gif2webp: Improved compression for lossy animated WebP

We reduce transparency by turning some transparent pixels into
corresponding RGB values from previous canvas.

This improves compression by about 23%.

Change-Id: I02d70a43a1d0906ac09a7e2dc510be3b2d38f593

examples/gif2webp.c

@@ -166,6 +166,11 @@ static int OptimizeAndEncodeFrame(
     // Update prev_canvas by simply copying from 'curr'.
     WebPUtilCopyPixels(curr, prev_canvas);
   } else {
+    if (!config->lossless) {
+      // For lossy compression, it's better to replace transparent pixels of
+      // 'curr' with actual RGB values, whenever possible.
+      WebPUtilReduceTransparency(prev_canvas, &rect, curr);
+    }
     if (!WebPFrameCacheShouldTryKeyFrame(cache)) {
       // Add this as a frame rectangle.
       if (!WebPFrameCacheAddFrame(cache, config, info, sub_image, NULL, NULL)) {

examples/gif2webp_util.c

@@ -287,6 +287,25 @@ void WebPUtilBlendPixels(const WebPPicture* const src,
   }
 }
 
+void WebPUtilReduceTransparency(const WebPPicture* const src,
+                                const WebPFrameRect* const rect,
+                                WebPPicture* const dst) {
+  int j;
+  assert(src->width == dst->width && src->height == dst->height);
+  for (j = rect->y_offset; j < rect->y_offset + rect->height; ++j) {
+    int i;
+    for (i = rect->x_offset; i < rect->x_offset + rect->width; ++i) {
+      const uint32_t src_pixel = src->argb[j * src->argb_stride + i];
+      const int src_alpha = src_pixel >> 24;
+      const uint32_t dst_pixel = dst->argb[j * dst->argb_stride + i];
+      const int dst_alpha = dst_pixel >> 24;
+      if (dst_alpha == 0 && src_alpha == 0xff) {
+        dst->argb[j * dst->argb_stride + i] = src_pixel;
+      }
+    }
+  }
+}
+
 //------------------------------------------------------------------------------
 // Key frame related utilities.
 

examples/gif2webp_util.h

@@ -86,6 +86,11 @@ void WebPUtilBlendPixels(const struct WebPPicture* const src,
                          const WebPFrameRect* const src_rect,
                          struct WebPPicture* const dst);
 
+// Replace transparent pixels within 'dst_rect' of 'dst' by those in the 'src'.
+void WebPUtilReduceTransparency(const struct WebPPicture* const src,
+                                const WebPFrameRect* const dst_rect,
+                                struct WebPPicture* const dst);
+
 //------------------------------------------------------------------------------
 // Key frame related.