these functions like it should have been.  You can
+ * only call this function once.  If you desire to have an image for
+ * each pass of a interlaced image, use png_read_rows() instead.
+ *
+ * [*] png_handle_alpha() does not exist yet, as of this version of libpng
+ */
+void PNGAPI
+png_read_image(png_structrp png_ptr, png_bytepp image)
+{
+   png_uint_32 i, image_height;
+   int pass, j;
+   png_bytepp rp;
+
+   png_debug(1, "in png_read_image");
+
+   if (png_ptr == NULL)
+      return;
+
+#ifdef PNG_READ_INTERLACING_SUPPORTED
+   if ((png_ptr->flags & PNG_FLAG_ROW_INIT) == 0)
+   {
+      pass = png_set_interlace_handling(png_ptr);
+      /* And make sure transforms are initialized. */
+      png_start_read_image(png_ptr);
+   }
+   else
+   {
+      if (png_ptr->interlaced != 0 &&
+          (png_ptr->transformations & PNG_INTERLACE) == 0)
+      {
+         /* Caller called png_start_read_image or png_read_update_info without
+          * first turning on the PNG_INTERLACE transform.  We can fix this here,
+          * but the caller should do it!
+          */
+         png_warning(png_ptr, "Interlace handling should be turned on when "
+            "using png_read_image");
+         /* Make sure this is set correctly */
+         png_ptr->num_rows = png_ptr->height;
+      }
+
+      /* Obtain the pass number, which also turns on the PNG_INTERLACE flag in
+       * the above error case.
+       */
+      pass = png_set_interlace_handling(png_ptr);
+   }
+#else
+   if (png_ptr->interlaced)
+      png_error(png_ptr,
+          "Cannot read interlaced image -- interlace handler disabled");
+
+   pass = 1;
+#endif
+
+   image_height=png_ptr->height;
+
+   for (j = 0; j < pass; j++)
+   {
+      rp = image;
+      for (i = 0; i < image_height; i++)
+      {
+         png_read_row(png_ptr, *rp, NULL);
+         rp++;
+      }
+   }
+}
+#endif /* SEQUENTIAL_READ */
+
+#ifdef PNG_SEQUENTIAL_READ_SUPPORTED
+/* Read the end of the PNG file.  Will not read past the end of the
+ * file, will verify the end is accurate, and will read any comments
+ * or time information at the end of the file, if info is not NULL.
+ */
+void PNGAPI
+png_read_end(png_structrp png_ptr, png_inforp info_ptr)
+{
+#ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED
+   int keep;
+#endif
+
+   png_debug(1, "in png_read_end");
+
+   if (png_ptr == NULL)
+      return;
+
+   /* If png_read_end is called in the middle of reading the rows there may
+    * still be pending IDAT data and an owned zstream.  Deal with this here.
+    */
+#ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED
+   if (png_chunk_unknown_handling(png_ptr, png_IDAT) == 0)
+#endif
+      png_read_finish_IDAT(png_ptr);
+
+#ifdef PNG_READ_CHECK_FOR_INVALID_INDEX_SUPPORTED
+   /* Report invalid palette index; added at libng-1.5.10 */
+   if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE &&
+      png_ptr->num_palette_max > png_ptr->num_palette)
+     png_benign_error(png_ptr, "Read palette index exceeding num_palette");
+#endif
+
+   do
+   {
+      png_uint_32 length = png_read_chunk_header(png_ptr);
+      png_uint_32 chunk_name = png_ptr->chunk_name;
+
+      if (chunk_name == png_IEND)
+         png_handle_IEND(png_ptr, info_ptr, length);
+
+      else if (chunk_name == png_IHDR)
+         png_handle_IHDR(png_ptr, info_ptr, length);
+
+      else if (info_ptr == NULL)
+         png_crc_finish(png_ptr, length);
+
+#ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED
+      else if ((keep = png_chunk_unknown_handling(png_ptr, chunk_name)) != 0)
+      {
+         if (chunk_name == png_IDAT)
+         {
+            if ((length > 0) ||
+                (png_ptr->mode & PNG_HAVE_CHUNK_AFTER_IDAT) != 0)
+               png_benign_error(png_ptr, "Too many IDATs found");
+         }
+         png_handle_unknown(png_ptr, info_ptr, length, keep);
+         if (chunk_name == png_PLTE)
+            png_ptr->mode |= PNG_HAVE_PLTE;
+      }
+#endif
+
+      else if (chunk_name == png_IDAT)
+      {
+         /* Zero length IDATs are legal after the last IDAT has been
+          * read, but not after other chunks have been read.
+          */
+         if ((length > 0) || (png_ptr->mode & PNG_HAVE_CHUNK_AFTER_IDAT) != 0)
+            png_benign_error(png_ptr, "Too many IDATs found");
+
+         png_crc_finish(png_ptr, length);
+      }
+      else if (chunk_name == png_PLTE)
+         png_handle_PLTE(png_ptr, info_ptr, length);
+
+#ifdef PNG_READ_bKGD_SUPPORTED
+      else if (chunk_name == png_bKGD)
+         png_handle_bKGD(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_cHRM_SUPPORTED
+      else if (chunk_name == png_cHRM)
+         png_handle_cHRM(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_gAMA_SUPPORTED
+      else if (chunk_name == png_gAMA)
+         png_handle_gAMA(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_hIST_SUPPORTED
+      else if (chunk_name == png_hIST)
+         png_handle_hIST(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_oFFs_SUPPORTED
+      else if (chunk_name == png_oFFs)
+         png_handle_oFFs(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_pCAL_SUPPORTED
+      else if (chunk_name == png_pCAL)
+         png_handle_pCAL(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_sCAL_SUPPORTED
+      else if (chunk_name == png_sCAL)
+         png_handle_sCAL(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_pHYs_SUPPORTED
+      else if (chunk_name == png_pHYs)
+         png_handle_pHYs(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_sBIT_SUPPORTED
+      else if (chunk_name == png_sBIT)
+         png_handle_sBIT(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_sRGB_SUPPORTED
+      else if (chunk_name == png_sRGB)
+         png_handle_sRGB(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_iCCP_SUPPORTED
+      else if (chunk_name == png_iCCP)
+         png_handle_iCCP(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_sPLT_SUPPORTED
+      else if (chunk_name == png_sPLT)
+         png_handle_sPLT(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_tEXt_SUPPORTED
+      else if (chunk_name == png_tEXt)
+         png_handle_tEXt(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_tIME_SUPPORTED
+      else if (chunk_name == png_tIME)
+         png_handle_tIME(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_tRNS_SUPPORTED
+      else if (chunk_name == png_tRNS)
+         png_handle_tRNS(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_zTXt_SUPPORTED
+      else if (chunk_name == png_zTXt)
+         png_handle_zTXt(png_ptr, info_ptr, length);
+#endif
+
+#ifdef PNG_READ_iTXt_SUPPORTED
+      else if (chunk_name == png_iTXt)
+         png_handle_iTXt(png_ptr, info_ptr, length);
+#endif
+
+      else
+         png_handle_unknown(png_ptr, info_ptr, length,
+            PNG_HANDLE_CHUNK_AS_DEFAULT);
+   } while ((png_ptr->mode & PNG_HAVE_IEND) == 0);
+}
+#endif /* SEQUENTIAL_READ */
+
+/* Free all memory used in the read struct */
+static void
+png_read_destroy(png_structrp png_ptr)
+{
+   png_debug(1, "in png_read_destroy");
+
+#ifdef PNG_READ_GAMMA_SUPPORTED
+   png_destroy_gamma_table(png_ptr);
+#endif
+
+   png_free(png_ptr, png_ptr->big_row_buf);
+   png_ptr->big_row_buf = NULL;
+   png_free(png_ptr, png_ptr->big_prev_row);
+   png_ptr->big_prev_row = NULL;
+   png_free(png_ptr, png_ptr->read_buffer);
+   png_ptr->read_buffer = NULL;
+
+#ifdef PNG_READ_QUANTIZE_SUPPORTED
+   png_free(png_ptr, png_ptr->palette_lookup);
+   png_ptr->palette_lookup = NULL;
+   png_free(png_ptr, png_ptr->quantize_index);
+   png_ptr->quantize_index = NULL;
+#endif
+
+   if ((png_ptr->free_me & PNG_FREE_PLTE) != 0)
+   {
+      png_zfree(png_ptr, png_ptr->palette);
+      png_ptr->palette = NULL;
+   }
+   png_ptr->free_me &= ~PNG_FREE_PLTE;
+
+#if defined(PNG_tRNS_SUPPORTED) || \
+    defined(PNG_READ_EXPAND_SUPPORTED) || defined(PNG_READ_BACKGROUND_SUPPORTED)
+   if ((png_ptr->free_me & PNG_FREE_TRNS) != 0)
+   {
+      png_free(png_ptr, png_ptr->trans_alpha);
+      png_ptr->trans_alpha = NULL;
+   }
+   png_ptr->free_me &= ~PNG_FREE_TRNS;
+#endif
+
+   inflateEnd(&png_ptr->zstream);
+
+#ifdef PNG_PROGRESSIVE_READ_SUPPORTED
+   png_free(png_ptr, png_ptr->save_buffer);
+   png_ptr->save_buffer = NULL;
+#endif
+
+#if defined(PNG_STORE_UNKNOWN_CHUNKS_SUPPORTED) && \
+   defined(PNG_READ_UNKNOWN_CHUNKS_SUPPORTED)
+   png_free(png_ptr, png_ptr->unknown_chunk.data);
+   png_ptr->unknown_chunk.data = NULL;
+#endif
+
+#ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTED
+   png_free(png_ptr, png_ptr->chunk_list);
+   png_ptr->chunk_list = NULL;
+#endif
+
+   /* NOTE: the 'setjmp' buffer may still be allocated and the memory and error
+    * callbacks are still set at this point.  They are required to complete the
+    * destruction of the png_struct itself.
+    */
+}
+
+/* Free all memory used by the read */
+void PNGAPI
+png_destroy_read_struct(png_structpp png_ptr_ptr, png_infopp info_ptr_ptr,
+    png_infopp end_info_ptr_ptr)
+{
+   png_structrp png_ptr = NULL;
+
+   png_debug(1, "in png_destroy_read_struct");
+
+   if (png_ptr_ptr != NULL)
+      png_ptr = *png_ptr_ptr;
+
+   if (png_ptr == NULL)
+      return;
+
+   /* libpng 1.6.0: use the API to destroy info structs to ensure consistent
+    * behavior.  Prior to 1.6.0 libpng did extra 'info' destruction in this API.
+    * The extra was, apparently, unnecessary yet this hides memory leak bugs.
+    */
+   png_destroy_info_struct(png_ptr, end_info_ptr_ptr);
+   png_destroy_info_struct(png_ptr, info_ptr_ptr);
+
+   *png_ptr_ptr = NULL;
+   png_read_destroy(png_ptr);
+   png_destroy_png_struct(png_ptr);
+}
+
+void PNGAPI
+png_set_read_status_fn(png_structrp png_ptr, png_read_status_ptr read_row_fn)
+{
+   if (png_ptr == NULL)
+      return;
+
+   png_ptr->read_row_fn = read_row_fn;
+}
+
+
+#ifdef PNG_SEQUENTIAL_READ_SUPPORTED
+#ifdef PNG_INFO_IMAGE_SUPPORTED
+void PNGAPI
+png_read_png(png_structrp png_ptr, png_inforp info_ptr,
+                           int transforms,
+                           voidp params)
+{
+   if (png_ptr == NULL || info_ptr == NULL)
+      return;
+
+   /* png_read_info() gives us all of the information from the
+    * PNG file before the first IDAT (image data chunk).
+    */
+   png_read_info(png_ptr, info_ptr);
+   if (info_ptr->height > PNG_UINT_32_MAX/(sizeof (png_bytep)))
+      png_error(png_ptr, "Image is too high to process with png_read_png()");
+
+   /* -------------- image transformations start here ------------------- */
+   /* libpng 1.6.10: add code to cause a png_app_error if a selected TRANSFORM
+    * is not implemented.  This will only happen in de-configured (non-default)
+    * libpng builds.  The results can be unexpected - png_read_png may return
+    * short or mal-formed rows because the transform is skipped.
+    */
+
+   /* Tell libpng to strip 16-bit/color files down to 8 bits per color.
+    */
+   if ((transforms & PNG_TRANSFORM_SCALE_16) != 0)
+      /* Added at libpng-1.5.4. "strip_16" produces the same result that it
+       * did in earlier versions, while "scale_16" is now more accurate.
+       */
+#ifdef PNG_READ_SCALE_16_TO_8_SUPPORTED
+      png_set_scale_16(png_ptr);
+#else
+      png_app_error(png_ptr, "PNG_TRANSFORM_SCALE_16 not supported");
+#endif
+
+   /* If both SCALE and STRIP are required pngrtran will effectively cancel the
+    * latter by doing SCALE first.  This is ok and allows apps not to check for
+    * which is supported to get the right answer.
+    */
+   if ((transforms & PNG_TRANSFORM_STRIP_16) != 0)
+#ifdef PNG_READ_STRIP_16_TO_8_SUPPORTED
+      png_set_strip_16(png_ptr);
+#else
+      png_app_error(png_ptr, "PNG_TRANSFORM_STRIP_16 not supported");
+#endif
+
+   /* Strip alpha bytes from the input data without combining with
+    * the background (not recommended).
+    */
+   if ((transforms & PNG_TRANSFORM_STRIP_ALPHA) != 0)
+#ifdef PNG_READ_STRIP_ALPHA_SUPPORTED
+      png_set_strip_alpha(png_ptr);
+#else
+      png_app_error(png_ptr, "PNG_TRANSFORM_STRIP_ALPHA not supported");
+#endif
+
+   /* Extract multiple pixels with bit depths of 1, 2, or 4 from a single
+    * byte into separate bytes (useful for paletted and grayscale images).
+    */
+   if ((transforms & PNG_TRANSFORM_PACKING) != 0)
+#ifdef PNG_READ_PACK_SUPPORTED
+      png_set_packing(png_ptr);
+#else
+      png_app_error(png_ptr, "PNG_TRANSFORM_PACKING not supported");
+#endif
+
+   /* Change the order of packed pixels to least significant bit first
+    * (not useful if you are using png_set_packing).
+    */
+   if ((transforms & PNG_TRANSFORM_PACKSWAP) != 0)
+#ifdef PNG_READ_PACKSWAP_SUPPORTED
+      png_set_packswap(png_ptr);
+#else
+      png_app_error(png_ptr, "PNG_TRANSFORM_PACKSWAP not supported");
+#endif
+
+   /* Expand paletted colors into true RGB triplets
+    * Expand grayscale images to full 8 bits from 1, 2, or 4 bits/pixel
+    * Expand paletted or RGB images with transparency to full alpha
+    * channels so the data will be available as RGBA quartets.
+    */
+   if ((transforms & PNG_TRANSFORM_EXPAND) != 0)
+#ifdef PNG_READ_EXPAND_SUPPORTED
+      png_set_expand(png_ptr);
+#else
+      png_app_error(png_ptr, "PNG_TRANSFORM_EXPAND not supported");
+#endif
+
+   /* We don't handle background color or gamma transformation or quantizing.
