[web] Implement SceneBuilder.pushColorFilter for html (flutter/engine#20802)

* Implement Color filter layer
* Add test with BlendMode color
* update licenses file
* Move blend functions into color_filter.dart
* dartfmt

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -465,6 +465,7 @@ FILE: ../../../flutter/lib/web_ui/lib/src/engine/history.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/html/backdrop_filter.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/html/canvas.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/html/clip.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/engine/html/color_filter.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/html/debug_canvas_reuse_overlay.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/html/image_filter.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/html/offset.dart

engine/src/flutter/lib/web_ui/dev/goldens_lock.yaml

@@ -1,2 +1,2 @@
 repository: https://github.com/flutter/goldens.git
-revision: f26d68c3596eece3d40112e9dff01dc55d9bae97
+revision: 8a831253654d151635ee9cfb71389c257413de5d

engine/src/flutter/lib/web_ui/lib/src/engine.dart

@@ -67,6 +67,7 @@ part 'engine/history.dart';
 part 'engine/html/backdrop_filter.dart';
 part 'engine/html/canvas.dart';
 part 'engine/html/clip.dart';
+part 'engine/html/color_filter.dart';
 part 'engine/html/debug_canvas_reuse_overlay.dart';
 part 'engine/html/image_filter.dart';
 part 'engine/html/offset.dart';

engine/src/flutter/lib/web_ui/lib/src/engine/bitmap_canvas.dart

@@ -573,57 +573,10 @@ class BitmapCanvas extends EngineCanvas {
       ui.Color? filterColor, ui.BlendMode colorFilterBlendMode,
       SurfacePaintData paint) {
     // For srcIn blendMode, we use an svg filter to apply to image element.
-    String? svgFilter;
-    switch (colorFilterBlendMode) {
-      case ui.BlendMode.srcIn:
-      case ui.BlendMode.srcATop:
-        svgFilter = _srcInColorFilterToSvg(filterColor);
-        break;
-      case ui.BlendMode.srcOut:
-        svgFilter = _srcOutColorFilterToSvg(filterColor);
-        break;
-      case ui.BlendMode.xor:
-        svgFilter = _xorColorFilterToSvg(filterColor);
-        break;
-      case ui.BlendMode.plus:
-        // Porter duff source + destination.
-        svgFilter = _compositeColorFilterToSvg(filterColor, 0, 1, 1, 0);
-        break;
-      case ui.BlendMode.modulate:
-        // Porter duff source * destination but preserves alpha.
-        svgFilter = _modulateColorFilterToSvg(filterColor!);
-        break;
-      case ui.BlendMode.overlay:
-        // Since overlay is the same as hard-light by swapping layers,
-        // pass hard-light blend function.
-        svgFilter = _blendColorFilterToSvg(filterColor, 'hard-light',
-            swapLayers: true);
-        break;
-      // Several of the filters below (although supported) do not render the
-      // same (close but not exact) as native flutter when used as blend mode
-      // for a background-image with a background color. They only look
-      // identical when feBlend is used within an svg filter definition.
-      //
-      // Saturation filter uses destination when source is transparent.
-      // cMax = math.max(r, math.max(b, g));
-      // cMin = math.min(r, math.min(b, g));
-      // delta = cMax - cMin;
-      // lightness = (cMax + cMin) / 2.0;
-      // saturation = delta / (1.0 - (2 * lightness - 1.0).abs());
-      case ui.BlendMode.saturation:
-      case ui.BlendMode.colorDodge:
-      case ui.BlendMode.colorBurn:
-      case ui.BlendMode.hue:
-      case ui.BlendMode.color:
-      case ui.BlendMode.luminosity:
-        svgFilter = _blendColorFilterToSvg(filterColor,
-            _stringForBlendMode(colorFilterBlendMode));
-        break;
-      default:
-        break;
-    }
+    String? svgFilter = svgFilterFromBlendMode(filterColor,
+        colorFilterBlendMode);
     final html.Element filterElement =
-    html.Element.html(svgFilter, treeSanitizer: _NullTreeSanitizer());
+        html.Element.html(svgFilter, treeSanitizer: _NullTreeSanitizer());
     rootElement.append(filterElement);
     _children.add(filterElement);
     final html.HtmlElement imgElement = _reuseOrCreateImage(image);
@@ -1013,109 +966,3 @@ String _maskFilterToCanvasFilter(ui.MaskFilter? maskFilter) {
   }
 }
 
