From 896dd975d19a7df8790cbd82b7d53f13d63587e3 Mon Sep 17 00:00:00 2001
From: Damien Lespiau <damien.lespiau@intel.com>
Date: Sun, 6 Jun 2010 22:57:02 +0100
Subject: [PATCH] cogl-color: Add cogl_color_unpremultiply()

For some operations on pre-multiplied colors (say, replace the alpha
value), you need to unpremultiply the color.

This patch provides the counterpart to cogl_color_premultiply().
---
 cogl/cogl-color.c                    | 11 +++++++++++
 cogl/cogl-color.h                    | 13 +++++++++++++
 doc/reference/cogl/cogl-sections.txt |  1 +
 3 files changed, 25 insertions(+)

diff --git a/cogl/cogl-color.c b/cogl/cogl-color.c
index a89534e49..544d874fc 100644
--- a/cogl/cogl-color.c
+++ b/cogl/cogl-color.c
@@ -163,6 +163,17 @@ cogl_color_premultiply (CoglColor *color)
   color->blue = (color->blue * color->alpha + 128) / 255;
 }
 
+void
+cogl_color_unpremultiply (CoglColor *color)
+{
+  if (color->alpha != 0)
+    {
+      color->red = (color->red * 255) / color->alpha;
+      color->green = (color->green * 255) / color->alpha;
+      color->blue = (color->blue * 255) / color->alpha;
+    }
+}
+
 gboolean
 cogl_color_equal (gconstpointer v1, gconstpointer v2)
 {
diff --git a/cogl/cogl-color.h b/cogl/cogl-color.h
index 8f3458bbf..1e1a14da7 100644
--- a/cogl/cogl-color.h
+++ b/cogl/cogl-color.h
@@ -299,6 +299,19 @@ cogl_color_get_alpha (const CoglColor *color);
 void
 cogl_color_premultiply (CoglColor *color);
 
+/**
+ * cogl_color_unpremultiply:
+ * @color: the color to unpremultiply
+ *
+ * Converts a pre-multiplied color to a non-premultiplied color. For
+ * example, semi-transparent red is (0.5, 0, 0, 0.5) when premultiplied
+ * and (1.0, 0, 0, 0.5) when non-premultiplied.
+ *
+ * Since: 1.4
+ */
+void
+cogl_color_unpremultiply (CoglColor *color);
+
 /**
  * cogl_color_equal:
  * @v1: a #CoglColor
diff --git a/doc/reference/cogl/cogl-sections.txt b/doc/reference/cogl/cogl-sections.txt
index af4685d2d..f23a7b1ea 100644
--- a/doc/reference/cogl/cogl-sections.txt
+++ b/doc/reference/cogl/cogl-sections.txt
@@ -384,6 +384,7 @@ cogl_color_get_alpha_float
 
 <SUBSECTION>
 cogl_color_premultiply
+cogl_color_unpremultiply
 cogl_color_equal
 </SECTION>