[cairo-commit] libpixman/src ictrap.c,1.13,1.14

[Carl Worth]

Committed by: cworth

Update of /cvs/cairo/libpixman/src
In directory gabe:/tmp/cvs-serv4218/src

Modified Files:
	ictrap.c 
Log Message:

        * src/ictrap.c (IcRasterizeTrapezoid): Force alpha to 0 when the
        left edge is entirely above top. (This is a workaround for some
        bizarre edge effects in the current algorithm).


Index: ictrap.c
===================================================================
RCS file: /cvs/cairo/libpixman/src/ictrap.c,v
retrieving revision 1.13
retrieving revision 1.14
diff -u -d -r1.13 -r1.14
--- ictrap.c	16 Apr 2004 15:32:53 -0000	1.13
+++ ictrap.c	13 Jan 2005 22:53:14 -0000	1.14
@@ -1227,8 +1227,18 @@
 	 */
 	while (left.row.top.y == pixel_y && pixel_x < first_right_x) 
 	{
-	    alpha = (RectAlpha (pixel_y, y, y_next, depth)
-		     - PixelAlpha(pixel_x, pixel_y, y, y_next, &left, depth));
+	    /* When the left edge is entirely above top we "know" the
+	     * alpha is 0. Due to a subtle edge effect, the
+	     * calculations below return a non-zero result in some
+	     * situations. This defect in the algorithm is bad enough
+	     * that we plan to discard the current approach
+	     * entirely. But in the meantime, we do want to have the
+	     * correct alpha == 0 in these cases. */
+	    if (left.lower.y < y)
+		alpha = 0;
+	    else
+		alpha = (RectAlpha (pixel_y, y, y_next, depth)
+			 - PixelAlpha(pixel_x, pixel_y, y, y_next, &left, depth));
 	    if (alpha > 0)
 	    {
 		if (0 <= pixel_x && pixel_x < buf_width_fixed)