IMG_webp: Incorrect frame composition in animated WebP loading

src/IMG_webp.c

@@ -347,6 +347,13 @@ IMG_Animation *IMG_LoadWEBPAnimation_RW(SDL_RWops *src)
                           (bgcolor >> 24) & 0xFF);
 #endif
 
+    // Initialize both canvases - use bgcolor for non-alpha format, transparency for alpha
+    if (features.has_alpha) {
+        SDL_FillRect(canvas, NULL, SDL_MapRGBA(canvas->format, 0, 0, 0, 0));
+    } else {
+        SDL_FillRect(canvas, NULL, bgcolor);
+    }
+
     SDL_zero(iter);
     if (lib.WebPDemuxGetFrame(demuxer, 1, &iter)) {
         do {
@@ -357,8 +364,21 @@ IMG_Animation *IMG_LoadWEBPAnimation_RW(SDL_RWops *src)
                 continue;
             }
 
-            if (dispose_method == WEBP_MUX_DISPOSE_BACKGROUND) {
-                SDL_FillRect(canvas, NULL, bgcolor);
+            // Set up destination rect for current frame
+            dst.x = iter.x_offset;
+            dst.y = iter.y_offset;
+            dst.w = iter.width;
+            dst.h = iter.height;
+
+            // Handle disposal and prepare region for new frame
+            if (iter.dispose_method == WEBP_MUX_DISPOSE_BACKGROUND ||
+                iter.blend_method == WEBP_MUX_NO_BLEND) {
+                if (features.has_alpha) {
+                    SDL_FillRect(canvas, &dst, SDL_MapRGBA(canvas->format, 0, 0, 0, 0));
+                }
+                else {
+                    SDL_FillRect(canvas, &dst, bgcolor);
+                }
             }
 
             curr = SDL_CreateRGBSurfaceWithFormat(0, iter.width, iter.height, 0, SDL_PIXELFORMAT_RGBA32);
@@ -367,30 +387,29 @@ IMG_Animation *IMG_LoadWEBPAnimation_RW(SDL_RWops *src)
             }
 
             if (!lib.WebPDecodeRGBAInto(iter.fragment.bytes,
-                                        iter.fragment.size,
-                                        (uint8_t *)curr->pixels,
-                                        curr->pitch * curr->h,
-                                        curr->pitch)) {
+                iter.fragment.size,
+                (uint8_t*)curr->pixels,
+                curr->pitch * curr->h,
+                curr->pitch)) {
                 error = "WebPDecodeRGBAInto() failed";
                 SDL_FreeSurface(curr);
                 goto error;
             }
 
+            // Set blend mode based on the frame's blend method
             if (iter.blend_method == WEBP_MUX_BLEND) {
                 SDL_SetSurfaceBlendMode(curr, SDL_BLENDMODE_BLEND);
-            } else {
+            }
+            else {
                 SDL_SetSurfaceBlendMode(curr, SDL_BLENDMODE_NONE);
             }
-            dst.x = iter.x_offset;
-            dst.y = iter.y_offset;
-            dst.w = iter.width;
-            dst.h = iter.height;
+
             SDL_BlitSurface(curr, NULL, canvas, &dst);
             SDL_FreeSurface(curr);
 
+            // Store complete frame state
             anim->frames[frame_idx] = SDL_DuplicateSurface(canvas);
             anim->delays[frame_idx] = iter.duration;
-            dispose_method = iter.dispose_method;
 
         } while (lib.WebPDemuxNextFrame(&iter));