From a5ebd143d60bbf897684c60127dc926bbe4643c6 Mon Sep 17 00:00:00 2001
From: Urvang Joshi <urvang@google.com>
Date: Thu, 21 Mar 2013 15:52:02 -0700
Subject: [PATCH] gif2webp: Bgcolor fix for a special case

When transparent color index and background color index are same,
we should set background color to 0x00ffffff (transparent).

For this, we delay setting the background color until we have read the
first frame.

Change-Id: I443609b9c7697a2b94a66992460cff8465b3c127
---
 examples/gif2webp.c | 41 +++++++++++++++++++++++------------------
 1 file changed, 23 insertions(+), 18 deletions(-)

diff --git a/examples/gif2webp.c b/examples/gif2webp.c
index 2407a74b..db1332dd 100644
--- a/examples/gif2webp.c
+++ b/examples/gif2webp.c
@@ -117,17 +117,20 @@ static int ReadSubImage(GifFileType* gif, WebPPicture* pic, WebPPicture* view) {
   return ok;
 }
 
-static int GetColorFromIndex(const ColorMapObject* const color_map, GifWord idx,
-                             uint32_t* const argb) {
-  assert(color_map != NULL && color_map->Colors != NULL);
-  if (idx >= color_map->ColorCount) {
-    return 0;  // Invalid index.
+static int GetBackgroundColor(const ColorMapObject* const color_map,
+                              GifWord bgcolor_idx, uint32_t* const bgcolor) {
+  if (transparent_index != -1 && bgcolor_idx == transparent_index) {
+    *bgcolor = TRANSPARENT_COLOR;  // Special case.
+    return 1;
+  } else if (color_map == NULL || color_map->Colors == NULL
+             || bgcolor_idx >= color_map->ColorCount) {
+    return 0;  // Invalid color map or index.
   } else {
-    const GifColorType color = color_map->Colors[idx];
-    *argb = (0xff        << 24)
-          | (color.Red   << 16)
-          | (color.Green <<  8)
-          | (color.Blue  <<  0);
+    const GifColorType color = color_map->Colors[bgcolor_idx];
+    *bgcolor = (0xff        << 24)
+             | (color.Red   << 16)
+             | (color.Green <<  8)
+             | (color.Blue  <<  0);
     return 1;
   }
 }
@@ -199,6 +202,7 @@ int main(int argc, const char *argv[]) {
   WebPMuxFrameInfo frame;
   WebPMuxAnimParams anim = { WHITE_COLOR, 0 };
 
+  int is_first_frame = 1;
   int done;
   int c;
   int quiet = 0;
@@ -283,14 +287,6 @@ int main(int argc, const char *argv[]) {
   picture.custom_ptr = &memory;
   if (!WebPPictureAlloc(&picture)) goto End;
 
-  if (gif->SColorMap != NULL &&
-      !GetColorFromIndex(gif->SColorMap, gif->SBackGroundColor,
-                         &anim.bgcolor)) {
-    fprintf(stderr, "GIF decode warning: invalid background color index. "
-            "Assuming white background.\n");
-  }
-  ClearPicture(&picture, anim.bgcolor);
-
   mux = WebPMuxNew();
   if (mux == NULL) {
     fprintf(stderr, "ERROR: could not create a mux object.\n");
@@ -379,6 +375,15 @@ int main(int argc, const char *argv[]) {
                                  : WEBP_MUX_DISPOSE_NONE;
             }
             transparent_index = (flags & GIF_TRANSPARENT_MASK) ? data[4] : -1;
+            if (is_first_frame) {
+              if (!GetBackgroundColor(gif->SColorMap, gif->SBackGroundColor,
+                                      &anim.bgcolor)) {
+                fprintf(stderr, "GIF decode warning: invalid background color "
+                        "index. Assuming white background.\n");
+              }
+              ClearPicture(&picture, anim.bgcolor);
+              is_first_frame = 0;
+            }
             break;
           }
           case PLAINTEXT_EXT_FUNC_CODE: {