Merge pull request #23 from SDWebImage/fix_animated_webp_frame_blend_issue

Recheck the animated canvas calculation logic, fix the issue of that calculation

Author: dreampiggy

SDWebImageWebPCoder/Classes/SDImageWebPCoder.m

@@ -810,12 +810,10 @@ - (UIImage *)safeAnimatedImageFrameAtIndex:(NSUInteger)index {
     // But when one frame's dispose method is `WEBP_MUX_DISPOSE_BACKGROUND`, the canvas is cleared after the frame decoded. And subsequent frames are not effected by that frame.
     // So, we calculate each frame's `blendFromIndex`. Then directly draw canvas from that index, instead of always from 0 index.
 
-    if (_currentBlendIndex + 1 == index) {
+    if (_currentBlendIndex != NSNotFound && _currentBlendIndex + 1 == index) {
         // If the request index is subsequence of current blend index, it does not matter what dispose method is. The canvas is always ready.
-        _currentBlendIndex = index;
-        NSUInteger startIndex = index;
         // libwebp's index start with 1
-        if (!WebPDemuxGetFrame(_demux, (int)(startIndex + 1), &iter)) {
+        if (!WebPDemuxGetFrame(_demux, (int)(index + 1), &iter)) {
             WebPDemuxReleaseIterator(&iter);
             return nil;
         }
@@ -824,7 +822,6 @@ - (UIImage *)safeAnimatedImageFrameAtIndex:(NSUInteger)index {
         if (_currentBlendIndex != NSNotFound) {
             CGContextClearRect(_canvas, CGRectMake(0, 0, _canvasWidth, _canvasHeight));
         }
-        _currentBlendIndex = index;
 
         // Then, loop from the blend from index, draw each of previous frames on the canvas.
         // We use do while loop to call `WebPDemuxNextFrame`(fast), until the endIndex meet.
@@ -843,7 +840,13 @@ - (UIImage *)safeAnimatedImageFrameAtIndex:(NSUInteger)index {
                 }
             } while ((size_t)iter.frame_num < endIndex && WebPDemuxNextFrame(&iter));
         }
+        // libwebp's index start with 1
+        if (!WebPDemuxGetFrame(_demux, (int)(index + 1), &iter)) {
+            WebPDemuxReleaseIterator(&iter);
+            return nil;
+        }
     }
+    _currentBlendIndex = index;
 
     // Now the canvas is ready, which respects of dispose method behavior. Just do normal decoding and produce image.
     CGImageRef imageRef = [self sd_drawnWebpImageWithCanvas:_canvas iterator:iter colorSpace:_colorSpace];