[patch] gl: fix gradient color texture generation

Hi, Chris and Soren

Please discard my previous patch on offsetting to get gradient color.  The pixman behavior is correct, except we have problem if first few stops offset at 0 - then we should pick the first stop's color for the first pixel.  To do that, we need to increase the first stop offset a little such that pixman gradient walker picks the first stop color when it starts to walk through.

Here is the patch, test case linear-gradient-large

From c749d938a71ff9045e264f618b09bb99ced16ee8 Mon Sep 17 00:00:00 2001
From: Henry Song <[email protected]>
Date: Thu, 26 Jul 2012 14:10:19 -0700
Subject: [PATCH] gl: fix gradient color generation if first few stops are
 offset at 0

---
 src/cairo-gl-gradient.c |   19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/cairo-gl-gradient.c b/src/cairo-gl-gradient.c
index 76920da..8cf3d6b 100644
--- a/src/cairo-gl-gradient.c
+++ b/src/cairo-gl-gradient.c
@@ -97,6 +97,7 @@ _cairo_gl_gradient_render (const cairo_gl_context_t    *ctx,
     pixman_point_fixed_t p1, p2;
     unsigned int i;
     pixman_format_code_t gradient_pixman_format;
+    double offset = 0.00001;
 
     /*
      * Ensure that the order of the gradient's components in memory is BGRA.
@@ -116,7 +117,23 @@ _cairo_gl_gradient_render (const cairo_gl_context_t    *ctx,
 	    return _cairo_error (CAIRO_STATUS_NO_MEMORY);
     }
 
-    for (i = 0; i < n_stops; i++) {
+    /* pixman always picks color at the right most side of stops if
+     * multiple stops have a same offset.  This is actually a correct 
+     * behaviro.  However, this means if we have two
+     * first stops at (0, 1, 1, 1, 1) and (0, 0, 0, 0, 1), the first
+     * pixel generated is black color instead of white color.  To
+     * compensate that we need to increase the first stop offset a little
+     * such that pixman picks up the first stop at first pixel.
+     */
+    if (n_stops > 2 && (stops[0].offset == stops[1].offset) && 
+	(stops[0].offset == 0)) {
+	pixman_stops[0].x = _cairo_fixed_16_16_from_double (stops[0].offset + offset);
+	pixman_stops[0].color.red   = stops[0].color.red_short;
+	pixman_stops[0].color.green = stops[0].color.green_short;
+	pixman_stops[0].color.blue  = stops[0].color.blue_short;
+	pixman_stops[0].color.alpha = stops[0].color.alpha_short;
+    }
+    for (i = 1; i < n_stops; i++) {
 	pixman_stops[i].x = _cairo_fixed_16_16_from_double (stops[i].offset);
 	pixman_stops[i].color.red   = stops[i].color.red_short;
 	pixman_stops[i].color.green = stops[i].color.green_short;
-- 
1.7.9.5
 
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