Implement the APNG_DISPOSE_OP_BACKGROUND disposal method in the animated PNG decoder (flutter/engine#42933)

engine/src/flutter/lib/ui/painting/image_generator.h

@@ -44,6 +44,9 @@ class ImageGenerator {
     /// How this frame should be modified before decoding the next one.
     SkCodecAnimation::DisposalMethod disposal_method;
 
+    /// The region of the frame that is affected by the disposal method.
+    std::optional<SkIRect> disposal_rect;
+
     /// How this frame should be blended with the previous frame.
     SkCodecAnimation::Blend blend_mode;
   };

engine/src/flutter/lib/ui/painting/image_generator_apng.cc

@@ -404,6 +404,10 @@ APNGImageGenerator::DemuxNextImage(const void* buffer_p,
       default:
         return std::make_pair(std::nullopt, nullptr);
     }
+
+    SkIRect frame_rect = SkIRect::MakeXYWH(
+        control_data->get_x_offset(), control_data->get_y_offset(),
+        control_data->get_width(), control_data->get_height());
     switch (control_data->get_dispose_op()) {
       case 0:  // APNG_DISPOSE_OP_NONE
         frame_info.disposal_method = SkCodecAnimation::DisposalMethod::kKeep;
@@ -411,6 +415,7 @@ APNGImageGenerator::DemuxNextImage(const void* buffer_p,
       case 1:  // APNG_DISPOSE_OP_BACKGROUND
         frame_info.disposal_method =
             SkCodecAnimation::DisposalMethod::kRestoreBGColor;
+        frame_info.disposal_rect = frame_rect;
         break;
       case 2:  // APNG_DISPOSE_OP_PREVIOUS
         frame_info.disposal_method =
@@ -547,8 +552,10 @@ bool APNGImageGenerator::DemuxNextImageInternal() {
   }
 
   if (images_.size() > first_frame_index_ &&
-      last_frame_info->disposal_method ==
-          SkCodecAnimation::DisposalMethod::kKeep) {
+      (last_frame_info->disposal_method ==
+           SkCodecAnimation::DisposalMethod::kKeep ||
+       last_frame_info->disposal_method ==
+           SkCodecAnimation::DisposalMethod::kRestoreBGColor)) {
     // Mark the required frame as the previous frame in all cases.
     image->frame_info->required_frame = images_.size() - 1;
   } else if (images_.size() > (first_frame_index_ + 1) &&

engine/src/flutter/lib/ui/painting/multi_frame_codec.cc

@@ -94,6 +94,9 @@ MultiFrameCodec::State::GetNextFrameImage(
       // Copy the previous frame's output buffer into the current frame as the
       // starting point.
       bitmap.writePixels(lastRequiredFrame_->pixmap());
+      if (restoreBGColorRect_.has_value()) {
+        bitmap.erase(SK_ColorTRANSPARENT, restoreBGColorRect_.value());
+      }
     }
   }
 
@@ -133,6 +136,13 @@ MultiFrameCodec::State::GetNextFrameImage(
     lastRequiredFrameIndex_ = nextFrameIndex_;
   }
 
+  if (frameInfo.disposal_method ==
+      SkCodecAnimation::DisposalMethod::kRestoreBGColor) {
+    restoreBGColorRect_ = frameInfo.disposal_rect;
+  } else {
+    restoreBGColorRect_.reset();
+  }
+
 #if IMPELLER_SUPPORTS_RENDERING
   if (is_impeller_enabled_) {
     // This is safe regardless of whether the GPU is available or not because

engine/src/flutter/lib/ui/painting/multi_frame_codec.h

@@ -54,10 +54,13 @@ class MultiFrameCodec : public Codec {
     int nextFrameIndex_;
     // The last decoded frame that's required to decode any subsequent frames.
     std::optional<SkBitmap> lastRequiredFrame_;
-
     // The index of the last decoded required frame.
     int lastRequiredFrameIndex_ = -1;
 
+    // The rectangle that should be cleared if the previous frame's disposal
+    // method was kRestoreBGColor.
+    std::optional<SkIRect> restoreBGColorRect_;
+
     std::pair<sk_sp<DlImage>, std::string> GetNextFrameImage(
         fml::WeakPtr<GrDirectContext> resourceContext,
         const std::shared_ptr<const fml::SyncSwitch>& gpu_disable_sync_switch,

engine/src/flutter/testing/dart/codec_test.dart

@@ -204,8 +204,6 @@ void main() {
   });
 
   test('Animated apng alpha type handling', () async {
-    // https://github.com/flutter/engine/pull/42153
-
     final Uint8List data = File(
       path.join('flutter', 'lib', 'ui', 'fixtures', 'alpha_animated.apng'),
     ).readAsBytesSync();
@@ -220,6 +218,29 @@ void main() {
     imageData = (await image.toByteData())!;
     expect(imageData.getUint32(0), 0x99000099);
   });
+
+  test('Animated apng background color restore', () async {
+    final Uint8List data = File(
+      path.join('flutter', 'lib', 'ui', 'fixtures', 'dispose_op_background.apng'),
+    ).readAsBytesSync();
+    final ui.Codec codec = await ui.instantiateImageCodec(data);
+
+    // First frame is solid red
+    ui.Image image = (await codec.getNextFrame()).image;
+    ByteData imageData = (await image.toByteData())!;
+    expect(imageData.getUint32(0), 0xFF0000FF);
+
+    // Third frame is blue in the lower right corner.
+    await codec.getNextFrame();
+    image = (await codec.getNextFrame()).image;
+    imageData = (await image.toByteData())!;
+    expect(imageData.getUint32(imageData.lengthInBytes - 4), 0x0000FFFF);
+
+    // Fourth frame is transparent in the lower right corner
+    image = (await codec.getNextFrame()).image;
+    imageData = (await image.toByteData())!;
+    expect(imageData.getUint32(imageData.lengthInBytes - 4), 0x00000000);
+  });
 }
 
 /// Returns a File handle to a file in the skia/resources directory.