anim_decode: fix alpha blending with big-endian

images are decoded in RGBA/BGRA, but represented as uint32_t during the
blend process; this fixes the channel extraction

Bug: webp:548
Change-Id: Ie74aa43d8f87d3552d5afc0abba466335f5d1617
(cherry picked from commit e4886716d3bde3cc67e74ad956e5bd50112cd8f0)

1 files changed, 24 insertions, 16 deletions

diff --git a/src/demux/anim_decode.c b/src/demux/anim_decode.c
index 2bf4dcff..e077ffb5 100644
--- a/src/demux/anim_decode.c
+++ b/src/demux/anim_decode.c
@@ -23,6 +23,14 @@
 
 #define NUM_CHANNELS 4
 
+// Channel extraction from a uint32_t representation of a uint8_t RGBA/BGRA
+// buffer.
+#ifdef WORDS_BIGENDIAN
+#define CHANNEL_SHIFT(i) (24 - (i) * 8)
+#else
+#define CHANNEL_SHIFT(i) ((i) * 8)
+#endif
+
 typedef void (*BlendRowFunc)(uint32_t* const, const uint32_t* const, int);
 static void BlendPixelRowNonPremult(uint32_t* const src,
                                     const uint32_t* const dst, int num_pixels);
@@ -209,35 +217,35 @@ static uint8_t BlendChannelNonPremult(uint32_t src, uint8_t src_a,
   const uint8_t dst_channel = (dst >> shift) & 0xff;
   const uint32_t blend_unscaled = src_channel * src_a + dst_channel * dst_a;
   assert(blend_unscaled < (1ULL << 32) / scale);
-  return (blend_unscaled * scale) >> 24;
+  return (blend_unscaled * scale) >> CHANNEL_SHIFT(3);
 }
 
 // Blend 'src' over 'dst' assuming they are NOT pre-multiplied by alpha.
 static uint32_t BlendPixelNonPremult(uint32_t src, uint32_t dst) {
-  const uint8_t src_a = (src >> 24) & 0xff;
+  const uint8_t src_a = (src >> CHANNEL_SHIFT(3)) & 0xff;
 
   if (src_a == 0) {
     return dst;
   } else {
-    const uint8_t dst_a = (dst >> 24) & 0xff;
+    const uint8_t dst_a = (dst >> CHANNEL_SHIFT(3)) & 0xff;
     // This is the approximate integer arithmetic for the actual formula:
     // dst_factor_a = (dst_a * (255 - src_a)) / 255.
     const uint8_t dst_factor_a = (dst_a * (256 - src_a)) >> 8;
     const uint8_t blend_a = src_a + dst_factor_a;
     const uint32_t scale = (1UL << 24) / blend_a;
 
-    const uint8_t blend_r =
-        BlendChannelNonPremult(src, src_a, dst, dst_factor_a, scale, 0);
-    const uint8_t blend_g =
-        BlendChannelNonPremult(src, src_a, dst, dst_factor_a, scale, 8);
-    const uint8_t blend_b =
-        BlendChannelNonPremult(src, src_a, dst, dst_factor_a, scale, 16);
+    const uint8_t blend_r = BlendChannelNonPremult(
+        src, src_a, dst, dst_factor_a, scale, CHANNEL_SHIFT(0));
+    const uint8_t blend_g = BlendChannelNonPremult(
+        src, src_a, dst, dst_factor_a, scale, CHANNEL_SHIFT(1));
+    const uint8_t blend_b = BlendChannelNonPremult(
+        src, src_a, dst, dst_factor_a, scale, CHANNEL_SHIFT(2));
     assert(src_a + dst_factor_a < 256);
 
-    return (blend_r << 0) |
-           (blend_g << 8) |
-           (blend_b << 16) |
-           ((uint32_t)blend_a << 24);
+    return ((uint32_t)blend_r << CHANNEL_SHIFT(0)) |
+           ((uint32_t)blend_g << CHANNEL_SHIFT(1)) |
+           ((uint32_t)blend_b << CHANNEL_SHIFT(2)) |
+           ((uint32_t)blend_a << CHANNEL_SHIFT(3));
   }
 }
 
@@ -247,7 +255,7 @@ static void BlendPixelRowNonPremult(uint32_t* const src,
                                     const uint32_t* const dst, int num_pixels) {
   int i;
   for (i = 0; i < num_pixels; ++i) {
-    const uint8_t src_alpha = (src[i] >> 24) & 0xff;
+    const uint8_t src_alpha = (src[i] >> CHANNEL_SHIFT(3)) & 0xff;
     if (src_alpha != 0xff) {
       src[i] = BlendPixelNonPremult(src[i], dst[i]);
     }
@@ -264,7 +272,7 @@ static WEBP_INLINE uint32_t ChannelwiseMultiply(uint32_t pix, uint32_t scale) {
 
 // Blend 'src' over 'dst' assuming they are pre-multiplied by alpha.
 static uint32_t BlendPixelPremult(uint32_t src, uint32_t dst) {
-  const uint8_t src_a = (src >> 24) & 0xff;
+  const uint8_t src_a = (src >> CHANNEL_SHIFT(3)) & 0xff;
   return src + ChannelwiseMultiply(dst, 256 - src_a);
 }
 
@@ -274,7 +282,7 @@ static void BlendPixelRowPremult(uint32_t* const src, const uint32_t* const dst,
                                  int num_pixels) {
   int i;
   for (i = 0; i < num_pixels; ++i) {
-    const uint8_t src_alpha = (src[i] >> 24) & 0xff;
+    const uint8_t src_alpha = (src[i] >> CHANNEL_SHIFT(3)) & 0xff;
     if (src_alpha != 0xff) {
       src[i] = BlendPixelPremult(src[i], dst[i]);
     }