+    */
+
+   /* Invert monochrome files to have 0 as white and 1 as black
+    */
+   if ((transforms & PNG_TRANSFORM_INVERT_MONO) != 0)
+#ifdef PNG_READ_INVERT_SUPPORTED
+      png_set_invert_mono(png_ptr);
+#else
+      png_app_error(png_ptr, "PNG_TRANSFORM_INVERT_MONO not supported");
+#endif
+
+   /* If you want to shift the pixel values from the range [0,255] or
+    * [0,65535] to the original [0,7] or [0,31], or whatever range the
+    * colors were originally in:
+    */
+   if ((transforms & PNG_TRANSFORM_SHIFT) != 0)
+#ifdef PNG_READ_SHIFT_SUPPORTED
+      if ((info_ptr->valid & PNG_INFO_sBIT) != 0)
+         png_set_shift(png_ptr, &info_ptr->sig_bit);
+#else
+      png_app_error(png_ptr, "PNG_TRANSFORM_SHIFT not supported");
+#endif
+
+   /* Flip the RGB pixels to BGR (or RGBA to BGRA) */
+   if ((transforms & PNG_TRANSFORM_BGR) != 0)
+#ifdef PNG_READ_BGR_SUPPORTED
+      png_set_bgr(png_ptr);
+#else
+      png_app_error(png_ptr, "PNG_TRANSFORM_BGR not supported");
+#endif
+
+   /* Swap the RGBA or GA data to ARGB or AG (or BGRA to ABGR) */
+   if ((transforms & PNG_TRANSFORM_SWAP_ALPHA) != 0)
+#ifdef PNG_READ_SWAP_ALPHA_SUPPORTED
+      png_set_swap_alpha(png_ptr);
+#else
+      png_app_error(png_ptr, "PNG_TRANSFORM_SWAP_ALPHA not supported");
+#endif
+
+   /* Swap bytes of 16-bit files to least significant byte first */
+   if ((transforms & PNG_TRANSFORM_SWAP_ENDIAN) != 0)
+#ifdef PNG_READ_SWAP_SUPPORTED
+      png_set_swap(png_ptr);
+#else
+      png_app_error(png_ptr, "PNG_TRANSFORM_SWAP_ENDIAN not supported");
+#endif
+
+/* Added at libpng-1.2.41 */
+   /* Invert the alpha channel from opacity to transparency */
+   if ((transforms & PNG_TRANSFORM_INVERT_ALPHA) != 0)
+#ifdef PNG_READ_INVERT_ALPHA_SUPPORTED
+      png_set_invert_alpha(png_ptr);
+#else
+      png_app_error(png_ptr, "PNG_TRANSFORM_INVERT_ALPHA not supported");
+#endif
+
+/* Added at libpng-1.2.41 */
+   /* Expand grayscale image to RGB */
+   if ((transforms & PNG_TRANSFORM_GRAY_TO_RGB) != 0)
+#ifdef PNG_READ_GRAY_TO_RGB_SUPPORTED
+      png_set_gray_to_rgb(png_ptr);
+#else
+      png_app_error(png_ptr, "PNG_TRANSFORM_GRAY_TO_RGB not supported");
+#endif
+
+/* Added at libpng-1.5.4 */
+   if ((transforms & PNG_TRANSFORM_EXPAND_16) != 0)
+#ifdef PNG_READ_EXPAND_16_SUPPORTED
+      png_set_expand_16(png_ptr);
+#else
+      png_app_error(png_ptr, "PNG_TRANSFORM_EXPAND_16 not supported");
+#endif
+
+   /* We don't handle adding filler bytes */
+
+   /* We use png_read_image and rely on that for interlace handling, but we also
+    * call png_read_update_info therefore must turn on interlace handling now:
+    */
+   (void)png_set_interlace_handling(png_ptr);
+
+   /* Optional call to gamma correct and add the background to the palette
+    * and update info structure.  REQUIRED if you are expecting libpng to
+    * update the palette for you (i.e., you selected such a transform above).
+    */
+   png_read_update_info(png_ptr, info_ptr);
+
+   /* -------------- image transformations end here ------------------- */
+
+   png_free_data(png_ptr, info_ptr, PNG_FREE_ROWS, 0);
+   if (info_ptr->row_pointers == NULL)
+   {
+      png_uint_32 iptr;
+
+      info_ptr->row_pointers = png_voidcast(png_bytepp, png_malloc(png_ptr,
+          info_ptr->height * (sizeof (png_bytep))));
+
+      for (iptr=0; iptr<info_ptr->height; iptr++)
+         info_ptr->row_pointers[iptr] = NULL;
+
+      info_ptr->free_me |= PNG_FREE_ROWS;
+
+      for (iptr = 0; iptr < info_ptr->height; iptr++)
+         info_ptr->row_pointers[iptr] = png_voidcast(png_bytep,
+             png_malloc(png_ptr, info_ptr->rowbytes));
+   }
+
+   png_read_image(png_ptr, info_ptr->row_pointers);
+   info_ptr->valid |= PNG_INFO_IDAT;
+
+   /* Read rest of file, and get additional chunks in info_ptr - REQUIRED */
+   png_read_end(png_ptr, info_ptr);
+
+   PNG_UNUSED(params)
+}
+#endif /* INFO_IMAGE */
+#endif /* SEQUENTIAL_READ */
+
+#ifdef PNG_SIMPLIFIED_READ_SUPPORTED
+/* SIMPLIFIED READ
+ *
+ * This code currently relies on the sequential reader, though it could easily
+ * be made to work with the progressive one.
+ */
+/* Arguments to png_image_finish_read: */
+
+/* Encoding of PNG data (used by the color-map code) */
+#  define P_NOTSET  0 /* File encoding not yet known */
+#  define P_sRGB    1 /* 8-bit encoded to sRGB gamma */
+#  define P_LINEAR  2 /* 16-bit linear: not encoded, NOT pre-multiplied! */
+#  define P_FILE    3 /* 8-bit encoded to file gamma, not sRGB or linear */
+#  define P_LINEAR8 4 /* 8-bit linear: only from a file value */
+
+/* Color-map processing: after libpng has run on the PNG image further
+ * processing may be needed to convert the data to color-map indices.
+ */
+#define PNG_CMAP_NONE      0
+#define PNG_CMAP_GA        1 /* Process GA data to a color-map with alpha */
+#define PNG_CMAP_TRANS     2 /* Process GA data to a background index */
+#define PNG_CMAP_RGB       3 /* Process RGB data */
+#define PNG_CMAP_RGB_ALPHA 4 /* Process RGBA data */
+
+/* The following document where the background is for each processing case. */
+#define PNG_CMAP_NONE_BACKGROUND      256
+#define PNG_CMAP_GA_BACKGROUND        231
+#define PNG_CMAP_TRANS_BACKGROUND     254
+#define PNG_CMAP_RGB_BACKGROUND       256
+#define PNG_CMAP_RGB_ALPHA_BACKGROUND 216
+
+typedef struct
+{
+   /* Arguments: */
+   png_imagep image;
+   png_voidp  buffer;
+   png_int_32 row_stride;
+   png_voidp  colormap;
+   png_const_colorp background;
+   /* Local variables: */
+   png_voidp       local_row;
+   png_voidp       first_row;
+   ptrdiff_t       row_bytes;           /* step between rows */
+   int             file_encoding;       /* E_ values above */
+   png_fixed_point gamma_to_linear;     /* For P_FILE, reciprocal of gamma */
+   int             colormap_processing; /* PNG_CMAP_ values above */
+} png_image_read_control;
+
+/* Do all the *safe* initialization - 'safe' means that png_error won't be
+ * called, so setting up the jmp_buf is not required.  This means that anything
+ * called from here must *not* call png_malloc - it has to call png_malloc_warn
+ * instead so that control is returned safely back to this routine.
+ */
+static int
+png_image_read_init(png_imagep image)
+{
+   if (image->opaque == NULL)
+   {
+      png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, image,
+          png_safe_error, png_safe_warning);
+
+      /* And set the rest of the structure to NULL to ensure that the various
+       * fields are consistent.
+       */
+      memset(image, 0, (sizeof *image));
+      image->version = PNG_IMAGE_VERSION;
+
+      if (png_ptr != NULL)
+      {
+         png_infop info_ptr = png_create_info_struct(png_ptr);
+
+         if (info_ptr != NULL)
+         {
+            png_controlp control = png_voidcast(png_controlp,
+               png_malloc_warn(png_ptr, (sizeof *control)));
+
+            if (control != NULL)
+            {
+               memset(control, 0, (sizeof *control));
+
+               control->png_ptr = png_ptr;
+               control->info_ptr = info_ptr;
+               control->for_write = 0;
+
+               image->opaque = control;
+               return 1;
+            }
+
+            /* Error clean up */
+            png_destroy_info_struct(png_ptr, &info_ptr);
+         }
+
+         png_destroy_read_struct(&png_ptr, NULL, NULL);
+      }
+
+      return png_image_error(image, "png_image_read: out of memory");
+   }
+
+   return png_image_error(image, "png_image_read: opaque pointer not NULL");
+}
+
+/* Utility to find the base format of a PNG file from a png_struct. */
+static png_uint_32
+png_image_format(png_structrp png_ptr)
+{
+   png_uint_32 format = 0;
+
+   if ((png_ptr->color_type & PNG_COLOR_MASK_COLOR) != 0)
+      format |= PNG_FORMAT_FLAG_COLOR;
+
+   if ((png_ptr->color_type & PNG_COLOR_MASK_ALPHA) != 0)
+      format |= PNG_FORMAT_FLAG_ALPHA;
+
+   /* Use png_ptr here, not info_ptr, because by examination png_handle_tRNS
+    * sets the png_struct fields; that's all we are interested in here.  The
+    * precise interaction with an app call to png_set_tRNS and PNG file reading
+    * is unclear.
+    */
+   else if (png_ptr->num_trans > 0)
+      format |= PNG_FORMAT_FLAG_ALPHA;
+
+   if (png_ptr->bit_depth == 16)
+      format |= PNG_FORMAT_FLAG_LINEAR;
+
+   if ((png_ptr->color_type & PNG_COLOR_MASK_PALETTE) != 0)
+      format |= PNG_FORMAT_FLAG_COLORMAP;
+
+   return format;
+}
+
+/* Is the given gamma significantly different from sRGB?  The test is the same
+ * one used in pngrtran.c when deciding whether to do gamma correction.  The
+ * arithmetic optimizes the division by using the fact that the inverse of the
+ * file sRGB gamma is 2.2
+ */
+static int
+png_gamma_not_sRGB(png_fixed_point g)
+{
+   if (g < PNG_FP_1)
+   {
+      /* An uninitialized gamma is assumed to be sRGB for the simplified API. */
+      if (g == 0)
+         return 0;
+
+      return png_gamma_significant((g * 11 + 2)/5 /* i.e. *2.2, rounded */);
+   }
+
+   return 1;
+}
+
+/* Do the main body of a 'png_image_begin_read' function; read the PNG file
+ * header and fill in all the information.  This is executed in a safe context,
+ * unlike the init routine above.
+ */
+static int
+png_image_read_header(png_voidp argument)
+{
+   png_imagep image = png_voidcast(png_imagep, argument);
+   png_structrp png_ptr = image->opaque->png_ptr;
+   png_inforp info_ptr = image->opaque->info_ptr;
+
+   png_set_benign_errors(png_ptr, 1/*warn*/);
+   png_read_info(png_ptr, info_ptr);
+
+   /* Do this the fast way; just read directly out of png_struct. */
+   image->width = png_ptr->width;
+   image->height = png_ptr->height;
+
+   {
+      png_uint_32 format = png_image_format(png_ptr);
+
+      image->format = format;
+
+#ifdef PNG_COLORSPACE_SUPPORTED
+      /* Does the colorspace match sRGB?  If there is no color endpoint
+       * (colorant) information assume yes, otherwise require the
+       * 'ENDPOINTS_MATCHP_sRGB' colorspace flag to have been set.  If the
+       * colorspace has been determined to be invalid ignore it.
+       */
+      if ((format & PNG_FORMAT_FLAG_COLOR) != 0 && ((png_ptr->colorspace.flags
+         & (PNG_COLORSPACE_HAVE_ENDPOINTS|PNG_COLORSPACE_ENDPOINTS_MATCH_sRGB|
+            PNG_COLORSPACE_INVALID)) == PNG_COLORSPACE_HAVE_ENDPOINTS))
+         image->flags |= PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB;
+#endif
+   }
+
+   /* We need the maximum number of entries regardless of the format the
+    * application sets here.
+    */
+   {
+      png_uint_32 cmap_entries;
+
+      switch (png_ptr->color_type)
+      {
+         case PNG_COLOR_TYPE_GRAY:
+            cmap_entries = 1U << png_ptr->bit_depth;
+            break;
+
+         case PNG_COLOR_TYPE_PALETTE:
+            cmap_entries = png_ptr->num_palette;
+            break;
+
+         default:
+            cmap_entries = 256;
+            break;
+      }
+
+      if (cmap_entries > 256)
+         cmap_entries = 256;
+
+      image->colormap_entries = cmap_entries;
+   }
+
+   return 1;
+}
+
+#ifdef PNG_STDIO_SUPPORTED
+int PNGAPI
+png_image_begin_read_from_stdio(png_imagep image, FILE* file)
+{
+   if (image != NULL && image->version == PNG_IMAGE_VERSION)
+   {
+      if (file != NULL)
+      {
+         if (png_image_read_init(image) != 0)
+         {
+            /* This is slightly evil, but png_init_io doesn't do anything other
+             * than this and we haven't changed the standard IO functions so
+             * this saves a 'safe' function.
