vwebp: clear canvas at the beginning of each loop

this is in line with the recommendation in the spec, cf.,
5603947 webp-container-spec: clarify background clear on loop

(cherry picked from commit 1579de3cae534938cd61b16dfabdab7c8eb3d359)

Change-Id: Id3910395b05a1a1f2804be841b61f97bd4bac593

1 files changed, 12 insertions, 5 deletions

diff --git a/examples/vwebp.c b/examples/vwebp.c
index 70520e42..b515b27a 100644
--- a/examples/vwebp.c
+++ b/examples/vwebp.c
@@ -80,6 +80,16 @@ static void ClearParams(void) {
   kParams.dmux = NULL;
 }
 
+// Sets the previous frame to the dimensions of the canvas and has it dispose
+// to background to cause the canvas to be cleared.
+static void ClearPreviousFrame(void) {
+  WebPIterator* const prev = &kParams.prev_frame;
+  prev->width = kParams.canvas_width;
+  prev->height = kParams.canvas_height;
+  prev->x_offset = prev->y_offset = 0;
+  prev->dispose_method = WEBP_MUX_DISPOSE_BACKGROUND;
+}
+
 // -----------------------------------------------------------------------------
 // Color profile handling
 static int ApplyColorProfile(const WebPData* const profile,
@@ -180,6 +190,7 @@ static void decode_callback(int what) {
           --kParams.loop_count;
           kParams.done = (kParams.loop_count == 0);
           if (kParams.done) return;
+          ClearPreviousFrame();
         } else {
           kParams.decoding_error = 1;
           kParams.done = 1;
@@ -394,7 +405,6 @@ int main(int argc, char *argv[]) {
   int c;
   WebPDecoderConfig* const config = &kParams.config;
   WebPIterator* const curr = &kParams.curr_frame;
-  WebPIterator* const prev = &kParams.prev_frame;
 
   if (!WebPInitDecoderConfig(config)) {
     fprintf(stderr, "Library version mismatch!\n");
@@ -485,10 +495,7 @@ int main(int argc, char *argv[]) {
     printf("Canvas: %d x %d\n", kParams.canvas_width, kParams.canvas_height);
   }
 
-  prev->width = kParams.canvas_width;
-  prev->height = kParams.canvas_height;
-  prev->x_offset = prev->y_offset = 0;
-  prev->dispose_method = WEBP_MUX_DISPOSE_BACKGROUND;
+  ClearPreviousFrame();
 
   memset(&kParams.iccp, 0, sizeof(kParams.iccp));
   kParams.has_color_profile =