gif2webp: introduce -loop_compatibility option

This patch fixes the compatibility for loop-count handling.
This aims at addressing the change in Chrome handling of loop-count
prior to M63.

Before M63: loop-count interpretation was aligned to GIF's behaviour in
Chrome, but incompatible with WebP's spec. In particular, you couldn't
loop exactly once.

Post-M63: loop-count in WebP is really the total number of loops. Gif2webp
will convert correctly from a GIF source by adjusting the loop count.

Note: The Chrome version can be retrieved from the User-Agent
string (chrome://version).
An M63 version will contain the pattern:
   Chrome/63.x.xxxx.xx
for instance.

Change-Id: Ie6dc13227e6498f4d7af2f09247913648997648a

README

@@ -477,6 +477,8 @@ Options:
   -metadata <string> ..... comma separated list of metadata to
                            copy from the input to the output if present
                            Valid values: all, none, icc, xmp (default)
+  -loop_compatibility .... use compatibility mode for Chrome
+                           version prior to M62 (inclusive)
   -mt .................... use multi-threading if available
 
   -version ............... print version number and exit

examples/gif2webp.c

@@ -72,8 +72,10 @@ static void Help(void) {
   printf("  -metadata <string> ..... comma separated list of metadata to\n");
   printf("                           ");
   printf("copy from the input to the output if present\n");
-  printf("                           "
-         "Valid values: all, none, icc, xmp (default)\n");
+  printf("                           ");
+  printf("Valid values: all, none, icc, xmp (default)\n");
+  printf("  -loop_compatibility .... use compatibility mode for Chrome\n");
+  printf("                           version prior to M62 (inclusive)\n");
   printf("  -mt .................... use multi-threading if available\n");
   printf("\n");
   printf("  -version ............... print version number and exit\n");
@@ -115,8 +117,9 @@ int main(int argc, const char *argv[]) {
   int stored_icc = 0;         // Whether we have already stored an ICC profile.
   WebPData xmp_data;
   int stored_xmp = 0;         // Whether we have already stored an XMP profile.
-  int loop_count = 0;
+  int loop_count = 0;         // default: infinite
   int stored_loop_count = 0;  // Whether we have found an explicit loop count.
+  int loop_compatibility = 0;
   WebPMux* mux = NULL;
 
   int default_kmin = 1;  // Whether to use default kmin value.
@@ -151,6 +154,8 @@ int main(int argc, const char *argv[]) {
     } else if (!strcmp(argv[c], "-mixed")) {
       enc_options.allow_mixed = 1;
       config.lossless = 0;
+    } else if (!strcmp(argv[c], "-loop_compatibility")) {
+      loop_compatibility = 1;
     } else if (!strcmp(argv[c], "-q") && c < argc - 1) {
       config.quality = ExUtilGetFloat(argv[++c], &parse_error);
     } else if (!strcmp(argv[c], "-m") && c < argc - 1) {
@@ -386,7 +391,7 @@ int main(int argc, const char *argv[]) {
               if (verbose) {
                 fprintf(stderr, "Loop count: %d\n", loop_count);
               }
-              stored_loop_count = (loop_count != 0);
+              stored_loop_count = loop_compatibility ? (loop_count != 0) : 1;
             } else {  // An extension containing metadata.
               // We only store the first encountered chunk of each type, and
               // only if requested by the user.
@@ -443,6 +448,20 @@ int main(int argc, const char *argv[]) {
     goto End;
   }
 
+  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;
+    } else if (loop_count > 0) {
+      // adapt GIF's semantic to WebP's (except in the infinite-loop case)
+      loop_count += 1;
+    }
+  }
+  // loop_count of 0 is the default (infinite), so no need to signal it
+  if (loop_count == 0) stored_loop_count = 0;
+
   if (stored_loop_count || stored_icc || stored_xmp) {
     // Re-mux to add loop count and/or metadata as needed.
     mux = WebPMuxCreate(&webp_data, 1);

man/gif2webp.1

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH GIF2WEBP 1 "January 25, 2017"
+.TH GIF2WEBP 1 "September 20, 2017"
 .SH NAME
 gif2webp \- Convert a GIF image to WebP
 .SH SYNOPSIS
@@ -109,6 +109,9 @@ the range of 20 to 50.
 .TP
 .B \-mt
 Use multi-threading for encoding, if possible.
+.B \-loop_compatibility
+If enabled, handle the loop information in a compatible fashion for Chrome
+version prior to M62 (inclusive) and Firefox.
 .TP
 .B \-v
 Print extra information.