gif2webp: fix transcode of loop count=65535

with loop_compatibility disabled (the default), non-zero loop counts
will be incremented by 1 for browser rendering compatibility. the max,
65535, is a special case as the muxer will fail if it is exceeded; avoid
increasing the limit in this case. this isn't 100% correct, but should
be close enough given the high number of iterations.

BUG=webp:382

Change-Id: Icde3e98a58e9ee89604a72fafda30ab71060dec5
(cherry picked from commit af0e4fbb06)

examples/anim_diff.c

@@ -143,8 +143,18 @@ static int CompareAnimatedImagePair(const AnimatedImage* const img1,
   if (!ok) return 0;  // These are fatal failures, can't proceed.
 
   if (is_multi_frame_image) {  // Checks relevant for multi-frame images only.
-    ok = CompareValues(img1->loop_count, img2->loop_count,
-                       "Loop count mismatch") && ok;
+    int max_loop_count_workaround = 0;
+    // Transcodes to webp increase the gif loop count by 1 for compatibility.
+    // When the gif has the maximum value the webp value will be off by one.
+    if ((img1->format == ANIM_GIF && img1->loop_count == 65536 &&
+         img2->format == ANIM_WEBP && img2->loop_count == 65535) ||
+        (img1->format == ANIM_WEBP && img1->loop_count == 65535 &&
+         img2->format == ANIM_GIF && img2->loop_count == 65536)) {
+      max_loop_count_workaround = 1;
+    }
+    ok = (max_loop_count_workaround ||
+          CompareValues(img1->loop_count, img2->loop_count,
+                        "Loop count mismatch")) && ok;
     ok = CompareBackgroundColor(img1->bgcolor, img2->bgcolor,
                                 premultiply) && ok;
   }

examples/anim_util.c

@@ -275,6 +275,7 @@ static int ReadAnimatedWebP(const char filename[],
     prev_frame_timestamp = timestamp;
   }
   ok = dump_ok;
+  if (ok) image->format = ANIM_WEBP;
 
  End:
   WebPAnimDecoderDelete(dec);
@@ -687,6 +688,7 @@ static int ReadAnimatedGIF(const char filename[], AnimatedImage* const image,
       }
     }
   }
+  image->format = ANIM_GIF;
   DGifCloseFile(gif, NULL);
   return 1;
 }

examples/anim_util.h

@@ -22,6 +22,11 @@
 extern "C" {
 #endif
 
+typedef enum {
+  ANIM_GIF,
+  ANIM_WEBP
+} AnimatedFileFormat;
+
 typedef struct {
   uint8_t* rgba;         // Decoded and reconstructed full frame.
   int duration;          // Frame duration in milliseconds.
@@ -29,6 +34,7 @@ typedef struct {
 } DecodedFrame;
 
 typedef struct {
+  AnimatedFileFormat format;
   uint32_t canvas_width;
   uint32_t canvas_height;
   uint32_t bgcolor;

examples/gif2webp.c

@@ -478,7 +478,7 @@ int main(int argc, const char *argv[]) {
         stored_loop_count = 1;
         loop_count = 1;
       }
-    } else if (loop_count > 0) {
+    } else if (loop_count > 0 && loop_count < 65535) {
       // adapt GIF's semantic to WebP's (except in the infinite-loop case)
       loop_count += 1;
     }