+             */
+            image->opaque->png_ptr->io_ptr = file;
+            return png_safe_execute(image, png_image_read_header, image);
+         }
+      }
+
+      else
+         return png_image_error(image,
+            "png_image_begin_read_from_stdio: invalid argument");
+   }
+
+   else if (image != NULL)
+      return png_image_error(image,
+         "png_image_begin_read_from_stdio: incorrect PNG_IMAGE_VERSION");
+
+   return 0;
+}
+
+int PNGAPI
+png_image_begin_read_from_file(png_imagep image, const char *file_name)
+{
+   if (image != NULL && image->version == PNG_IMAGE_VERSION)
+   {
+      if (file_name != NULL)
+      {
+         FILE *fp = fopen(file_name, "rb");
+
+         if (fp != NULL)
+         {
+            if (png_image_read_init(image) != 0)
+            {
+               image->opaque->png_ptr->io_ptr = fp;
+               image->opaque->owned_file = 1;
+               return png_safe_execute(image, png_image_read_header, image);
+            }
+
+            /* Clean up: just the opened file. */
+            (void)fclose(fp);
+         }
+
+         else
+            return png_image_error(image, strerror(errno));
+      }
+
+      else
+         return png_image_error(image,
+            "png_image_begin_read_from_file: invalid argument");
+   }
+
+   else if (image != NULL)
+      return png_image_error(image,
+         "png_image_begin_read_from_file: incorrect PNG_IMAGE_VERSION");
+
+   return 0;
+}
+#endif /* STDIO */
+
+static void PNGCBAPI
+png_image_memory_read(png_structp png_ptr, png_bytep out, png_size_t need)
+{
+   if (png_ptr != NULL)
+   {
+      png_imagep image = png_voidcast(png_imagep, png_ptr->io_ptr);
+      if (image != NULL)
+      {
+         png_controlp cp = image->opaque;
+         if (cp != NULL)
+         {
+            png_const_bytep memory = cp->memory;
+            png_size_t size = cp->size;
+
+            if (memory != NULL && size >= need)
+            {
+               memcpy(out, memory, need);
+               cp->memory = memory + need;
+               cp->size = size - need;
+               return;
+            }
+
+            png_error(png_ptr, "read beyond end of data");
+         }
+      }
+
+      png_error(png_ptr, "invalid memory read");
+   }
+}
+
+int PNGAPI png_image_begin_read_from_memory(png_imagep image,
+   png_const_voidp memory, png_size_t size)
+{
+   if (image != NULL && image->version == PNG_IMAGE_VERSION)
+   {
+      if (memory != NULL && size > 0)
+      {
+         if (png_image_read_init(image) != 0)
+         {
+            /* Now set the IO functions to read from the memory buffer and
+             * store it into io_ptr.  Again do this in-place to avoid calling a
+             * libpng function that requires error handling.
+             */
+            image->opaque->memory = png_voidcast(png_const_bytep, memory);
+            image->opaque->size = size;
+            image->opaque->png_ptr->io_ptr = image;
+            image->opaque->png_ptr->read_data_fn = png_image_memory_read;
+
+            return png_safe_execute(image, png_image_read_header, image);
+         }
+      }
+
+      else
+         return png_image_error(image,
+            "png_image_begin_read_from_memory: invalid argument");
+   }
+
+   else if (image != NULL)
+      return png_image_error(image,
+         "png_image_begin_read_from_memory: incorrect PNG_IMAGE_VERSION");
+
+   return 0;
+}
+
+/* Utility function to skip chunks that are not used by the simplified image
+ * read functions and an appropriate macro to call it.
+ */
+#ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED
+static void
+png_image_skip_unused_chunks(png_structrp png_ptr)
+{
+   /* Prepare the reader to ignore all recognized chunks whose data will not
+    * be used, i.e., all chunks recognized by libpng except for those
+    * involved in basic image reading:
+    *
+    *    IHDR, PLTE, IDAT, IEND
+    *
+    * Or image data handling:
+    *
+    *    tRNS, bKGD, gAMA, cHRM, sRGB, [iCCP] and sBIT.
+    *
+    * This provides a small performance improvement and eliminates any
+    * potential vulnerability to security problems in the unused chunks.
+    *
+    * At present the iCCP chunk data isn't used, so iCCP chunk can be ignored
+    * too.  This allows the simplified API to be compiled without iCCP support,
+    * however if the support is there the chunk is still checked to detect
+    * errors (which are unfortunately quite common.)
+    */
+   {
+         static PNG_CONST png_byte chunks_to_process[] = {
+            98,  75,  71,  68, '\0',  /* bKGD */
+            99,  72,  82,  77, '\0',  /* cHRM */
+           103,  65,  77,  65, '\0',  /* gAMA */
+#        ifdef PNG_READ_iCCP_SUPPORTED
+           105,  67,  67,  80, '\0',  /* iCCP */
+#        endif
+           115,  66,  73,  84, '\0',  /* sBIT */
+           115,  82,  71,  66, '\0',  /* sRGB */
+           };
+
+       /* Ignore unknown chunks and all other chunks except for the
+        * IHDR, PLTE, tRNS, IDAT, and IEND chunks.
+        */
+       png_set_keep_unknown_chunks(png_ptr, PNG_HANDLE_CHUNK_NEVER,
+         NULL, -1);
+
+       /* But do not ignore image data handling chunks */
+       png_set_keep_unknown_chunks(png_ptr, PNG_HANDLE_CHUNK_AS_DEFAULT,
+         chunks_to_process, (int)/*SAFE*/(sizeof chunks_to_process)/5);
+    }
+}
+
+#  define PNG_SKIP_CHUNKS(p) png_image_skip_unused_chunks(p)
+#else
+#  define PNG_SKIP_CHUNKS(p) ((void)0)
+#endif /* HANDLE_AS_UNKNOWN */
+
+/* The following macro gives the exact rounded answer for all values in the
+ * range 0..255 (it actually divides by 51.2, but the rounding still generates
+ * the correct numbers 0..5
+ */
+#define PNG_DIV51(v8) (((v8) * 5 + 130) >> 8)
+
+/* Utility functions to make particular color-maps */
+static void
+set_file_encoding(png_image_read_control *display)
+{
+   png_fixed_point g = display->image->opaque->png_ptr->colorspace.gamma;
+   if (png_gamma_significant(g) != 0)
+   {
+      if (png_gamma_not_sRGB(g) != 0)
+      {
+         display->file_encoding = P_FILE;
+         display->gamma_to_linear = png_reciprocal(g);
+      }
+
+      else
+         display->file_encoding = P_sRGB;
+   }
+
+   else
+      display->file_encoding = P_LINEAR8;
+}
+
+static unsigned int
+decode_gamma(png_image_read_control *display, png_uint_32 value, int encoding)
+{
+   if (encoding == P_FILE) /* double check */
+      encoding = display->file_encoding;
+
+   if (encoding == P_NOTSET) /* must be the file encoding */
+   {
+      set_file_encoding(display);
+      encoding = display->file_encoding;
+   }
+
+   switch (encoding)
+   {
+      case P_FILE:
+         value = png_gamma_16bit_correct(value*257, display->gamma_to_linear);
+         break;
+
+      case P_sRGB:
+         value = png_sRGB_table[value];
+         break;
+
+      case P_LINEAR:
+         break;
+
+      case P_LINEAR8:
+         value *= 257;
+         break;
+
+#ifdef __GNUC__
+      default:
+         png_error(display->image->opaque->png_ptr,
+            "unexpected encoding (internal error)");
+#endif
+   }
+
+   return value;
+}
+
+static png_uint_32
+png_colormap_compose(png_image_read_control *display,
+   png_uint_32 foreground, int foreground_encoding, png_uint_32 alpha,
+   png_uint_32 background, int encoding)
+{
+   /* The file value is composed on the background, the background has the given
+    * encoding and so does the result, the file is encoded with P_FILE and the
+    * file and alpha are 8-bit values.  The (output) encoding will always be
+    * P_LINEAR or P_sRGB.
+    */
+   png_uint_32 f = decode_gamma(display, foreground, foreground_encoding);
+   png_uint_32 b = decode_gamma(display, background, encoding);
+
+   /* The alpha is always an 8-bit value (it comes from the palette), the value
+    * scaled by 255 is what PNG_sRGB_FROM_LINEAR requires.
+    */
+   f = f * alpha + b * (255-alpha);
+
+   if (encoding == P_LINEAR)
+   {
+      /* Scale to 65535; divide by 255, approximately (in fact this is extremely
+       * accurate, it divides by 255.00000005937181414556, with no overflow.)
+       */
+      f *= 257; /* Now scaled by 65535 */
+      f += f >> 16;
+      f = (f+32768) >> 16;
+   }
+
+   else /* P_sRGB */
+      f = PNG_sRGB_FROM_LINEAR(f);
+
+   return f;
+}
+
+/* NOTE: P_LINEAR values to this routine must be 16-bit, but P_FILE values must
+ * be 8-bit.
+ */
+static void
+png_create_colormap_entry(png_image_read_control *display,
+   png_uint_32 ip, png_uint_32 red, png_uint_32 green, png_uint_32 blue,
+   png_uint_32 alpha, int encoding)
+{
+   png_imagep image = display->image;
+   const int output_encoding = (image->format & PNG_FORMAT_FLAG_LINEAR) != 0 ?
+      P_LINEAR : P_sRGB;
+   const int convert_to_Y = (image->format & PNG_FORMAT_FLAG_COLOR) == 0 &&
+      (red != green || green != blue);
+
+   if (ip > 255)
+      png_error(image->opaque->png_ptr, "color-map index out of range");
+
+   /* Update the cache with whether the file gamma is significantly different
+    * from sRGB.
+    */
+   if (encoding == P_FILE)
+   {
+      if (display->file_encoding == P_NOTSET)
+         set_file_encoding(display);
+
+      /* Note that the cached value may be P_FILE too, but if it is then the
+       * gamma_to_linear member has been set.
+       */
+      encoding = display->file_encoding;
+   }
+
+   if (encoding == P_FILE)
+   {
+      png_fixed_point g = display->gamma_to_linear;
+
+      red = png_gamma_16bit_correct(red*257, g);
+      green = png_gamma_16bit_correct(green*257, g);
+      blue = png_gamma_16bit_correct(blue*257, g);
+
+      if (convert_to_Y != 0 || output_encoding == P_LINEAR)
+      {
+         alpha *= 257;
+         encoding = P_LINEAR;
+      }
+
+      else
+      {
+         red = PNG_sRGB_FROM_LINEAR(red * 255);
+         green = PNG_sRGB_FROM_LINEAR(green * 255);
+         blue = PNG_sRGB_FROM_LINEAR(blue * 255);
+         encoding = P_sRGB;
+      }
+   }
+
+   else if (encoding == P_LINEAR8)
+   {
+      /* This encoding occurs quite frequently in test cases because PngSuite
+       * includes a gAMA 1.0 chunk with most images.
+       */
+      red *= 257;
+      green *= 257;
+      blue *= 257;
+      alpha *= 257;
+      encoding = P_LINEAR;
+   }
+
+   else if (encoding == P_sRGB &&
+       (convert_to_Y  != 0 || output_encoding == P_LINEAR))
+   {
+      /* The values are 8-bit sRGB values, but must be converted to 16-bit
+       * linear.
+       */
+      red = png_sRGB_table[red];
+      green = png_sRGB_table[green];
+      blue = png_sRGB_table[blue];
+      alpha *= 257;
+      encoding = P_LINEAR;
+   }
+
+   /* This is set if the color isn't gray but the output is. */
+   if (encoding == P_LINEAR)
+   {
+      if (convert_to_Y != 0)
+      {
+         /* NOTE: these values are copied from png_do_rgb_to_gray */
+         png_uint_32 y = (png_uint_32)6968 * red  + (png_uint_32)23434 * green +
+            (png_uint_32)2366 * blue;
+
+         if (output_encoding == P_LINEAR)
+            y = (y + 16384) >> 15;
+
+         else
+         {
+            /* y is scaled by 32768, we need it scaled by 255: */
+            y = (y + 128) >> 8;
+            y *= 255;
+            y = PNG_sRGB_FROM_LINEAR((y + 64) >> 7);
+            alpha = PNG_DIV257(alpha);
+            encoding = P_sRGB;
+         }
+
+         blue = red = green = y;
+      }
+
+      else if (output_encoding == P_sRGB)
+      {
+         red = PNG_sRGB_FROM_LINEAR(red * 255);
+         green = PNG_sRGB_FROM_LINEAR(green * 255);
+         blue = PNG_sRGB_FROM_LINEAR(blue * 255);
+         alpha = PNG_DIV257(alpha);
+         encoding = P_sRGB;
+      }
+   }
+
+   if (encoding != output_encoding)
+      png_error(image->opaque->png_ptr, "bad encoding (internal error)");
+
+   /* Store the value. */
+   {
+#     ifdef PNG_FORMAT_AFIRST_SUPPORTED
+         const int afirst = (image->format & PNG_FORMAT_FLAG_AFIRST) != 0 &&
+            (image->format & PNG_FORMAT_FLAG_ALPHA) != 0;
+#     else
+#        define afirst 0
+#     endif
+#     ifdef PNG_FORMAT_BGR_SUPPORTED
+         const int bgr = (image->format & PNG_FORMAT_FLAG_BGR) != 0 ? 2 : 0;
+#     else
+#        define bgr 0
+#     endif
+
+      if (output_encoding == P_LINEAR)
+      {
+         png_uint_16p entry = png_voidcast(png_uint_16p, display->colormap);
+
+         entry += ip * PNG_IMAGE_SAMPLE_CHANNELS(image->format);
+
+         /* The linear 16-bit values must be pre-multiplied by the alpha channel
+          * value, if less than 65535 (this is, effectively, composite on black
+          * if the alpha channel is removed.)
+          */
+         switch (PNG_IMAGE_SAMPLE_CHANNELS(image->format))
+         {
+            case 4:
+               entry[afirst ? 0 : 3] = (png_uint_16)alpha;
+               /* FALL THROUGH */
+
+            case 3:
+               if (alpha < 65535)
+               {
+                  if (alpha > 0)
+                  {
+                     blue = (blue * alpha + 32767U)/65535U;
+                     green = (green * alpha + 32767U)/65535U;
+                     red = (red * alpha + 32767U)/65535U;
+                  }
+
+                  else
+                     red = green = blue = 0;
+               }
+               entry[afirst + (2 ^ bgr)] = (png_uint_16)blue;
+               entry[afirst + 1] = (png_uint_16)green;
+               entry[afirst + bgr] = (png_uint_16)red;
+               break;
+
+            case 2:
+               entry[1 ^ afirst] = (png_uint_16)alpha;
+               /* FALL THROUGH */
+
+            case 1:
+               if (alpha < 65535)
+               {
+                  if (alpha > 0)
+                     green = (green * alpha + 32767U)/65535U;
+
+                  else
+                     green = 0;
+               }
+               entry[afirst] = (png_uint_16)green;
+               break;
+
+            default:
+               break;
+         }
+      }
+
+      else /* output encoding is P_sRGB */
+      {
+         png_bytep entry = png_voidcast(png_bytep, display->colormap);
+
+         entry += ip * PNG_IMAGE_SAMPLE_CHANNELS(image->format);
+
+         switch (PNG_IMAGE_SAMPLE_CHANNELS(image->format))
+         {
+            case 4:
+               entry[afirst ? 0 : 3] = (png_byte)alpha;
+            case 3:
+               entry[afirst + (2 ^ bgr)] = (png_byte)blue;
+               entry[afirst + 1] = (png_byte)green;
+               entry[afirst + bgr] = (png_byte)red;
+               break;
+
+            case 2:
+               entry[1 ^ afirst] = (png_byte)alpha;
+            case 1:
+               entry[afirst] = (png_byte)green;
+               break;
+
+            default:
+               break;
+         }
+      }
+
+#     ifdef afirst
+#        undef afirst
+#     endif
+#     ifdef bgr
+#        undef bgr
+#     endif
+   }
+}
+
+static int
+make_gray_file_colormap(png_image_read_control *display)
+{
+   unsigned int i;
+
+   for (i=0; i<256; ++i)
+      png_create_colormap_entry(display, i, i, i, i, 255, P_FILE);
+
+   return i;
+}
+
+static int
+make_gray_colormap(png_image_read_control *display)
+{
+   unsigned int i;
+
+   for (i=0; i<256; ++i)
+      png_create_colormap_entry(display, i, i, i, i, 255, P_sRGB);
+
+   return i;
+}
+#define PNG_GRAY_COLORMAP_ENTRIES 256
+
+static int
+make_ga_colormap(png_image_read_control *display)
+{
+   unsigned int i, a;
+
+   /* Alpha is retained, the output will be a color-map with entries
+    * selected by six levels of alpha.  One transparent entry, 6 gray
+    * levels for all the intermediate alpha values, leaving 230 entries
+    * for the opaque grays.  The color-map entries are the six values
+    * [0..5]*51, the GA processing uses PNG_DIV51(value) to find the
+    * relevant entry.
