Support DisposalMethod::kRestorePrevious in MultiFrameCodec and fix the apng problem.

lib/ui/painting/image_generator_apng.cc

@@ -538,19 +538,19 @@ bool APNGImageGenerator::DemuxNextImageInternal() {
   if (!last_frame_info.has_value()) {
     return false;
   }
-  if (last_frame_info->disposal_method ==
-      SkCodecAnimation::DisposalMethod::kRestorePrevious) {
-    FML_DLOG(INFO)
-        << "DisposalMethod::kRestorePrevious is not supported by the "
-           "MultiFrameCodec. Falling back to DisposalMethod::kRestoreBGColor "
-           " behavior instead.";
-  }
 
   if (images_.size() > first_frame_index_ &&
       last_frame_info->disposal_method ==
           SkCodecAnimation::DisposalMethod::kKeep) {
     // 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) &&
+             last_frame_info->disposal_method ==
+                 SkCodecAnimation::DisposalMethod::kRestorePrevious) {
+    // Mark the required frame as the last previous frame
+    // It is not valid if there are 2 or above frames set |disposal_method| to
+    // |kRestorePrevious|. But it also works in MultiFrameCodec.
+    image->frame_info->required_frame = images_.size() - 2;
   }
 
   // Calling SkCodec::getInfo at least once prior to decoding is mandatory.

lib/ui/painting/multi_frame_codec.cc

@@ -116,9 +116,9 @@ MultiFrameCodec::State::GetNextFrameImage(
   std::optional<unsigned int> prior_frame_index = std::nullopt;
 
   if (requiredFrameIndex != SkCodec::kNoFrame) {
-    // We currently assume that frames can only ever depend on the immediately
-    // previous frame, if any. This means that
-    // `DisposalMethod::kRestorePrevious` is not supported.
+    // We are here when the frame said |disposal_method| is
+    // `DisposalMethod::kKeep` or `DisposalMethod::kRestorePrevious` and
+    // |requiredFrameIndex| is set to ex-frame or ex-ex-frame.
     if (lastRequiredFrame_ == nullptr) {
       FML_DLOG(INFO)
           << "Frame " << nextFrameIndex_ << " depends on frame "
@@ -146,9 +146,27 @@ MultiFrameCodec::State::GetNextFrameImage(
     return std::make_pair(nullptr, decode_error);
   }
 
-  // Hold onto this if we need it to decode future frames.
-  if (frameInfo.disposal_method == SkCodecAnimation::DisposalMethod::kKeep ||
-      lastRequiredFrame_) {
+  const bool keep_current_frame =
+      frameInfo.disposal_method == SkCodecAnimation::DisposalMethod::kKeep;
+  const bool restore_previous_frame =
+      frameInfo.disposal_method ==
+      SkCodecAnimation::DisposalMethod::kRestorePrevious;
+  const bool previous_frame_available = !!lastRequiredFrame_;
+
+  // Store the current frame in `lastRequiredFrame_` if the frame's disposal
+  // method indicates we should do so.
+  // * When the disposal method is "Keep", the stored frame should always be
+  //   overwritten with the new frame we just crafted.
+  // * When the disposal method is "RestorePrevious", the previously stored
+  //   frame should be retained and used as the backdrop for the next frame
+  //   again. If there isn't already a stored frame, that means we haven't
+  //   rendered any frames yet! When this happens, we just fall back to "Keep"
+  //   behavior and store the current frame as the backdrop of the next frame.
+
+  if (keep_current_frame ||
+      (previous_frame_available && !restore_previous_frame)) {
+    // Replace the stored frame. The `lastRequiredFrame_` will get used as the
+    // starting backdrop for the next frame.
     lastRequiredFrame_ = std::make_unique<SkBitmap>(bitmap);
     lastRequiredFrameIndex_ = nextFrameIndex_;
   }

testing/dart/codec_test.dart

@@ -177,6 +177,31 @@ void main() {
     expect(imageData.buffer.asUint8List(), goldenData);
 
   });
+
+  test('Animated apng can reuse pre-pre-frame', () async {
+    // https://github.com/flutter/engine/pull/42153
+
+    final Uint8List data = File(
+      path.join('flutter', 'lib', 'ui', 'fixtures', '2_dispose_op_restore_previous.apng'),
+    ).readAsBytesSync();
+    final ui.Codec codec = await ui.instantiateImageCodec(data);
+
+    // Capture the 67,68,69 frames of animation and then compare the pixels.
+    late ui.FrameInfo frameInfo;
+    for (int i = 0; i < 70; i++) {
+      frameInfo = await codec.getNextFrame();
+      if (i >= 67) {
+        final ui.Image image = frameInfo.image;
+        final ByteData imageData = (await image.toByteData(format: ui.ImageByteFormat.png))!;
+
+        final Uint8List goldenData = File(
+          path.join('flutter', 'lib', 'ui', 'fixtures', '2_dispose_op_restore_previous.apng.$i.png'),
+        ).readAsBytesSync();
+
+        expect(imageData.buffer.asUint8List(), goldenData);
+      }
+    }
+  });
 }
 
 /// Returns a File handle to a file in the skia/resources directory.