From 3c74c645ca241f9e481ab441cedaf28cebaf7716 Mon Sep 17 00:00:00 2001
From: Pascal Massimino <pascal.massimino@gmail.com>
Date: Wed, 20 Sep 2017 14:08:31 -0700
Subject: [PATCH] gif2webp: handle 1-frame case properly + fix anim_diff

follow-up to commit b4e046778

Change-Id: I3c7617d09682262ca7929e6a3e56777b163cce45
---
 examples/anim_util.c | 12 +++++++++---
 examples/gif2webp.c  | 20 +++++++++++++-------
 2 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/examples/anim_util.c b/examples/anim_util.c
index c95deb16..d1a48887 100644
--- a/examples/anim_util.c
+++ b/examples/anim_util.c
@@ -425,6 +425,11 @@ static uint32_t GetBackgroundColorGIF(GifFileType* gif) {
 }
 
 // Find appropriate app extension and get loop count from the next extension.
+// We use Chrome's interpretation of the 'loop_count' semantics:
+//   if not present -> loop once
+//   if present and loop_count == 0, return 0 ('infinite').
+//   if present and loop_count != 0, it's the number of *extra* loops
+//     so we need to return loop_count + 1 as total loop number.
 static uint32_t GetLoopCountGIF(const GifFileType* const gif) {
   int i;
   for (i = 0; i < gif->ImageCount; ++i) {
@@ -442,12 +447,13 @@ static uint32_t GetLoopCountGIF(const GifFileType* const gif) {
       if (signature_is_ok &&
           eb2->Function == CONTINUE_EXT_FUNC_CODE && eb2->ByteCount >= 3 &&
           eb2->Bytes[0] == 1) {
-        return ((uint32_t)(eb2->Bytes[2]) << 8) +
-               ((uint32_t)(eb2->Bytes[1]) << 0);
+        const uint32_t extra_loop = ((uint32_t)(eb2->Bytes[2]) << 8) +
+                                    ((uint32_t)(eb2->Bytes[1]) << 0);
+        return (extra_loop > 0) ? extra_loop + 1 : 0;
       }
     }
   }
-  return 0;  // Default.
+  return 1;  // Default.
 }
 
 // Get duration of 'n'th frame in milliseconds.
diff --git a/examples/gif2webp.c b/examples/gif2webp.c
index 7287bea8..0a7c7a88 100644
--- a/examples/gif2webp.c
+++ b/examples/gif2webp.c
@@ -106,7 +106,7 @@ int main(int argc, const char *argv[]) {
   WebPAnimEncoderOptions enc_options;
   WebPConfig config;
 
-  int is_first_frame = 1;     // Whether we are processing the first frame.
+  int frame_number = 0;     // Whether we are processing the first frame.
   int done;
   int c;
   int quiet = 0;
@@ -282,7 +282,7 @@ int main(int argc, const char *argv[]) {
 
         if (!DGifGetImageDesc(gif)) goto End;
 
-        if (is_first_frame) {
+        if (frame_number == 0) {
           if (verbose) {
             printf("Canvas screen: %d x %d\n", gif->SWidth, gif->SHeight);
           }
@@ -324,7 +324,6 @@ int main(int argc, const char *argv[]) {
                     "a memory error.\n");
             goto End;
           }
-          is_first_frame = 0;
         }
 
         // Some even more broken GIF can have sub-rect with zero width/height.
@@ -341,7 +340,11 @@ int main(int argc, const char *argv[]) {
         GIFBlendFrames(&frame, &gif_rect, &curr_canvas);
 
         if (!WebPAnimEncoderAdd(enc, &curr_canvas, frame_timestamp, &config)) {
-          fprintf(stderr, "%s\n", WebPAnimEncoderGetError(enc));
+          fprintf(stderr, "Error while adding frame #%d: %s\n", frame_number,
+                  WebPAnimEncoderGetError(enc));
+          goto End;
+        } else {
+          ++frame_number;
         }
 
         // Update canvases.
@@ -451,9 +454,12 @@ int main(int argc, const char *argv[]) {
   if (!loop_compatibility) {
     if (!stored_loop_count) {
       // if no loop-count element is seen, the default is '1' (loop-once)
-      // and we need to signal it explicitly in WebP.
-      stored_loop_count = 1;
-      loop_count = 1;
+      // and we need to signal it explicitly in WebP. Note however that
+      // in case there's a single frame, we still don't need to store it.
+      if (frame_number > 1) {
+        stored_loop_count = 1;
+        loop_count = 1;
+      }
     } else if (loop_count > 0) {
       // adapt GIF's semantic to WebP's (except in the infinite-loop case)
       loop_count += 1;