+    *
+    * if (alpha > 229) // opaque
+    * {
+    *    // The 231 entries are selected to make the math below work:
+    *    base = 0;
+    *    entry = (231 * gray + 128) >> 8;
+    * }
+    * else if (alpha < 26) // transparent
+    * {
+    *    base = 231;
+    *    entry = 0;
+    * }
+    * else // partially opaque
+    * {
+    *    base = 226 + 6 * PNG_DIV51(alpha);
+    *    entry = PNG_DIV51(gray);
+    * }
+    */
+   i = 0;
+   while (i < 231)
+   {
+      unsigned int gray = (i * 256 + 115) / 231;
+      png_create_colormap_entry(display, i++, gray, gray, gray, 255, P_sRGB);
+   }
+
+   /* 255 is used here for the component values for consistency with the code
+    * that undoes premultiplication in pngwrite.c.
+    */
+   png_create_colormap_entry(display, i++, 255, 255, 255, 0, P_sRGB);
+
+   for (a=1; a<5; ++a)
+   {
+      unsigned int g;
+
+      for (g=0; g<6; ++g)
+         png_create_colormap_entry(display, i++, g*51, g*51, g*51, a*51,
+            P_sRGB);
+   }
+
+   return i;
+}
+
+#define PNG_GA_COLORMAP_ENTRIES 256
+
+static int
+make_rgb_colormap(png_image_read_control *display)
+{
+   unsigned int i, r;
+
+   /* Build a 6x6x6 opaque RGB cube */
+   for (i=r=0; r<6; ++r)
+   {
+      unsigned int g;
+
+      for (g=0; g<6; ++g)
+      {
+         unsigned int b;
+
+         for (b=0; b<6; ++b)
+            png_create_colormap_entry(display, i++, r*51, g*51, b*51, 255,
+               P_sRGB);
+      }
+   }
+
+   return i;
+}
+
+#define PNG_RGB_COLORMAP_ENTRIES 216
+
+/* Return a palette index to the above palette given three 8-bit sRGB values. */
+#define PNG_RGB_INDEX(r,g,b) \
+   ((png_byte)(6 * (6 * PNG_DIV51(r) + PNG_DIV51(g)) + PNG_DIV51(b)))
+
+static int
+png_image_read_colormap(png_voidp argument)
+{
+   png_image_read_control *display =
+      png_voidcast(png_image_read_control*, argument);
+   const png_imagep image = display->image;
+
+   const png_structrp png_ptr = image->opaque->png_ptr;
+   const png_uint_32 output_format = image->format;
+   const int output_encoding = (output_format & PNG_FORMAT_FLAG_LINEAR) != 0 ?
+      P_LINEAR : P_sRGB;
+
+   unsigned int cmap_entries;
+   unsigned int output_processing;        /* Output processing option */
+   unsigned int data_encoding = P_NOTSET; /* Encoding libpng must produce */
+
+   /* Background information; the background color and the index of this color
+    * in the color-map if it exists (else 256).
+    */
+   unsigned int background_index = 256;
+   png_uint_32 back_r, back_g, back_b;
+
+   /* Flags to accumulate things that need to be done to the input. */
+   int expand_tRNS = 0;
+
+   /* Exclude the NYI feature of compositing onto a color-mapped buffer; it is
+    * very difficult to do, the results look awful, and it is difficult to see
+    * what possible use it is because the application can't control the
+    * color-map.
+    */
+   if (((png_ptr->color_type & PNG_COLOR_MASK_ALPHA) != 0 ||
+         png_ptr->num_trans > 0) /* alpha in input */ &&
+      ((output_format & PNG_FORMAT_FLAG_ALPHA) == 0) /* no alpha in output */)
+   {
+      if (output_encoding == P_LINEAR) /* compose on black */
+         back_b = back_g = back_r = 0;
+
+      else if (display->background == NULL /* no way to remove it */)
+         png_error(png_ptr,
+            "a background color must be supplied to remove alpha/transparency");
+
+      /* Get a copy of the background color (this avoids repeating the checks
+       * below.)  The encoding is 8-bit sRGB or 16-bit linear, depending on the
+       * output format.
+       */
+      else
+      {
+         back_g = display->background->green;
+         if ((output_format & PNG_FORMAT_FLAG_COLOR) != 0)
+         {
+            back_r = display->background->red;
+            back_b = display->background->blue;
+         }
+         else
+            back_b = back_r = back_g;
+      }
+   }
+
+   else if (output_encoding == P_LINEAR)
+      back_b = back_r = back_g = 65535;
+
+   else
+      back_b = back_r = back_g = 255;
+
+   /* Default the input file gamma if required - this is necessary because
+    * libpng assumes that if no gamma information is present the data is in the
+    * output format, but the simplified API deduces the gamma from the input
+    * format.
+    */
+   if ((png_ptr->colorspace.flags & PNG_COLORSPACE_HAVE_GAMMA) == 0)
+   {
+      /* Do this directly, not using the png_colorspace functions, to ensure
+       * that it happens even if the colorspace is invalid (though probably if
+       * it is the setting will be ignored)  Note that the same thing can be
+       * achieved at the application interface with png_set_gAMA.
+       */
+      if (png_ptr->bit_depth == 16 &&
+         (image->flags & PNG_IMAGE_FLAG_16BIT_sRGB) == 0)
+         png_ptr->colorspace.gamma = PNG_GAMMA_LINEAR;
+
+      else
+         png_ptr->colorspace.gamma = PNG_GAMMA_sRGB_INVERSE;
+
+      png_ptr->colorspace.flags |= PNG_COLORSPACE_HAVE_GAMMA;
+   }
+
+   /* Decide what to do based on the PNG color type of the input data.  The
+    * utility function png_create_colormap_entry deals with most aspects of the
+    * output transformations; this code works out how to produce bytes of
+    * color-map entries from the original format.
+    */
+   switch (png_ptr->color_type)
+   {
+      case PNG_COLOR_TYPE_GRAY:
+         if (png_ptr->bit_depth <= 8)
+         {
+            /* There at most 256 colors in the output, regardless of
+             * transparency.
+             */
+            unsigned int step, i, val, trans = 256/*ignore*/, back_alpha = 0;
+
+            cmap_entries = 1U << png_ptr->bit_depth;
+            if (cmap_entries > image->colormap_entries)
+               png_error(png_ptr, "gray[8] color-map: too few entries");
+
+            step = 255 / (cmap_entries - 1);
+            output_processing = PNG_CMAP_NONE;
+
+            /* If there is a tRNS chunk then this either selects a transparent
+             * value or, if the output has no alpha, the background color.
+             */
+            if (png_ptr->num_trans > 0)
+            {
+               trans = png_ptr->trans_color.gray;
+
+               if ((output_format & PNG_FORMAT_FLAG_ALPHA) == 0)
+                  back_alpha = output_encoding == P_LINEAR ? 65535 : 255;
+            }
+
+            /* png_create_colormap_entry just takes an RGBA and writes the
+             * corresponding color-map entry using the format from 'image',
+             * including the required conversion to sRGB or linear as
+             * appropriate.  The input values are always either sRGB (if the
+             * gamma correction flag is 0) or 0..255 scaled file encoded values
+             * (if the function must gamma correct them).
+             */
+            for (i=val=0; i<cmap_entries; ++i, val += step)
+            {
+               /* 'i' is a file value.  While this will result in duplicated
+                * entries for 8-bit non-sRGB encoded files it is necessary to
+                * have non-gamma corrected values to do tRNS handling.
+                */
+               if (i != trans)
+                  png_create_colormap_entry(display, i, val, val, val, 255,
+                     P_FILE/*8-bit with file gamma*/);
+
+               /* Else this entry is transparent.  The colors don't matter if
+                * there is an alpha channel (back_alpha == 0), but it does no
+                * harm to pass them in; the values are not set above so this
+                * passes in white.
+                *
+                * NOTE: this preserves the full precision of the application
+                * supplied background color when it is used.
+                */
+               else
+                  png_create_colormap_entry(display, i, back_r, back_g, back_b,
+                     back_alpha, output_encoding);
+            }
+
+            /* We need libpng to preserve the original encoding. */
+            data_encoding = P_FILE;
+
+            /* The rows from libpng, while technically gray values, are now also
+             * color-map indices; however, they may need to be expanded to 1
+             * byte per pixel.  This is what png_set_packing does (i.e., it
+             * unpacks the bit values into bytes.)
+             */
+            if (png_ptr->bit_depth < 8)
+               png_set_packing(png_ptr);
+         }
+
+         else /* bit depth is 16 */
+         {
+            /* The 16-bit input values can be converted directly to 8-bit gamma
+             * encoded values; however, if a tRNS chunk is present 257 color-map
+             * entries are required.  This means that the extra entry requires
+             * special processing; add an alpha channel, sacrifice gray level
+             * 254 and convert transparent (alpha==0) entries to that.
+             *
+             * Use libpng to chop the data to 8 bits.  Convert it to sRGB at the
+             * same time to minimize quality loss.  If a tRNS chunk is present
+             * this means libpng must handle it too; otherwise it is impossible
+             * to do the exact match on the 16-bit value.
+             *
+             * If the output has no alpha channel *and* the background color is
+             * gray then it is possible to let libpng handle the substitution by
+             * ensuring that the corresponding gray level matches the background
+             * color exactly.
+             */
+            data_encoding = P_sRGB;
+
+            if (PNG_GRAY_COLORMAP_ENTRIES > image->colormap_entries)
+               png_error(png_ptr, "gray[16] color-map: too few entries");
+
+            cmap_entries = make_gray_colormap(display);
+
+            if (png_ptr->num_trans > 0)
+            {
+               unsigned int back_alpha;
+
+               if ((output_format & PNG_FORMAT_FLAG_ALPHA) != 0)
+                  back_alpha = 0;
+
+               else
+               {
+                  if (back_r == back_g && back_g == back_b)
+                  {
+                     /* Background is gray; no special processing will be
+                      * required.
+                      */
+                     png_color_16 c;
+                     png_uint_32 gray = back_g;
+
+                     if (output_encoding == P_LINEAR)
+                     {
+                        gray = PNG_sRGB_FROM_LINEAR(gray * 255);
+
+                        /* And make sure the corresponding palette entry
+                         * matches.
+                         */
+                        png_create_colormap_entry(display, gray, back_g, back_g,
+                           back_g, 65535, P_LINEAR);
+                     }
+
+                     /* The background passed to libpng, however, must be the
+                      * sRGB value.
+                      */
+                     c.index = 0; /*unused*/
+                     c.gray = c.red = c.green = c.blue = (png_uint_16)gray;
+
+                     /* NOTE: does this work without expanding tRNS to alpha?
+                      * It should be the color->gray case below apparently
+                      * doesn't.
+                      */
+                     png_set_background_fixed(png_ptr, &c,
+                        PNG_BACKGROUND_GAMMA_SCREEN, 0/*need_expand*/,
+                        0/*gamma: not used*/);
+
+                     output_processing = PNG_CMAP_NONE;
+                     break;
+                  }
+#ifdef __COVERITY__
+                 /* Coverity claims that output_encoding cannot be 2 (P_LINEAR)
+                  * here.
+                  */
+                  back_alpha = 255;
+#else
+                  back_alpha = output_encoding == P_LINEAR ? 65535 : 255;
+#endif
+               }
+
+               /* output_processing means that the libpng-processed row will be
+                * 8-bit GA and it has to be processing to single byte color-map
+                * values.  Entry 254 is replaced by either a completely
+                * transparent entry or by the background color at full
+                * precision (and the background color is not a simple gray
+                * level in this case.)
+                */
+               expand_tRNS = 1;
+               output_processing = PNG_CMAP_TRANS;
+               background_index = 254;
+
+               /* And set (overwrite) color-map entry 254 to the actual
+                * background color at full precision.
+                */
+               png_create_colormap_entry(display, 254, back_r, back_g, back_b,
+                  back_alpha, output_encoding);
+            }
+
+            else
+               output_processing = PNG_CMAP_NONE;
+         }
+         break;
+
+      case PNG_COLOR_TYPE_GRAY_ALPHA:
+         /* 8-bit or 16-bit PNG with two channels - gray and alpha.  A minimum
+          * of 65536 combinations.  If, however, the alpha channel is to be
+          * removed there are only 256 possibilities if the background is gray.
+          * (Otherwise there is a subset of the 65536 possibilities defined by
+          * the triangle between black, white and the background color.)
+          *
+          * Reduce 16-bit files to 8-bit and sRGB encode the result.  No need to
+          * worry about tRNS matching - tRNS is ignored if there is an alpha
+          * channel.
+          */
+         data_encoding = P_sRGB;
+
+         if ((output_format & PNG_FORMAT_FLAG_ALPHA) != 0)
+         {
+            if (PNG_GA_COLORMAP_ENTRIES > image->colormap_entries)
+               png_error(png_ptr, "gray+alpha color-map: too few entries");
+
+            cmap_entries = make_ga_colormap(display);
+
+            background_index = PNG_CMAP_GA_BACKGROUND;
+            output_processing = PNG_CMAP_GA;
+         }
+
+         else /* alpha is removed */
+         {
+            /* Alpha must be removed as the PNG data is processed when the
+             * background is a color because the G and A channels are
+             * independent and the vector addition (non-parallel vectors) is a
+             * 2-D problem.
+             *
+             * This can be reduced to the same algorithm as above by making a
+             * colormap containing gray levels (for the opaque grays), a
+             * background entry (for a transparent pixel) and a set of four six
+             * level color values, one set for each intermediate alpha value.
+             * See the comments in make_ga_colormap for how this works in the
+             * per-pixel processing.
+             *
+             * If the background is gray, however, we only need a 256 entry gray
+             * level color map.  It is sufficient to make the entry generated
+             * for the background color be exactly the color specified.
+             */
+            if ((output_format & PNG_FORMAT_FLAG_COLOR) == 0 ||
+               (back_r == back_g && back_g == back_b))
+            {
+               /* Background is gray; no special processing will be required. */
+               png_color_16 c;
+               png_uint_32 gray = back_g;
+
+               if (PNG_GRAY_COLORMAP_ENTRIES > image->colormap_entries)
+                  png_error(png_ptr, "gray-alpha color-map: too few entries");
+
+               cmap_entries = make_gray_colormap(display);
+
+               if (output_encoding == P_LINEAR)
+               {
+                  gray = PNG_sRGB_FROM_LINEAR(gray * 255);
+
+                  /* And make sure the corresponding palette entry matches. */
+                  png_create_colormap_entry(display, gray, back_g, back_g,
+                     back_g, 65535, P_LINEAR);
+               }
+
+               /* The background passed to libpng, however, must be the sRGB
+                * value.
