From dae9729e880ef633bde1cbfa5c565e5114410cf1 Mon Sep 17 00:00:00 2001
From: Robin Watts <robin.watts@artifex.com>
Date: Sat, 12 Feb 2011 14:50:40 +0000
Subject: Calculate the y sub pixel position correctly.

---
 draw/imagesmooth.c | 36 +++++++++++++++++++++++++-----------
 1 file changed, 25 insertions(+), 11 deletions(-)

(limited to 'draw')

diff --git a/draw/imagesmooth.c b/draw/imagesmooth.c
index 67b5f481..e34c07b2 100644
--- a/draw/imagesmooth.c
+++ b/draw/imagesmooth.c
@@ -1004,12 +1004,15 @@ 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.
+	/* Note that the x and y sub pixel offsets here are different.
+	 * The (x,y) position given describes where the bottom left corner
+	 * of the source image should be mapped to (i.e. where (0,h) in image
+	 * space ends up, not the more logical and sane (0,0)). Also there
+	 * are differences in the 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
@@ -1017,8 +1020,11 @@ fz_smoothscalepixmap(fz_pixmap *src, float x, float y, float w, float h)
 	 *
 	 * 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.
+	 * y is always R.ymax - r.ymax.
 	 */
+	/* dst_x_int is calculated to be the left of the scaled image, and
+	 * x (the sub_pixel_offset) is the distance in from either the left
+	 * or right pixel expanded edge. */
 	flip_x = (w < 0);
 	if (flip_x)
 	{
@@ -1037,14 +1043,22 @@ fz_smoothscalepixmap(fz_pixmap *src, float x, float y, float w, float h)
 		dst_w_int = (int)ceilf(x + w);
 	}
 	flip_y = (h < 0);
+	/* dst_y_int is calculated to be the bottom of the scaled image, but
+	 * y (the sub pixel offset) has to end up being the value at the top.
+	 */
 	if (flip_y)
 	{
 		h = -h;
-		y -= h;
+		dst_y_int = floor(y-h);
+		dst_h_int = (int)ceilf(y) - dst_y_int;
+	} else {
+		dst_y_int = floor(y);
+		y += h;
+		dst_h_int = (int)ceilf(y) - dst_y_int;
 	}
-	dst_y_int = floor(y);
-	y -= (float)dst_y_int;
-	dst_h_int = (int)ceilf(y + h);
+	/* y is the top edge position in floats. We want it to be the
+	 * distance down from the next pixel boundary. */
+	y = ceilf(y) - y;
 
 	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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