From 7ec9552c225d113b6d0efd4d7bb0ef0f36c798f9 Mon Sep 17 00:00:00 2001
From: hui su <huisu@google.com>
Date: Thu, 1 Sep 2016 15:11:44 -0700
Subject: [PATCH] Make gif transparent color to be transparent black

Change it from transparent white to transparent black, which matches
the transparent color assumed in Webp dispose-to-background method.

Also pre-multiply background colors before comparison in anim_diff,
just as what is done with regular pixel values.

Change-Id: I5a790522df21619c666ce499f73e42294ed276f2
(cherry picked from commit 43bd89587987cdb37611f065210d5a260c9462fb)
---
 examples/anim_diff.c | 18 ++++++++++++++++--
 examples/anim_util.c |  2 +-
 examples/gifdec.c    |  2 +-
 3 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/examples/anim_diff.c b/examples/anim_diff.c
index a91c8b34..011a6581 100644
--- a/examples/anim_diff.c
+++ b/examples/anim_diff.c
@@ -110,6 +110,20 @@ static int CompareValues(uint32_t a, uint32_t b, const char* output_str) {
   return 1;
 }
 
+static int CompareBackgroundColor(uint32_t bg1, uint32_t bg2, int premultiply) {
+  if (premultiply) {
+    const int alpha1 = (bg1 >> 24) & 0xff;
+    const int alpha2 = (bg2 >> 24) & 0xff;
+    if (alpha1 == 0 && alpha2 == 0) return 1;
+  }
+  if (bg1 != bg2) {
+    fprintf(stderr, "Background color mismatch: 0x%08x vs 0x%08x\n",
+            bg1, bg2);
+    return 0;
+  }
+  return 1;
+}
+
 // Note: As long as frame durations and reconstructed frames are identical, it
 // is OK for other aspects like offsets, dispose/blend method to vary.
 static int CompareAnimatedImagePair(const AnimatedImage* const img1,
@@ -131,8 +145,8 @@ static int CompareAnimatedImagePair(const AnimatedImage* const img1,
   if (is_multi_frame_image) {  // Checks relevant for multi-frame images only.
     ok = CompareValues(img1->loop_count, img2->loop_count,
                        "Loop count mismatch") && ok;
-    ok = CompareValues(img1->bgcolor, img2->bgcolor,
-                       "Background color mismatch") && ok;
+    ok = CompareBackgroundColor(img1->bgcolor, img2->bgcolor,
+                                premultiply) && ok;
   }
 
   for (i = 0; i < img1->num_frames; ++i) {
diff --git a/examples/anim_util.c b/examples/anim_util.c
index 417fec2f..73fc7b97 100644
--- a/examples/anim_util.c
+++ b/examples/anim_util.c
@@ -396,7 +396,7 @@ static uint32_t GetBackgroundColorGIF(GifFileType* gif) {
   const ColorMapObject* const color_map = gif->SColorMap;
   if (transparent_index != NO_TRANSPARENT_COLOR &&
       gif->SBackGroundColor == transparent_index) {
-    return 0x00ffffff;  // Special case: transparent white.
+    return 0x00000000;  // Special case: transparent black.
   } else if (color_map == NULL || color_map->Colors == NULL
              || gif->SBackGroundColor >= color_map->ColorCount) {
     return 0xffffffff;  // Invalid: assume white.
diff --git a/examples/gifdec.c b/examples/gifdec.c
index 8c36af61..c535a407 100644
--- a/examples/gifdec.c
+++ b/examples/gifdec.c
@@ -21,7 +21,7 @@
 #include "webp/encode.h"
 #include "webp/mux_types.h"
 
-#define GIF_TRANSPARENT_COLOR 0x00ffffff
+#define GIF_TRANSPARENT_COLOR 0x00000000
 #define GIF_WHITE_COLOR       0xffffffff
 #define GIF_TRANSPARENT_MASK  0x01
 #define GIF_DISPOSE_MASK      0x07