+                */
+               c.index = 0; /*unused*/
+               c.gray = c.red = c.green = c.blue = (png_uint_16)gray;
+
+               png_set_background_fixed(png_ptr, &c,
+                  PNG_BACKGROUND_GAMMA_SCREEN, 0/*need_expand*/,
+                  0/*gamma: not used*/);
+
+               output_processing = PNG_CMAP_NONE;
+            }
+
+            else
+            {
+               png_uint_32 i, a;
+
+               /* This is the same as png_make_ga_colormap, above, except that
+                * the entries are all opaque.
+                */
+               if (PNG_GA_COLORMAP_ENTRIES > image->colormap_entries)
+                  png_error(png_ptr, "ga-alpha color-map: too few entries");
+
+               i = 0;
+               while (i < 231)
+               {
+                  png_uint_32 gray = (i * 256 + 115) / 231;
+                  png_create_colormap_entry(display, i++, gray, gray, gray,
+                     255, P_sRGB);
+               }
+
+               /* NOTE: this preserves the full precision of the application
+                * background color.
+                */
+               background_index = i;
+               png_create_colormap_entry(display, i++, back_r, back_g, back_b,
+#ifdef __COVERITY__
+                 /* Coverity claims that output_encoding cannot be 2 (P_LINEAR)
+                  * here.
+                  */ 255U,
+#else
+                  output_encoding == P_LINEAR ? 65535U : 255U,
+#endif
+                  output_encoding);
+
+               /* For non-opaque input composite on the sRGB background - this
+                * requires inverting the encoding for each component.  The input
+                * is still converted to the sRGB encoding because this is a
+                * reasonable approximate to the logarithmic curve of human
+                * visual sensitivity, at least over the narrow range which PNG
+                * represents.  Consequently 'G' is always sRGB encoded, while
+                * 'A' is linear.  We need the linear background colors.
+                */
+               if (output_encoding == P_sRGB) /* else already linear */
+               {
+                  /* This may produce a value not exactly matching the
+                   * background, but that's ok because these numbers are only
+                   * used when alpha != 0
+                   */
+                  back_r = png_sRGB_table[back_r];
+                  back_g = png_sRGB_table[back_g];
+                  back_b = png_sRGB_table[back_b];
+               }
+
+               for (a=1; a<5; ++a)
+               {
+                  unsigned int g;
+
+                  /* PNG_sRGB_FROM_LINEAR expects a 16-bit linear value scaled
+                   * by an 8-bit alpha value (0..255).
+                   */
+                  png_uint_32 alpha = 51 * a;
+                  png_uint_32 back_rx = (255-alpha) * back_r;
+                  png_uint_32 back_gx = (255-alpha) * back_g;
+                  png_uint_32 back_bx = (255-alpha) * back_b;
+
+                  for (g=0; g<6; ++g)
+                  {
+                     png_uint_32 gray = png_sRGB_table[g*51] * alpha;
+
+                     png_create_colormap_entry(display, i++,
+                        PNG_sRGB_FROM_LINEAR(gray + back_rx),
+                        PNG_sRGB_FROM_LINEAR(gray + back_gx),
+                        PNG_sRGB_FROM_LINEAR(gray + back_bx), 255, P_sRGB);
+                  }
+               }
+
+               cmap_entries = i;
+               output_processing = PNG_CMAP_GA;
+            }
+         }
+         break;
+
+      case PNG_COLOR_TYPE_RGB:
+      case PNG_COLOR_TYPE_RGB_ALPHA:
+         /* Exclude the case where the output is gray; we can always handle this
+          * with the cases above.
+          */
+         if ((output_format & PNG_FORMAT_FLAG_COLOR) == 0)
+         {
+            /* The color-map will be grayscale, so we may as well convert the
+             * input RGB values to a simple grayscale and use the grayscale
+             * code above.
+             *
+             * NOTE: calling this apparently damages the recognition of the
+             * transparent color in background color handling; call
+             * png_set_tRNS_to_alpha before png_set_background_fixed.
+             */
+            png_set_rgb_to_gray_fixed(png_ptr, PNG_ERROR_ACTION_NONE, -1,
+               -1);
+            data_encoding = P_sRGB;
+
+            /* The output will now be one or two 8-bit gray or gray+alpha
+             * channels.  The more complex case arises when the input has alpha.
+             */
+            if ((png_ptr->color_type == PNG_COLOR_TYPE_RGB_ALPHA ||
+               png_ptr->num_trans > 0) &&
+               (output_format & PNG_FORMAT_FLAG_ALPHA) != 0)
+            {
+               /* Both input and output have an alpha channel, so no background
+                * processing is required; just map the GA bytes to the right
+                * color-map entry.
+                */
+               expand_tRNS = 1;
+
+               if (PNG_GA_COLORMAP_ENTRIES > image->colormap_entries)
+                  png_error(png_ptr, "rgb[ga] color-map: too few entries");
+
+               cmap_entries = make_ga_colormap(display);
+               background_index = PNG_CMAP_GA_BACKGROUND;
+               output_processing = PNG_CMAP_GA;
+            }
+
+            else
+            {
+               /* Either the input or the output has no alpha channel, so there
+                * will be no non-opaque pixels in the color-map; it will just be
+                * grayscale.
+                */
+               if (PNG_GRAY_COLORMAP_ENTRIES > image->colormap_entries)
+                  png_error(png_ptr, "rgb[gray] color-map: too few entries");
+
+               /* Ideally this code would use libpng to do the gamma correction,
+                * but if an input alpha channel is to be removed we will hit the
+                * libpng bug in gamma+compose+rgb-to-gray (the double gamma
+                * correction bug).  Fix this by dropping the gamma correction in
+                * this case and doing it in the palette; this will result in
+                * duplicate palette entries, but that's better than the
+                * alternative of double gamma correction.
+                */
+               if ((png_ptr->color_type == PNG_COLOR_TYPE_RGB_ALPHA ||
+                  png_ptr->num_trans > 0) &&
+                  png_gamma_not_sRGB(png_ptr->colorspace.gamma) != 0)
+               {
+                  cmap_entries = make_gray_file_colormap(display);
+                  data_encoding = P_FILE;
+               }
+
+               else
+                  cmap_entries = make_gray_colormap(display);
+
+               /* But if the input has alpha or transparency it must be removed
+                */
+               if (png_ptr->color_type == PNG_COLOR_TYPE_RGB_ALPHA ||
+                  png_ptr->num_trans > 0)
+               {
+                  png_color_16 c;
+                  png_uint_32 gray = back_g;
+
+                  /* We need to ensure that the application background exists in
+                   * the colormap and that completely transparent pixels map to
+                   * it.  Achieve this simply by ensuring that the entry
+                   * selected for the background really is the background color.
+                   */
+                  if (data_encoding == P_FILE) /* from the fixup above */
+                  {
+                     /* The app supplied a gray which is in output_encoding, we
+                      * need to convert it to a value of the input (P_FILE)
+                      * encoding then set this palette entry to the required
+                      * output encoding.
+                      */
+                     if (output_encoding == P_sRGB)
+                        gray = png_sRGB_table[gray]; /* now P_LINEAR */
+
+                     gray = PNG_DIV257(png_gamma_16bit_correct(gray,
+                        png_ptr->colorspace.gamma)); /* now P_FILE */
+
+                     /* And make sure the corresponding palette entry contains
+                      * exactly the required sRGB value.
+                      */
+                     png_create_colormap_entry(display, gray, back_g, back_g,
+                        back_g, 0/*unused*/, output_encoding);
+                  }
+
+                  else if (output_encoding == P_LINEAR)
+                  {
+                     gray = PNG_sRGB_FROM_LINEAR(gray * 255);
+
+                     /* And make sure the corresponding palette entry matches.
+                      */
+                     png_create_colormap_entry(display, gray, back_g, back_g,
+                        back_g, 0/*unused*/, P_LINEAR);
+                  }
+
+                  /* The background passed to libpng, however, must be the
+                   * output (normally sRGB) value.
+                   */
+                  c.index = 0; /*unused*/
+                  c.gray = c.red = c.green = c.blue = (png_uint_16)gray;
+
+                  /* NOTE: the following is apparently a bug in libpng. Without
+                   * it the transparent color recognition in
+                   * png_set_background_fixed seems to go wrong.
+                   */
+                  expand_tRNS = 1;
+                  png_set_background_fixed(png_ptr, &c,
+                     PNG_BACKGROUND_GAMMA_SCREEN, 0/*need_expand*/,
+                     0/*gamma: not used*/);
+               }
+
+               output_processing = PNG_CMAP_NONE;
+            }
+         }
+
+         else /* output is color */
+         {
+            /* We could use png_quantize here so long as there is no transparent
+             * color or alpha; png_quantize ignores alpha.  Easier overall just
+             * to do it once and using PNG_DIV51 on the 6x6x6 reduced RGB cube.
+             * Consequently we always want libpng to produce sRGB data.
+             */
+            data_encoding = P_sRGB;
+
+            /* Is there any transparency or alpha? */
+            if (png_ptr->color_type == PNG_COLOR_TYPE_RGB_ALPHA ||
+               png_ptr->num_trans > 0)
+            {
+               /* Is there alpha in the output too?  If so all four channels are
+                * processed into a special RGB cube with alpha support.
+                */
+               if ((output_format & PNG_FORMAT_FLAG_ALPHA) != 0)
+               {
+                  png_uint_32 r;
+
+                  if (PNG_RGB_COLORMAP_ENTRIES+1+27 > image->colormap_entries)
+                     png_error(png_ptr, "rgb+alpha color-map: too few entries");
+
+                  cmap_entries = make_rgb_colormap(display);
+
+                  /* Add a transparent entry. */
+                  png_create_colormap_entry(display, cmap_entries, 255, 255,
+                     255, 0, P_sRGB);
+
+                  /* This is stored as the background index for the processing
+                   * algorithm.
+                   */
+                  background_index = cmap_entries++;
+
+                  /* Add 27 r,g,b entries each with alpha 0.5. */
+                  for (r=0; r<256; r = (r << 1) | 0x7f)
+                  {
+                     png_uint_32 g;
+
+                     for (g=0; g<256; g = (g << 1) | 0x7f)
+                     {
+                        png_uint_32 b;
+
+                        /* This generates components with the values 0, 127 and
+                         * 255
+                         */
+                        for (b=0; b<256; b = (b << 1) | 0x7f)
+                           png_create_colormap_entry(display, cmap_entries++,
+                              r, g, b, 128, P_sRGB);
+                     }
+                  }
+
+                  expand_tRNS = 1;
+                  output_processing = PNG_CMAP_RGB_ALPHA;
+               }
+
+               else
+               {
+                  /* Alpha/transparency must be removed.  The background must
+                   * exist in the color map (achieved by setting adding it after
+                   * the 666 color-map).  If the standard processing code will
+                   * pick up this entry automatically that's all that is
+                   * required; libpng can be called to do the background
+                   * processing.
+                   */
+                  unsigned int sample_size =
+                     PNG_IMAGE_SAMPLE_SIZE(output_format);
+                  png_uint_32 r, g, b; /* sRGB background */
+
+                  if (PNG_RGB_COLORMAP_ENTRIES+1+27 > image->colormap_entries)
+                     png_error(png_ptr, "rgb-alpha color-map: too few entries");
+
+                  cmap_entries = make_rgb_colormap(display);
+
+                  png_create_colormap_entry(display, cmap_entries, back_r,
+                        back_g, back_b, 0/*unused*/, output_encoding);
+
+                  if (output_encoding == P_LINEAR)
+                  {
+                     r = PNG_sRGB_FROM_LINEAR(back_r * 255);
+                     g = PNG_sRGB_FROM_LINEAR(back_g * 255);
+                     b = PNG_sRGB_FROM_LINEAR(back_b * 255);
+                  }
+
+                  else
+                  {
+                     r = back_r;
+                     g = back_g;
+                     b = back_g;
+                  }
+
+                  /* Compare the newly-created color-map entry with the one the
+                   * PNG_CMAP_RGB algorithm will use.  If the two entries don't
+                   * match, add the new one and set this as the background
+                   * index.
+                   */
+                  if (memcmp((png_const_bytep)display->colormap +
+                        sample_size * cmap_entries,
+                     (png_const_bytep)display->colormap +
+                        sample_size * PNG_RGB_INDEX(r,g,b),
+                     sample_size) != 0)
+                  {
+                     /* The background color must be added. */
+                     background_index = cmap_entries++;
+
+                     /* Add 27 r,g,b entries each with created by composing with
+                      * the background at alpha 0.5.
+                      */
+                     for (r=0; r<256; r = (r << 1) | 0x7f)
+                     {
+                        for (g=0; g<256; g = (g << 1) | 0x7f)
+                        {
+                           /* This generates components with the values 0, 127
+                            * and 255
+                            */
+                           for (b=0; b<256; b = (b << 1) | 0x7f)
+                              png_create_colormap_entry(display, cmap_entries++,
+                                 png_colormap_compose(display, r, P_sRGB, 128,
+                                    back_r, output_encoding),
+                                 png_colormap_compose(display, g, P_sRGB, 128,
+                                    back_g, output_encoding),
+                                 png_colormap_compose(display, b, P_sRGB, 128,
+                                    back_b, output_encoding),
+                                 0/*unused*/, output_encoding);
+                        }
+                     }
+
+                     expand_tRNS = 1;
+                     output_processing = PNG_CMAP_RGB_ALPHA;
+                  }
+
+                  else /* background color is in the standard color-map */
+                  {
+                     png_color_16 c;
+
+                     c.index = 0; /*unused*/
+                     c.red = (png_uint_16)back_r;
+                     c.gray = c.green = (png_uint_16)back_g;
+                     c.blue = (png_uint_16)back_b;
+
+                     png_set_background_fixed(png_ptr, &c,
+                        PNG_BACKGROUND_GAMMA_SCREEN, 0/*need_expand*/,
+                        0/*gamma: not used*/);
+
+                     output_processing = PNG_CMAP_RGB;
+                  }
+               }
+            }
+
+            else /* no alpha or transparency in the input */
+            {
+               /* Alpha in the output is irrelevant, simply map the opaque input
+                * pixels to the 6x6x6 color-map.
+                */
+               if (PNG_RGB_COLORMAP_ENTRIES > image->colormap_entries)
+                  png_error(png_ptr, "rgb color-map: too few entries");
+
+               cmap_entries = make_rgb_colormap(display);
+               output_processing = PNG_CMAP_RGB;
+            }
+         }
+         break;
+
+      case PNG_COLOR_TYPE_PALETTE:
+         /* It's already got a color-map.  It may be necessary to eliminate the
+          * tRNS entries though.
