Apply the transform of an image filter layer to paint bounds in the CanvasKit backend

Fixes flutter/flutter#128788

Author: jason-simmons

lib/web_ui/lib/src/engine/canvaskit/image_filter.dart

@@ -40,6 +40,8 @@ abstract class CkImageFilter implements CkManagedSkImageFilterConvertible {
       required ui.FilterQuality filterQuality}) = _CkMatrixImageFilter;
 
   CkImageFilter._();
+
+  Matrix4 get transform => Matrix4.identity();
 }
 
 class CkColorFilterImageFilter extends CkImageFilter {
@@ -149,6 +151,7 @@ class _CkMatrixImageFilter extends CkImageFilter {
   _CkMatrixImageFilter(
       {required Float64List matrix, required this.filterQuality})
       : matrix = Float64List.fromList(matrix),
+        _transform = Matrix4.fromFloat32List(toMatrix32(matrix)),
         super._() {
     final SkImageFilter skImageFilter = canvasKit.ImageFilter.MakeMatrixTransform(
       toSkMatrixFromFloat64(matrix),
@@ -160,6 +163,7 @@ class _CkMatrixImageFilter extends CkImageFilter {
 
   final Float64List matrix;
   final ui.FilterQuality filterQuality;
+  final Matrix4 _transform;
 
   late final UniqueRef<SkImageFilter> _ref;
 
@@ -183,4 +187,7 @@ class _CkMatrixImageFilter extends CkImageFilter {
 
   @override
   String toString() => 'ImageFilter.matrix($matrix, $filterQuality)';
+
+  @override
+  Matrix4 get transform => _transform;
 }

lib/web_ui/lib/src/engine/canvaskit/layer.dart

@@ -7,6 +7,7 @@ import 'package:ui/ui.dart' as ui;
 import '../vector_math.dart';
 import 'canvas.dart';
 import 'embedded_views.dart';
+import 'image_filter.dart';
 import 'n_way_canvas.dart';
 import 'painting.dart';
 import 'path.dart';
@@ -396,6 +397,17 @@ class ImageFilterEngineLayer extends ContainerLayer
   final ui.Offset _offset;
   final ui.ImageFilter _filter;
 
+  @override
+  void preroll(PrerollContext prerollContext, Matrix4 matrix) {
+    final Matrix4 transform = (_filter as CkImageFilter).transform;
+    final Matrix4 childMatrix = matrix.multiplied(transform);
+    prerollContext.mutatorsStack.pushTransform(transform);
+    final ui.Rect childPaintBounds =
+        prerollChildren(prerollContext, childMatrix);
+    paintBounds = transform.transformRect(childPaintBounds);
+    prerollContext.mutatorsStack.pop();
+  }
+
   @override
   void paint(PaintContext paintContext) {
     assert(needsPainting);

lib/web_ui/test/canvaskit/layer_test.dart

@@ -72,5 +72,31 @@ void testMain() {
         region: kDefaultRegion,
       );
     });
+
+    test('ImageFilter layer applies matrix in preroll', () async {
+      final CkPicture picture =
+          paintPicture(const ui.Rect.fromLTRB(0, 0, 100, 100), (CkCanvas canvas) {
+        canvas.drawRect(const ui.Rect.fromLTRB(0, 0, 100, 100),
+            CkPaint()..style = ui.PaintingStyle.fill);
+      });
+
+      final LayerSceneBuilder sb = LayerSceneBuilder();
+      sb.pushImageFilter(
+        ui.ImageFilter.matrix(
+          (
+            Matrix4.identity()
+              ..scale(0.5, 0.5)
+              ..translate(20)
+          ).toFloat64(),
+        ),
+      );
+      sb.addPicture(ui.Offset.zero, picture);
+
+      final LayerTree layerTree = sb.build().layerTree;
+      CanvasKitRenderer.instance.rasterizer.draw(layerTree);
+
+      final ImageFilterEngineLayer imageFilterLayer = layerTree.rootLayer.debugLayers.single as ImageFilterEngineLayer;
+      expect(imageFilterLayer.paintBounds, const ui.Rect.fromLTRB(10, 0, 60, 50));
+    });
   });
 }