[PATCH 1/2] image: add support for GIF transparency

[Julien Danjou <julien@danjou.info>]

Signed-off-by: Julien Danjou <julien@danjou.info>
---
 src/ChangeLog |    4 ++++
 src/image.c   |   27 +++++++++++++++++++++++----
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/src/ChangeLog b/src/ChangeLog
index 51645ac..3d9b6bf 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,7 @@
+2010-10-27  Julien Danjou  <julien@danjou.info>
+
+	* image.c (gif_load): Add support for transparency.
+
 2010-10-26  Juanma Barranquero  <lekktu@gmail.com>
 
 	* eval.c (init_eval_once): Set max_lisp_eval_depth to 600;
diff --git a/src/image.c b/src/image.c
index b7edf05..e9b85fa 100644
--- a/src/image.c
+++ b/src/image.c
@@ -7096,12 +7096,15 @@ gif_read_from_memory (GifFileType *file, GifByteType *buf, int len)
 static const int interlace_start[] = {0, 4, 2, 1};
 static const int interlace_increment[] = {8, 8, 4, 2};
 
+#define GIF_LOCAL_DESCRIPTOR_EXTENSION 249
+
 static int
 gif_load (struct frame *f, struct image *img)
 {
   Lisp_Object file, specified_file;
   Lisp_Object specified_data;
   int rc, width, height, x, y, i;
+  boolean transparent_p;
   XImagePtr ximg;
   ColorMapObject *gif_color_map;
   unsigned long pixel_colors[256];
@@ -7110,6 +7113,7 @@ gif_load (struct frame *f, struct image *img)
   int ino, image_height, image_width;
   gif_memory_source memsrc;
   unsigned char *raster;
+  unsigned int transparency_color_index;
 
   specified_file = image_spec_value (img->spec, QCfile, NULL);
   specified_data = image_spec_value (img->spec, QCdata, NULL);
@@ -7182,6 +7186,16 @@ gif_load (struct frame *f, struct image *img)
       return 0;
     }
 
+  for(i = 0; i < gif->SavedImages[ino].ExtensionBlockCount; i++)
+    if (gif->SavedImages[ino].ExtensionBlocks[i].Function == GIF_LOCAL_DESCRIPTOR_EXTENSION
+        && gif->SavedImages[ino].ExtensionBlocks[i].ByteCount == 4
+        /* Transparency enabled? */
+        && gif->SavedImages[ino].ExtensionBlocks[i].Bytes[0] & 1)
+    {
+        transparent_p = 1;
+        transparency_color_index = (unsigned char) gif->SavedImages[ino].ExtensionBlocks[i].Bytes[3];
+    }
+
   img->corners[TOP_CORNER] = gif->SavedImages[ino].ImageDesc.Top;
   img->corners[LEFT_CORNER] = gif->SavedImages[ino].ImageDesc.Left;
   image_height = gif->SavedImages[ino].ImageDesc.Height;
@@ -7220,10 +7234,15 @@ gif_load (struct frame *f, struct image *img)
   if (gif_color_map)
     for (i = 0; i < gif_color_map->ColorCount; ++i)
       {
-        int r = gif_color_map->Colors[i].Red << 8;
-        int g = gif_color_map->Colors[i].Green << 8;
-        int b = gif_color_map->Colors[i].Blue << 8;
-        pixel_colors[i] = lookup_rgb_color (f, r, g, b);
+        if (transparent_p && transparency_color_index == i)
+            pixel_colors[i] = FRAME_BACKGROUND_PIXEL (f);
+        else
+          {
+            int r = gif_color_map->Colors[i].Red << 8;
+            int g = gif_color_map->Colors[i].Green << 8;
+            int b = gif_color_map->Colors[i].Blue << 8;
+            pixel_colors[i] = lookup_rgb_color (f, r, g, b);
+          }
       }
 
 #ifdef COLOR_TABLE_SUPPORT
-- 
1.7.2.3