+          */
+         {
+            unsigned int num_trans = png_ptr->num_trans;
+            png_const_bytep trans = num_trans > 0 ? png_ptr->trans_alpha : NULL;
+            png_const_colorp colormap = png_ptr->palette;
+            const int do_background = trans != NULL &&
+               (output_format & PNG_FORMAT_FLAG_ALPHA) == 0;
+            unsigned int i;
+
+            /* Just in case: */
+            if (trans == NULL)
+               num_trans = 0;
+
+            output_processing = PNG_CMAP_NONE;
+            data_encoding = P_FILE; /* Don't change from color-map indices */
+            cmap_entries = png_ptr->num_palette;
+            if (cmap_entries > 256)
+               cmap_entries = 256;
+
+            if (cmap_entries > image->colormap_entries)
+               png_error(png_ptr, "palette color-map: too few entries");
+
+            for (i=0; i < cmap_entries; ++i)
+            {
+               if (do_background != 0 && i < num_trans && trans[i] < 255)
+               {
+                  if (trans[i] == 0)
+                     png_create_colormap_entry(display, i, back_r, back_g,
+                        back_b, 0, output_encoding);
+
+                  else
+                  {
+                     /* Must compose the PNG file color in the color-map entry
+                      * on the sRGB color in 'back'.
+                      */
+                     png_create_colormap_entry(display, i,
+                        png_colormap_compose(display, colormap[i].red, P_FILE,
+                           trans[i], back_r, output_encoding),
+                        png_colormap_compose(display, colormap[i].green, P_FILE,
+                           trans[i], back_g, output_encoding),
+                        png_colormap_compose(display, colormap[i].blue, P_FILE,
+                           trans[i], back_b, output_encoding),
+                        output_encoding == P_LINEAR ? trans[i] * 257U :
+                           trans[i],
+                        output_encoding);
+                  }
+               }
+
+               else
+                  png_create_colormap_entry(display, i, colormap[i].red,
+                     colormap[i].green, colormap[i].blue,
+                     i < num_trans ? trans[i] : 255U, P_FILE/*8-bit*/);
+            }
+
+            /* The PNG data may have indices packed in fewer than 8 bits, it
+             * must be expanded if so.
+             */
+            if (png_ptr->bit_depth < 8)
+               png_set_packing(png_ptr);
+         }
+         break;
+
+      default:
+         png_error(png_ptr, "invalid PNG color type");
+         /*NOT REACHED*/
+   }
+
+   /* Now deal with the output processing */
+   if (expand_tRNS != 0 && png_ptr->num_trans > 0 &&
+       (png_ptr->color_type & PNG_COLOR_MASK_ALPHA) == 0)
+      png_set_tRNS_to_alpha(png_ptr);
+
+   switch (data_encoding)
+   {
+      case P_sRGB:
+         /* Change to 8-bit sRGB */
+         png_set_alpha_mode_fixed(png_ptr, PNG_ALPHA_PNG, PNG_GAMMA_sRGB);
+         /* FALL THROUGH */
+
+      case P_FILE:
+         if (png_ptr->bit_depth > 8)
+            png_set_scale_16(png_ptr);
+         break;
+
+#ifdef __GNUC__
+      default:
+         png_error(png_ptr, "bad data option (internal error)");
+#endif
+   }
+
+   if (cmap_entries > 256 || cmap_entries > image->colormap_entries)
+      png_error(png_ptr, "color map overflow (BAD internal error)");
+
+   image->colormap_entries = cmap_entries;
+
+   /* Double check using the recorded background index */
+   switch (output_processing)
+   {
+      case PNG_CMAP_NONE:
+         if (background_index != PNG_CMAP_NONE_BACKGROUND)
+            goto bad_background;
+         break;
+
+      case PNG_CMAP_GA:
+         if (background_index != PNG_CMAP_GA_BACKGROUND)
+            goto bad_background;
+         break;
+
+      case PNG_CMAP_TRANS:
+         if (background_index >= cmap_entries ||
+            background_index != PNG_CMAP_TRANS_BACKGROUND)
+            goto bad_background;
+         break;
+
+      case PNG_CMAP_RGB:
+         if (background_index != PNG_CMAP_RGB_BACKGROUND)
+            goto bad_background;
+         break;
+
+      case PNG_CMAP_RGB_ALPHA:
+         if (background_index != PNG_CMAP_RGB_ALPHA_BACKGROUND)
+            goto bad_background;
+         break;
+
+      default:
+         png_error(png_ptr, "bad processing option (internal error)");
+
+      bad_background:
+         png_error(png_ptr, "bad background index (internal error)");
+   }
+
+   display->colormap_processing = output_processing;
+
+   return 1/*ok*/;
+}
+
+/* The final part of the color-map read called from png_image_finish_read. */
+static int
+png_image_read_and_map(png_voidp argument)
+{
+   png_image_read_control *display = png_voidcast(png_image_read_control*,
+      argument);
+   png_imagep image = display->image;
+   png_structrp png_ptr = image->opaque->png_ptr;
+   int passes;
+
+   /* Called when the libpng data must be transformed into the color-mapped
+    * form.  There is a local row buffer in display->local and this routine must
+    * do the interlace handling.
+    */
+   switch (png_ptr->interlaced)
+   {
+      case PNG_INTERLACE_NONE:
+         passes = 1;
+         break;
+
+      case PNG_INTERLACE_ADAM7:
+         passes = PNG_INTERLACE_ADAM7_PASSES;
+         break;
+
+      default:
+         png_error(png_ptr, "unknown interlace type");
+   }
+
+   {
+      png_uint_32  height = image->height;
+      png_uint_32  width = image->width;
+      int          proc = display->colormap_processing;
+      png_bytep    first_row = png_voidcast(png_bytep, display->first_row);
+      ptrdiff_t    step_row = display->row_bytes;
+      int pass;
+
+      for (pass = 0; pass < passes; ++pass)
+      {
+         unsigned int     startx, stepx, stepy;
+         png_uint_32      y;
+
+         if (png_ptr->interlaced == PNG_INTERLACE_ADAM7)
+         {
+            /* The row may be empty for a short image: */
+            if (PNG_PASS_COLS(width, pass) == 0)
+               continue;
+
+            startx = PNG_PASS_START_COL(pass);
+            stepx = PNG_PASS_COL_OFFSET(pass);
+            y = PNG_PASS_START_ROW(pass);
+            stepy = PNG_PASS_ROW_OFFSET(pass);
+         }
+
+         else
+         {
+            y = 0;
+            startx = 0;
+            stepx = stepy = 1;
+         }
+
+         for (; y<height; y += stepy)
+         {
+            png_bytep inrow = png_voidcast(png_bytep, display->local_row);
+            png_bytep outrow = first_row + y * step_row;
+            png_const_bytep end_row = outrow + width;
+
+            /* Read read the libpng data into the temporary buffer. */
+            png_read_row(png_ptr, inrow, NULL);
+
+            /* Now process the row according to the processing option, note
+             * that the caller verifies that the format of the libpng output
+             * data is as required.
+             */
+            outrow += startx;
+            switch (proc)
+            {
+               case PNG_CMAP_GA:
+                  for (; outrow < end_row; outrow += stepx)
+                  {
+                     /* The data is always in the PNG order */
+                     unsigned int gray = *inrow++;
+                     unsigned int alpha = *inrow++;
+                     unsigned int entry;
+
+                     /* NOTE: this code is copied as a comment in
+                      * make_ga_colormap above.  Please update the
+                      * comment if you change this code!
+                      */
+                     if (alpha > 229) /* opaque */
+                     {
+                        entry = (231 * gray + 128) >> 8;
+                     }
+                     else if (alpha < 26) /* transparent */
+                     {
+                        entry = 231;
+                     }
+                     else /* partially opaque */
+                     {
+                        entry = 226 + 6 * PNG_DIV51(alpha) + PNG_DIV51(gray);
+                     }
+
+                     *outrow = (png_byte)entry;
+                  }
+                  break;
+
+               case PNG_CMAP_TRANS:
+                  for (; outrow < end_row; outrow += stepx)
+                  {
+                     png_byte gray = *inrow++;
+                     png_byte alpha = *inrow++;
+
+                     if (alpha == 0)
+                        *outrow = PNG_CMAP_TRANS_BACKGROUND;
+
+                     else if (gray != PNG_CMAP_TRANS_BACKGROUND)
+                        *outrow = gray;
+
+                     else
+                        *outrow = (png_byte)(PNG_CMAP_TRANS_BACKGROUND+1);
+                  }
+                  break;
+
+               case PNG_CMAP_RGB:
+                  for (; outrow < end_row; outrow += stepx)
+                  {
+                     *outrow = PNG_RGB_INDEX(inrow[0], inrow[1], inrow[2]);
+                     inrow += 3;
+                  }
+                  break;
+
+               case PNG_CMAP_RGB_ALPHA:
+                  for (; outrow < end_row; outrow += stepx)
+                  {
+                     unsigned int alpha = inrow[3];
+
+                     /* Because the alpha entries only hold alpha==0.5 values
+                      * split the processing at alpha==0.25 (64) and 0.75
+                      * (196).
+                      */
+
+                     if (alpha >= 196)
+                        *outrow = PNG_RGB_INDEX(inrow[0], inrow[1],
+                           inrow[2]);
+
+                     else if (alpha < 64)
+                        *outrow = PNG_CMAP_RGB_ALPHA_BACKGROUND;
+
+                     else
+                     {
+                        /* Likewise there are three entries for each of r, g
+                         * and b.  We could select the entry by popcount on
+                         * the top two bits on those architectures that
+                         * support it, this is what the code below does,
+                         * crudely.
+                         */
+                        unsigned int back_i = PNG_CMAP_RGB_ALPHA_BACKGROUND+1;
+
+                        /* Here are how the values map:
+                         *
+                         * 0x00 .. 0x3f -> 0
+                         * 0x40 .. 0xbf -> 1
+                         * 0xc0 .. 0xff -> 2
+                         *
+                         * So, as above with the explicit alpha checks, the
+                         * breakpoints are at 64 and 196.
+                         */
+                        if (inrow[0] & 0x80) back_i += 9; /* red */
+                        if (inrow[0] & 0x40) back_i += 9;
+                        if (inrow[0] & 0x80) back_i += 3; /* green */
+                        if (inrow[0] & 0x40) back_i += 3;
+                        if (inrow[0] & 0x80) back_i += 1; /* blue */
+                        if (inrow[0] & 0x40) back_i += 1;
+
+                        *outrow = (png_byte)back_i;
+                     }
+
+                     inrow += 4;
+                  }
+                  break;
+
+               default:
+                  break;
+            }
+         }
+      }
+   }
+
+   return 1;
+}
+
+static int
+png_image_read_colormapped(png_voidp argument)
+{
+   png_image_read_control *display = png_voidcast(png_image_read_control*,
+      argument);
+   png_imagep image = display->image;
+   png_controlp control = image->opaque;
+   png_structrp png_ptr = control->png_ptr;
+   png_inforp info_ptr = control->info_ptr;
+
+   int passes = 0; /* As a flag */
+
+   PNG_SKIP_CHUNKS(png_ptr);
+
+   /* Update the 'info' structure and make sure the result is as required; first
+    * make sure to turn on the interlace handling if it will be required
+    * (because it can't be turned on *after* the call to png_read_update_info!)
+    */
+   if (display->colormap_processing == PNG_CMAP_NONE)
+      passes = png_set_interlace_handling(png_ptr);
+
+   png_read_update_info(png_ptr, info_ptr);
+
+   /* The expected output can be deduced from the colormap_processing option. */
+   switch (display->colormap_processing)
+   {
+      case PNG_CMAP_NONE:
+         /* Output must be one channel and one byte per pixel, the output
+          * encoding can be anything.
+          */
+         if ((info_ptr->color_type == PNG_COLOR_TYPE_PALETTE ||
+            info_ptr->color_type == PNG_COLOR_TYPE_GRAY) &&
+            info_ptr->bit_depth == 8)
+            break;
+
+         goto bad_output;
+
+      case PNG_CMAP_TRANS:
+      case PNG_CMAP_GA:
+         /* Output must be two channels and the 'G' one must be sRGB, the latter
+          * can be checked with an exact number because it should have been set
+          * to this number above!
+          */
+         if (info_ptr->color_type == PNG_COLOR_TYPE_GRAY_ALPHA &&
+            info_ptr->bit_depth == 8 &&
+            png_ptr->screen_gamma == PNG_GAMMA_sRGB &&
+            image->colormap_entries == 256)
+            break;
+
+         goto bad_output;
+
+      case PNG_CMAP_RGB:
+         /* Output must be 8-bit sRGB encoded RGB */
+         if (info_ptr->color_type == PNG_COLOR_TYPE_RGB &&
+            info_ptr->bit_depth == 8 &&
+            png_ptr->screen_gamma == PNG_GAMMA_sRGB &&
+            image->colormap_entries == 216)
+            break;
+
+         goto bad_output;
+
+      case PNG_CMAP_RGB_ALPHA:
+         /* Output must be 8-bit sRGB encoded RGBA */
+         if (info_ptr->color_type == PNG_COLOR_TYPE_RGB_ALPHA &&
+            info_ptr->bit_depth == 8 &&
+            png_ptr->screen_gamma == PNG_GAMMA_sRGB &&
+            image->colormap_entries == 244 /* 216 + 1 + 27 */)
+            break;
+
+         /* goto bad_output; */
+         /* FALL THROUGH */
+
+      default:
+      bad_output:
+         png_error(png_ptr, "bad color-map processing (internal error)");
+   }
+
+   /* Now read the rows.  Do this here if it is possible to read directly into
+    * the output buffer, otherwise allocate a local row buffer of the maximum
+    * size libpng requires and call the relevant processing routine safely.
+    */
+   {
+      png_voidp first_row = display->buffer;
+      ptrdiff_t row_bytes = display->row_stride;
+
+      /* The following expression is designed to work correctly whether it gives
+       * a signed or an unsigned result.
