From 07e6d886ae575d94c13c22bed4c9b9f907fb3def Mon Sep 17 00:00:00 2001
From: Robin Watts <robin.watts@artifex.com>
Date: Thu, 6 Jan 2011 12:07:39 +0000
Subject: Solve subpixel positioning problems when scaling rotated images.

---
 draw/imagesmooth.c | 34 +++++++++++++++++++++-------------
 1 file changed, 21 insertions(+), 13 deletions(-)

diff --git a/draw/imagesmooth.c b/draw/imagesmooth.c
index c564ccbf..3807de9b 100644
--- a/draw/imagesmooth.c
+++ b/draw/imagesmooth.c
@@ -27,6 +27,7 @@ static void debug_print(const char *fmt, ...)
 	vsprintf(text, fmt, args);
 	va_end(args);
 	OutputDebugStringA(text);
+	printf(text);
 }
 #else
 static void debug_print(const char *fmt, ...)
@@ -1003,6 +1004,21 @@ fz_smoothscalepixmap(fz_pixmap *src, float x, float y, float w, float h)
 
 	/* Find the destination bbox, width/height, and sub pixel offset,
 	 * allowing for whether we're flipping or not. */
+        /* Note that the x and y sub pixel offsets here are different, due to
+         * the different way we scale horizontally and vertically. When scaling
+         * rows horizontally, we always read forwards through the source, and
+         * store either forwards or in reverse as required. When scaling
+         * vertically, we always store out forwards, but may feed source rows
+         * in in a different order.
+         *
+         * Consider the image rectange 'r' to which the image is mapped,
+         * and the (possibly) larger rectangle 'R', given by expanding 'r' to
+         * complete pixels.
+         *
+         * x can either be r.xmin-R.xmin or R.xmax-r.xmax depending on whether
+         * the image is x flipped or not. Whatever happens 0 <= x < 1.
+         * y is always r.ymin - R.ymin.
+         */
 	flip_x = (w < 0);
 	if (flip_x)
 	{
@@ -1023,20 +1039,12 @@ fz_smoothscalepixmap(fz_pixmap *src, float x, float y, float w, float h)
 	flip_y = (h < 0);
 	if (flip_y)
 	{
-		float tmp;
 		h = -h;
-		dst_y_int = floor(y-h);
-		tmp = ceilf(y);
-		dst_h_int = (int)tmp;
-		y = tmp - y;
-		dst_h_int -= dst_y_int;
-	}
-	else
-	{
-		dst_y_int = floor(y);
-		y -= (float)dst_y_int;
-		dst_h_int = (int)ceilf(y + h);
-	}
+                y -= h;
+        }
+	dst_y_int = floor(y);
+	y -= (float)dst_y_int;
+	dst_h_int = (int)ceilf(y + h);
 
 	DBUG(("Result image: (%d,%d) at (%d,%d) (subpix=%g,%g)\n", dst_w_int, dst_h_int, dst_x_int, dst_y_int, x, y));
 
-- 
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