transparency support

Originally committed as revision 1808 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/gifdec.c

@@ -94,6 +94,7 @@ static int gif_video_probe(AVProbeData * pd)
 {
     const uint8_t *p, *p_end;
     int bits_per_pixel, has_global_palette, ext_code, ext_len;
+    int gce_flags, gce_disposal;
 
     if (pd->buf_size < 24 ||
 	memcmp(pd->buf, gif89a_sig, 6) != 0)
@@ -114,16 +115,28 @@ static int gif_video_probe(AVProbeData * pd)
         if (p >= p_end)
             return 0;
         ext_code = *p++;
-        /* if GCE extension found: it is likely to be an animation */
-        if (ext_code == 0xf9)
-            return AVPROBE_SCORE_MAX;
-        for(;;) {
         if (p >= p_end)
             return 0;
         ext_len = *p++;
+        if (ext_code == 0xf9) {
+            if (p >= p_end)
+                return 0;
+            /* if GCE extension found with gce_disposal != 0: it is
+               likely to be an animation */
+            gce_flags = *p++;
+            gce_disposal = (gce_flags >> 2) & 0x7;
+            if (gce_disposal != 0)
+                return AVPROBE_SCORE_MAX;
+            else
+                return 0;
+        }
+        for(;;) {
             if (ext_len == 0)
                 break;
             p += ext_len;
+            if (p >= p_end)
+                return 0;
+            ext_len = *p++;
         }
     }
     return 0;
@@ -320,6 +333,9 @@ static int gif_read_image(GifState *s)
         }
         for(; i < 256; i++)
             s->image_palette[i] = (0xff << 24);
+        /* handle transparency */
+        if (s->transparent_color_index >= 0)
+            s->image_palette[s->transparent_color_index] = 0;
         line = NULL;
     }