+       */
+      if (row_bytes < 0)
+      {
+         char *ptr = png_voidcast(char*, first_row);
+         ptr += (image->height-1) * (-row_bytes);
+         first_row = png_voidcast(png_voidp, ptr);
+      }
+
+      display->first_row = first_row;
+      display->row_bytes = row_bytes;
+   }
+
+   if (passes == 0)
+   {
+      int result;
+      png_voidp row = png_malloc(png_ptr, png_get_rowbytes(png_ptr, info_ptr));
+
+      display->local_row = row;
+      result = png_safe_execute(image, png_image_read_and_map, display);
+      display->local_row = NULL;
+      png_free(png_ptr, row);
+
+      return result;
+   }
+
+   else
+   {
+      png_alloc_size_t row_bytes = display->row_bytes;
+
+      while (--passes >= 0)
+      {
+         png_uint_32      y = image->height;
+         png_bytep        row = png_voidcast(png_bytep, display->first_row);
+
+         while (y-- > 0)
+         {
+            png_read_row(png_ptr, row, NULL);
+            row += row_bytes;
+         }
+      }
+
+      return 1;
+   }
+}
+
+/* Just the row reading part of png_image_read. */
+static int
+png_image_read_composite(png_voidp argument)
+{
+   png_image_read_control *display = png_voidcast(png_image_read_control*,
+      argument);
+   png_imagep image = display->image;
+   png_structrp png_ptr = image->opaque->png_ptr;
+   int passes;
+
+   switch (png_ptr->interlaced)
+   {
+      case PNG_INTERLACE_NONE:
+         passes = 1;
+         break;
+
+      case PNG_INTERLACE_ADAM7:
+         passes = PNG_INTERLACE_ADAM7_PASSES;
+         break;
+
+      default:
+         png_error(png_ptr, "unknown interlace type");
+   }
+
+   {
+      png_uint_32  height = image->height;
+      png_uint_32  width = image->width;
+      ptrdiff_t    step_row = display->row_bytes;
+      unsigned int channels =
+          (image->format & PNG_FORMAT_FLAG_COLOR) != 0 ? 3 : 1;
+      int pass;
+
+      for (pass = 0; pass < passes; ++pass)
+      {
+         unsigned int     startx, stepx, stepy;
+         png_uint_32      y;
+
+         if (png_ptr->interlaced == PNG_INTERLACE_ADAM7)
+         {
+            /* The row may be empty for a short image: */
+            if (PNG_PASS_COLS(width, pass) == 0)
+               continue;
+
+            startx = PNG_PASS_START_COL(pass) * channels;
+            stepx = PNG_PASS_COL_OFFSET(pass) * channels;
+            y = PNG_PASS_START_ROW(pass);
+            stepy = PNG_PASS_ROW_OFFSET(pass);
+         }
+
+         else
+         {
+            y = 0;
+            startx = 0;
+            stepx = channels;
+            stepy = 1;
+         }
+
+         for (; y<height; y += stepy)
+         {
+            png_bytep inrow = png_voidcast(png_bytep, display->local_row);
+            png_bytep outrow;
+            png_const_bytep end_row;
+
+            /* Read the row, which is packed: */
+            png_read_row(png_ptr, inrow, NULL);
+
+            outrow = png_voidcast(png_bytep, display->first_row);
+            outrow += y * step_row;
+            end_row = outrow + width * channels;
+
+            /* Now do the composition on each pixel in this row. */
+            outrow += startx;
+            for (; outrow < end_row; outrow += stepx)
+            {
+               png_byte alpha = inrow[channels];
+
+               if (alpha > 0) /* else no change to the output */
+               {
+                  unsigned int c;
+
+                  for (c=0; c<channels; ++c)
+                  {
+                     png_uint_32 component = inrow[c];
+
+                     if (alpha < 255) /* else just use component */
+                     {
+                        /* This is PNG_OPTIMIZED_ALPHA, the component value
+                         * is a linear 8-bit value.  Combine this with the
+                         * current outrow[c] value which is sRGB encoded.
+                         * Arithmetic here is 16-bits to preserve the output
+                         * values correctly.
+                         */
+                        component *= 257*255; /* =65535 */
+                        component += (255-alpha)*png_sRGB_table[outrow[c]];
+
+                        /* So 'component' is scaled by 255*65535 and is
+                         * therefore appropriate for the sRGB to linear
+                         * conversion table.
+                         */
+                        component = PNG_sRGB_FROM_LINEAR(component);
+                     }
+
+                     outrow[c] = (png_byte)component;
+                  }
+               }
+
+               inrow += channels+1; /* components and alpha channel */
+            }
+         }
+      }
+   }
+
+   return 1;
+}
+
+/* The do_local_background case; called when all the following transforms are to
+ * be done:
+ *
+ * PNG_RGB_TO_GRAY
+ * PNG_COMPOSITE
+ * PNG_GAMMA
+ *
+ * This is a work-around for the fact that both the PNG_RGB_TO_GRAY and
+ * PNG_COMPOSITE code performs gamma correction, so we get double gamma
+ * correction.  The fix-up is to prevent the PNG_COMPOSITE operation from
+ * happening inside libpng, so this routine sees an 8 or 16-bit gray+alpha
+ * row and handles the removal or pre-multiplication of the alpha channel.
+ */
+static int
+png_image_read_background(png_voidp argument)
+{
+   png_image_read_control *display = png_voidcast(png_image_read_control*,
+      argument);
+   png_imagep image = display->image;
+   png_structrp png_ptr = image->opaque->png_ptr;
+   png_inforp info_ptr = image->opaque->info_ptr;
+   png_uint_32 height = image->height;
+   png_uint_32 width = image->width;
+   int pass, passes;
+
+   /* Double check the convoluted logic below.  We expect to get here with
+    * libpng doing rgb to gray and gamma correction but background processing
+    * left to the png_image_read_background function.  The rows libpng produce
+    * might be 8 or 16-bit but should always have two channels; gray plus alpha.
+    */
+   if ((png_ptr->transformations & PNG_RGB_TO_GRAY) == 0)
+      png_error(png_ptr, "lost rgb to gray");
+
+   if ((png_ptr->transformations & PNG_COMPOSE) != 0)
+      png_error(png_ptr, "unexpected compose");
+
+   if (png_get_channels(png_ptr, info_ptr) != 2)
+      png_error(png_ptr, "lost/gained channels");
+
+   /* Expect the 8-bit case to always remove the alpha channel */
+   if ((image->format & PNG_FORMAT_FLAG_LINEAR) == 0 &&
+      (image->format & PNG_FORMAT_FLAG_ALPHA) != 0)
+      png_error(png_ptr, "unexpected 8-bit transformation");
+
+   switch (png_ptr->interlaced)
+   {
+      case PNG_INTERLACE_NONE:
+         passes = 1;
+         break;
+
+      case PNG_INTERLACE_ADAM7:
+         passes = PNG_INTERLACE_ADAM7_PASSES;
+         break;
+
+      default:
+         png_error(png_ptr, "unknown interlace type");
+   }
+
+   /* Use direct access to info_ptr here because otherwise the simplified API
+    * would require PNG_EASY_ACCESS_SUPPORTED (just for this.)  Note this is
+    * checking the value after libpng expansions, not the original value in the
+    * PNG.
+    */
+   switch (info_ptr->bit_depth)
+   {
+      case 8:
+         /* 8-bit sRGB gray values with an alpha channel; the alpha channel is
+          * to be removed by composing on a background: either the row if
+          * display->background is NULL or display->background->green if not.
+          * Unlike the code above ALPHA_OPTIMIZED has *not* been done.
+          */
+         {
+            png_bytep first_row = png_voidcast(png_bytep, display->first_row);
+            ptrdiff_t step_row = display->row_bytes;
+
+            for (pass = 0; pass < passes; ++pass)
+            {
+               png_bytep        row = png_voidcast(png_bytep,
+                                                   display->first_row);
+               unsigned int     startx, stepx, stepy;
+               png_uint_32      y;
+
+               if (png_ptr->interlaced == PNG_INTERLACE_ADAM7)
+               {
+                  /* The row may be empty for a short image: */
+                  if (PNG_PASS_COLS(width, pass) == 0)
+                     continue;
+
+                  startx = PNG_PASS_START_COL(pass);
+                  stepx = PNG_PASS_COL_OFFSET(pass);
+                  y = PNG_PASS_START_ROW(pass);
+                  stepy = PNG_PASS_ROW_OFFSET(pass);
+               }
+
+               else
+               {
+                  y = 0;
+                  startx = 0;
+                  stepx = stepy = 1;
+               }
+
+               if (display->background == NULL)
+               {
+                  for (; y<height; y += stepy)
+                  {
+                     png_bytep inrow = png_voidcast(png_bytep,
+                        display->local_row);
+                     png_bytep outrow = first_row + y * step_row;
+                     png_const_bytep end_row = outrow + width;
+
+                     /* Read the row, which is packed: */
+                     png_read_row(png_ptr, inrow, NULL);
+
+                     /* Now do the composition on each pixel in this row. */
+                     outrow += startx;
+                     for (; outrow < end_row; outrow += stepx)
+                     {
+                        png_byte alpha = inrow[1];
+
+                        if (alpha > 0) /* else no change to the output */
+                        {
+                           png_uint_32 component = inrow[0];
+
+                           if (alpha < 255) /* else just use component */
+                           {
+                              /* Since PNG_OPTIMIZED_ALPHA was not set it is
+                               * necessary to invert the sRGB transfer
+                               * function and multiply the alpha out.
+                               */
+                              component = png_sRGB_table[component] * alpha;
+                              component += png_sRGB_table[outrow[0]] *
+                                 (255-alpha);
+                              component = PNG_sRGB_FROM_LINEAR(component);
+                           }
+
+                           outrow[0] = (png_byte)component;
+                        }
+
+                        inrow += 2; /* gray and alpha channel */
+                     }
+                  }
+               }
+
+               else /* constant background value */
+               {
+                  png_byte background8 = display->background->green;
+                  png_uint_16 background = png_sRGB_table[background8];
+
+                  for (; y<height; y += stepy)
+                  {
+                     png_bytep inrow = png_voidcast(png_bytep,
+                        display->local_row);
+                     png_bytep outrow = first_row + y * step_row;
+                     png_const_bytep end_row = outrow + width;
+
+                     /* Read the row, which is packed: */
+                     png_read_row(png_ptr, inrow, NULL);
+
+                     /* Now do the composition on each pixel in this row. */
+                     outrow += startx;
+                     for (; outrow < end_row; outrow += stepx)
+                     {
+                        png_byte alpha = inrow[1];
+
+                        if (alpha > 0) /* else use background */
+                        {
+                           png_uint_32 component = inrow[0];
+
+                           if (alpha < 255) /* else just use component */
+                           {
+                              component = png_sRGB_table[component] * alpha;
+                              component += background * (255-alpha);
+                              component = PNG_sRGB_FROM_LINEAR(component);
+                           }
+
+                           outrow[0] = (png_byte)component;
+                        }
+
+                        else
+                           outrow[0] = background8;
+
+                        inrow += 2; /* gray and alpha channel */
+                     }
+
+                     row += display->row_bytes;
+                  }
+               }
+            }
+         }
+         break;
+
+      case 16:
+         /* 16-bit linear with pre-multiplied alpha; the pre-multiplication must
+          * still be done and, maybe, the alpha channel removed.  This code also
+          * handles the alpha-first option.
+          */
+         {
+            png_uint_16p first_row = png_voidcast(png_uint_16p,
+               display->first_row);
+            /* The division by two is safe because the caller passed in a
+             * stride which was multiplied by 2 (below) to get row_bytes.
+             */
+            ptrdiff_t    step_row = display->row_bytes / 2;
+            int preserve_alpha = (image->format & PNG_FORMAT_FLAG_ALPHA) != 0;
+            unsigned int outchannels = 1+preserve_alpha;
+            int swap_alpha = 0;
+
+#           ifdef PNG_SIMPLIFIED_READ_AFIRST_SUPPORTED
+               if (preserve_alpha != 0 &&
+                   (image->format & PNG_FORMAT_FLAG_AFIRST) != 0)
+                  swap_alpha = 1;
+#           endif
+
+            for (pass = 0; pass < passes; ++pass)
+            {
+               unsigned int     startx, stepx, stepy;
+               png_uint_32      y;
+
+               /* The 'x' start and step are adjusted to output components here.
+                */
+               if (png_ptr->interlaced == PNG_INTERLACE_ADAM7)
+               {
+                  /* The row may be empty for a short image: */
+                  if (PNG_PASS_COLS(width, pass) == 0)
+                     continue;
+
+                  startx = PNG_PASS_START_COL(pass) * outchannels;
+                  stepx = PNG_PASS_COL_OFFSET(pass) * outchannels;
+                  y = PNG_PASS_START_ROW(pass);
+                  stepy = PNG_PASS_ROW_OFFSET(pass);
+               }
+
+               else
+               {
+                  y = 0;
+                  startx = 0;
+                  stepx = outchannels;
+                  stepy = 1;
+               }
+
+               for (; y<height; y += stepy)
+               {
+                  png_const_uint_16p inrow;
+                  png_uint_16p outrow = first_row + y*step_row;
+                  png_uint_16p end_row = outrow + width * outchannels;
+
+                  /* Read the row, which is packed: */
+                  png_read_row(png_ptr, png_voidcast(png_bytep,
+                     display->local_row), NULL);
+                  inrow = png_voidcast(png_const_uint_16p, display->local_row);
+
+                  /* Now do the pre-multiplication on each pixel in this row.
+                   */
+                  outrow += startx;
+                  for (; outrow < end_row; outrow += stepx)
+                  {
+                     png_uint_32 component = inrow[0];
+                     png_uint_16 alpha = inrow[1];
+
+                     if (alpha > 0) /* else 0 */
+                     {
+                        if (alpha < 65535) /* else just use component */
+                        {
+                           component *= alpha;
+                           component += 32767;
+                           component /= 65535;
+                        }
+                     }
+
+                     else
+                        component = 0;
+
+                     outrow[swap_alpha] = (png_uint_16)component;
+                     if (preserve_alpha != 0)
+                        outrow[1 ^ swap_alpha] = alpha;
+
+                     inrow += 2; /* components and alpha channel */
+                  }
+               }
+            }
+         }
+         break;
+
+#ifdef __GNUC__
+      default:
+         png_error(png_ptr, "unexpected bit depth");
+#endif
+   }
+
+   return 1;
+}
+
+/* The guts of png_image_finish_read as a png_safe_execute callback. */
+static int
+png_image_read_direct(png_voidp argument)
+{
+   png_image_read_control *display = png_voidcast(png_image_read_control*,
+      argument);
+   png_imagep image = display->image;
+   png_structrp png_ptr = image->opaque->png_ptr;
+   png_inforp info_ptr = image->opaque->info_ptr;
+
+   png_uint_32 format = image->format;
+   int linear = (format & PNG_FORMAT_FLAG_LINEAR) != 0;
+   int do_local_compose = 0;
+   int do_local_background = 0; /* to avoid double gamma correction bug */
+   int passes = 0;
+
+   /* Add transforms to ensure the correct output format is produced then check
+    * that the required implementation support is there.  Always expand; always
+    * need 8 bits minimum, no palette and expanded tRNS.
+    */
+   png_set_expand(png_ptr);
+
+   /* Now check the format to see if it was modified. */
+   {
+      png_uint_32 base_format = png_image_format(png_ptr) &
+         ~PNG_FORMAT_FLAG_COLORMAP /* removed by png_set_expand */;
+      png_uint_32 change = format ^ base_format;
+      png_fixed_point output_gamma;
+      int mode; /* alpha mode */
+
+      /* Do this first so that we have a record if rgb to gray is happening. */
+      if ((change & PNG_FORMAT_FLAG_COLOR) != 0)
+      {
+         /* gray<->color transformation required. */
+         if ((format & PNG_FORMAT_FLAG_COLOR) != 0)
+            png_set_gray_to_rgb(png_ptr);
+
+         else
+         {
+            /* libpng can't do both rgb to gray and
+             * background/pre-multiplication if there is also significant gamma
+             * correction, because both operations require linear colors and
+             * the code only supports one transform doing the gamma correction.
+             * Handle this by doing the pre-multiplication or background
+             * operation in this code, if necessary.
+             *
+             * TODO: fix this by rewriting pngrtran.c (!)
+             *
+             * For the moment (given that fixing this in pngrtran.c is an
+             * enormous change) 'do_local_background' is used to indicate that
+             * the problem exists.
+             */
+            if ((base_format & PNG_FORMAT_FLAG_ALPHA) != 0)
+               do_local_background = 1/*maybe*/;
+
+            png_set_rgb_to_gray_fixed(png_ptr, PNG_ERROR_ACTION_NONE,
+               PNG_RGB_TO_GRAY_DEFAULT, PNG_RGB_TO_GRAY_DEFAULT);
+         }
+
+         change &= ~PNG_FORMAT_FLAG_COLOR;
+      }
+
+      /* Set the gamma appropriately, linear for 16-bit input, sRGB otherwise.
+       */
+      {
+         png_fixed_point input_gamma_default;
+
+         if ((base_format & PNG_FORMAT_FLAG_LINEAR) != 0 &&
+             (image->flags & PNG_IMAGE_FLAG_16BIT_sRGB) == 0)
+            input_gamma_default = PNG_GAMMA_LINEAR;
+         else
+            input_gamma_default = PNG_DEFAULT_sRGB;
+
+         /* Call png_set_alpha_mode to set the default for the input gamma; the
+          * output gamma is set by a second call below.
+          */
+         png_set_alpha_mode_fixed(png_ptr, PNG_ALPHA_PNG, input_gamma_default);
+      }
+
+      if (linear != 0)
+      {
+         /* If there *is* an alpha channel in the input it must be multiplied
+          * out; use PNG_ALPHA_STANDARD, otherwise just use PNG_ALPHA_PNG.
+          */
+         if ((base_format & PNG_FORMAT_FLAG_ALPHA) != 0)
+            mode = PNG_ALPHA_STANDARD; /* associated alpha */
+
+         else
+            mode = PNG_ALPHA_PNG;
+
+         output_gamma = PNG_GAMMA_LINEAR;
+      }
+
+      else
+      {
+         mode = PNG_ALPHA_PNG;
+         output_gamma = PNG_DEFAULT_sRGB;
+      }
+
+      /* If 'do_local_background' is set check for the presence of gamma
+       * correction; this is part of the work-round for the libpng bug
+       * described above.
+       *
+       * TODO: fix libpng and remove this.
+       */
+      if (do_local_background != 0)
+      {
+         png_fixed_point gtest;
+
+         /* This is 'png_gamma_threshold' from pngrtran.c; the test used for
+          * gamma correction, the screen gamma hasn't been set on png_struct
+          * yet; it's set below.  png_struct::gamma, however, is set to the
+          * final value.
+          */
+         if (png_muldiv(&gtest, output_gamma, png_ptr->colorspace.gamma,
+               PNG_FP_1) != 0 && png_gamma_significant(gtest) == 0)
+            do_local_background = 0;
+
+         else if (mode == PNG_ALPHA_STANDARD)
+         {
+            do_local_background = 2/*required*/;
+            mode = PNG_ALPHA_PNG; /* prevent libpng doing it */
+         }
+
+         /* else leave as 1 for the checks below */
+      }
+
+      /* If the bit-depth changes then handle that here. */
+      if ((change & PNG_FORMAT_FLAG_LINEAR) != 0)
+      {
+         if (linear != 0 /*16-bit output*/)
+            png_set_expand_16(png_ptr);
+
+         else /* 8-bit output */
+            png_set_scale_16(png_ptr);
+
+         change &= ~PNG_FORMAT_FLAG_LINEAR;
+      }
+
+      /* Now the background/alpha channel changes. */
+      if ((change & PNG_FORMAT_FLAG_ALPHA) != 0)
+      {
+         /* Removing an alpha channel requires composition for the 8-bit
+          * formats; for the 16-bit it is already done, above, by the
+          * pre-multiplication and the channel just needs to be stripped.
+          */
+         if ((base_format & PNG_FORMAT_FLAG_ALPHA) != 0)
+         {
+            /* If RGB->gray is happening the alpha channel must be left and the
+             * operation completed locally.
+             *
+             * TODO: fix libpng and remove this.
+             */
+            if (do_local_background != 0)
+               do_local_background = 2/*required*/;
+
+            /* 16-bit output: just remove the channel */
+            else if (linear != 0) /* compose on black (well, pre-multiply) */
+               png_set_strip_alpha(png_ptr);
+
+            /* 8-bit output: do an appropriate compose */
+            else if (display->background != NULL)
+            {
+               png_color_16 c;
+
+               c.index = 0; /*unused*/
+               c.red = display->background->red;
+               c.green = display->background->green;
+               c.blue = display->background->blue;
+               c.gray = display->background->green;
+
+               /* This is always an 8-bit sRGB value, using the 'green' channel
+                * for gray is much better than calculating the luminance here;
+                * we can get off-by-one errors in that calculation relative to
+                * the app expectations and that will show up in transparent
+                * pixels.
+                */
+               png_set_background_fixed(png_ptr, &c,
+                  PNG_BACKGROUND_GAMMA_SCREEN, 0/*need_expand*/,
+                  0/*gamma: not used*/);
+            }
+
+            else /* compose on row: implemented below. */
+            {
+               do_local_compose = 1;
+               /* This leaves the alpha channel in the output, so it has to be
+                * removed by the code below.  Set the encoding to the 'OPTIMIZE'
+                * one so the code only has to hack on the pixels that require
+                * composition.
+                */
+               mode = PNG_ALPHA_OPTIMIZED;
+            }
+         }
+
+         else /* output needs an alpha channel */
+         {
+            /* This is tricky because it happens before the swap operation has
+             * been accomplished; however, the swap does *not* swap the added
+             * alpha channel (weird API), so it must be added in the correct
+             * place.
+             */
+            png_uint_32 filler; /* opaque filler */
+            int where;
+
+            if (linear != 0)
+               filler = 65535;
+
+            else
+               filler = 255;
+
+#           ifdef PNG_FORMAT_AFIRST_SUPPORTED
+               if ((format & PNG_FORMAT_FLAG_AFIRST) != 0)
+               {
+                  where = PNG_FILLER_BEFORE;
+                  change &= ~PNG_FORMAT_FLAG_AFIRST;
+               }
+
+               else
+#           endif
+               where = PNG_FILLER_AFTER;
+
+            png_set_add_alpha(png_ptr, filler, where);
+         }
+
+         /* This stops the (irrelevant) call to swap_alpha below. */
+         change &= ~PNG_FORMAT_FLAG_ALPHA;
+      }
+
+      /* Now set the alpha mode correctly; this is always done, even if there is
+       * no alpha channel in either the input or the output because it correctly
+       * sets the output gamma.
+       */
+      png_set_alpha_mode_fixed(png_ptr, mode, output_gamma);
+
+#     ifdef PNG_FORMAT_BGR_SUPPORTED
+         if ((change & PNG_FORMAT_FLAG_BGR) != 0)
+         {
+            /* Check only the output format; PNG is never BGR; don't do this if
+             * the output is gray, but fix up the 'format' value in that case.
+             */
+            if ((format & PNG_FORMAT_FLAG_COLOR) != 0)
+               png_set_bgr(png_ptr);
+
+            else
+               format &= ~PNG_FORMAT_FLAG_BGR;
+
+            change &= ~PNG_FORMAT_FLAG_BGR;
+         }
+#     endif
+
+#     ifdef PNG_FORMAT_AFIRST_SUPPORTED
+         if ((change & PNG_FORMAT_FLAG_AFIRST) != 0)
+         {
+            /* Only relevant if there is an alpha channel - it's particularly
+             * important to handle this correctly because do_local_compose may
+             * be set above and then libpng will keep the alpha channel for this
+             * code to remove.
+             */
+            if ((format & PNG_FORMAT_FLAG_ALPHA) != 0)
+            {
+               /* Disable this if doing a local background,
+                * TODO: remove this when local background is no longer required.
+                */
+               if (do_local_background != 2)
+                  png_set_swap_alpha(png_ptr);
+            }
+
+            else
+               format &= ~PNG_FORMAT_FLAG_AFIRST;
+
+            change &= ~PNG_FORMAT_FLAG_AFIRST;
+         }
+#     endif
+
+      /* If the *output* is 16-bit then we need to check for a byte-swap on this
+       * architecture.
+       */
+      if (linear != 0)
+      {
+         PNG_CONST png_uint_16 le = 0x0001;
+
+         if ((*(png_const_bytep) & le) != 0)
+            png_set_swap(png_ptr);
+      }
+
+      /* If change is not now 0 some transformation is missing - error out. */
+      if (change != 0)
+         png_error(png_ptr, "png_read_image: unsupported transformation");
+   }
+
+   PNG_SKIP_CHUNKS(png_ptr);
+
+   /* Update the 'info' structure and make sure the result is as required; first
+    * make sure to turn on the interlace handling if it will be required
+    * (because it can't be turned on *after* the call to png_read_update_info!)
+    *
+    * TODO: remove the do_local_background fixup below.
+    */
+   if (do_local_compose == 0 && do_local_background != 2)
+      passes = png_set_interlace_handling(png_ptr);
+
+   png_read_update_info(png_ptr, info_ptr);
+
+   {
+      png_uint_32 info_format = 0;
+
+      if ((info_ptr->color_type & PNG_COLOR_MASK_COLOR) != 0)
+         info_format |= PNG_FORMAT_FLAG_COLOR;
+
+      if ((info_ptr->color_type & PNG_COLOR_MASK_ALPHA) != 0)
+      {
+         /* do_local_compose removes this channel below. */
+         if (do_local_compose == 0)
+         {
+            /* do_local_background does the same if required. */
+            if (do_local_background != 2 ||
+               (format & PNG_FORMAT_FLAG_ALPHA) != 0)
+               info_format |= PNG_FORMAT_FLAG_ALPHA;
+         }
+      }
+
+      else if (do_local_compose != 0) /* internal error */
+         png_error(png_ptr, "png_image_read: alpha channel lost");
+
+      if (info_ptr->bit_depth == 16)
+         info_format |= PNG_FORMAT_FLAG_LINEAR;
+
+#     ifdef PNG_FORMAT_BGR_SUPPORTED
+         if ((png_ptr->transformations & PNG_BGR) != 0)
+            info_format |= PNG_FORMAT_FLAG_BGR;
+#     endif
+
+#     ifdef PNG_FORMAT_AFIRST_SUPPORTED
+         if (do_local_background == 2)
+         {
+            if ((format & PNG_FORMAT_FLAG_AFIRST) != 0)
+               info_format |= PNG_FORMAT_FLAG_AFIRST;
+         }
+
+         if ((png_ptr->transformations & PNG_SWAP_ALPHA) != 0 ||
+            ((png_ptr->transformations & PNG_ADD_ALPHA) != 0 &&
+            (png_ptr->flags & PNG_FLAG_FILLER_AFTER) == 0))
+         {
+            if (do_local_background == 2)
+               png_error(png_ptr, "unexpected alpha swap transformation");
+
+            info_format |= PNG_FORMAT_FLAG_AFIRST;
+         }
+#     endif
+
+      /* This is actually an internal error. */
+      if (info_format != format)
+         png_error(png_ptr, "png_read_image: invalid transformations");
+   }
+
+   /* Now read the rows.  If do_local_compose is set then it is necessary to use
+    * a local row buffer.  The output will be GA, RGBA or BGRA and must be
+    * converted to G, RGB or BGR as appropriate.  The 'local_row' member of the
+    * display acts as a flag.
+    */
+   {
+      png_voidp first_row = display->buffer;
+      ptrdiff_t row_bytes = display->row_stride;
+
+      if (linear != 0)
+         row_bytes *= 2;
+
+      /* The following expression is designed to work correctly whether it gives
+       * a signed or an unsigned result.
+       */
+      if (row_bytes < 0)
+      {
+         char *ptr = png_voidcast(char*, first_row);
+         ptr += (image->height-1) * (-row_bytes);
+         first_row = png_voidcast(png_voidp, ptr);
+      }
+
+      display->first_row = first_row;
+      display->row_bytes = row_bytes;
+   }
+
+   if (do_local_compose != 0)
+   {
+      int result;
+      png_voidp row = png_malloc(png_ptr, png_get_rowbytes(png_ptr, info_ptr));
+
+      display->local_row = row;
+      result = png_safe_execute(image, png_image_read_composite, display);
+      display->local_row = NULL;
+      png_free(png_ptr, row);
+
+      return result;
+   }
+
+   else if (do_local_background == 2)
+   {
+      int result;
+      png_voidp row = png_malloc(png_ptr, png_get_rowbytes(png_ptr, info_ptr));
+
+      display->local_row = row;
+      result = png_safe_execute(image, png_image_read_background, display);
+      display->local_row = NULL;
+      png_free(png_ptr, row);
+
+      return result;
+   }
+
+   else
+   {
+      png_alloc_size_t row_bytes = display->row_bytes;
+
+      while (--passes >= 0)
+      {
+         png_uint_32      y = image->height;
+         png_bytep        row = png_voidcast(png_bytep, display->first_row);
+
+         while (y-- > 0)
+         {
+            png_read_row(png_ptr, row, NULL);
+            row += row_bytes;
+         }
+      }
+
+      return 1;
+   }
+}
+
+int PNGAPI
+png_image_finish_read(png_imagep image, png_const_colorp background,
+   void *buffer, png_int_32 row_stride, void *colormap)
+{
+   if (image != NULL && image->version == PNG_IMAGE_VERSION)
+   {
+      png_uint_32 check;
+
+      if (row_stride == 0)
+         row_stride = PNG_IMAGE_ROW_STRIDE(*image);
+
+      if (row_stride < 0)
+         check = -row_stride;
+
+      else
+         check = row_stride;
+
+      if (image->opaque != NULL && buffer != NULL &&
+         check >= PNG_IMAGE_ROW_STRIDE(*image))
+      {
+         if ((image->format & PNG_FORMAT_FLAG_COLORMAP) == 0 ||
+            (image->colormap_entries > 0 && colormap != NULL))
+         {
+            int result;
+            png_image_read_control display;
+
+            memset(&display, 0, (sizeof display));
+            display.image = image;
+            display.buffer = buffer;
+            display.row_stride = row_stride;
+            display.colormap = colormap;
+            display.background = background;
+            display.local_row = NULL;
+
+            /* Choose the correct 'end' routine; for the color-map case all the
+             * setup has already been done.
+             */
+            if ((image->format & PNG_FORMAT_FLAG_COLORMAP) != 0)
+               result =
+                  png_safe_execute(image, png_image_read_colormap, &display) &&
+                  png_safe_execute(image, png_image_read_colormapped, &display);
+
+            else
+               result =
+                  png_safe_execute(image, png_image_read_direct, &display);
+
+            png_image_free(image);
+            return result;
+         }
+
+         else
+            return png_image_error(image,
+               "png_image_finish_read[color-map]: no color-map");
+      }
+
+      else
+         return png_image_error(image,
+            "png_image_finish_read: invalid argument");
+   }
+
+   else if (image != NULL)
+      return png_image_error(image,
+         "png_image_finish_read: damaged PNG_IMAGE_VERSION");
+
+   return 0;
+}
+
+#endif /* SIMPLIFIED_READ */
+#endif /* READ */