-int _filterIdCounter = 0;
-
-// The color matrix for feColorMatrix element changes colors based on
-// the following:
-//
-// | R' |     | r1 r2 r3 r4 r5 |   | R |
-// | G' |     | g1 g2 g3 g4 g5 |   | G |
-// | B' |  =  | b1 b2 b3 b4 b5 | * | B |
-// | A' |     | a1 a2 a3 a4 a5 |   | A |
-// | 1  |     | 0  0  0  0  1  |   | 1 |
-//
-// R' = r1*R + r2*G + r3*B + r4*A + r5
-// G' = g1*R + g2*G + g3*B + g4*A + g5
-// B' = b1*R + b2*G + b3*B + b4*A + b5
-// A' = a1*R + a2*G + a3*B + a4*A + a5
-String _srcInColorFilterToSvg(ui.Color? color) {
-  _filterIdCounter += 1;
-  return '<svg width="0" height="0">'
-      '<filter id="_fcf$_filterIdCounter" '
-      'filterUnits="objectBoundingBox" x="0%" y="0%" width="100%" height="100%">'
-      '<feColorMatrix values="0 0 0 0 1 ' // Ignore input, set it to absolute.
-          '0 0 0 0 1 '
-          '0 0 0 0 1 '
-          '0 0 0 1 0" result="destalpha"/>' // Just take alpha channel of destination
-      '<feFlood flood-color="${colorToCssString(color)}" flood-opacity="1" result="flood">'
-      '</feFlood>'
-      '<feComposite in="flood" in2="destalpha" '
-      'operator="arithmetic" k1="1" k2="0" k3="0" k4="0" result="comp">'
-      '</feComposite>'
-      '</filter></svg>';
-}
-
-String _srcOutColorFilterToSvg(ui.Color? color) {
-  _filterIdCounter += 1;
-  return '<svg width="0" height="0">'
-      '<filter id="_fcf$_filterIdCounter" '
-      'filterUnits="objectBoundingBox" x="0%" y="0%" width="100%" height="100%">'
-      '<feFlood flood-color="${colorToCssString(color)}" flood-opacity="1" result="flood">'
-      '</feFlood>'
-      '<feComposite in="flood" in2="SourceGraphic" operator="out" result="comp">'
-      '</feComposite>'
-      '</filter></svg>';
-}
-
-String _xorColorFilterToSvg(ui.Color? color) {
-  _filterIdCounter += 1;
-  return '<svg width="0" height="0">'
-      '<filter id="_fcf$_filterIdCounter" '
-      'filterUnits="objectBoundingBox" x="0%" y="0%" width="100%" height="100%">'
-      '<feFlood flood-color="${colorToCssString(color)}" flood-opacity="1" result="flood">'
-      '</feFlood>'
-      '<feComposite in="flood" in2="SourceGraphic" operator="xor" result="comp">'
-      '</feComposite>'
-      '</filter></svg>';
-}
-
-// The source image and color are composited using :
-// result = k1 *in*in2 + k2*in + k3*in2 + k4.
-String _compositeColorFilterToSvg(ui.Color? color, double k1, double k2, double k3 , double k4) {
-  _filterIdCounter += 1;
-  return '<svg width="0" height="0">'
-      '<filter id="_fcf$_filterIdCounter" '
-      'filterUnits="objectBoundingBox" x="0%" y="0%" width="100%" height="100%">'
-      '<feFlood flood-color="${colorToCssString(color)}" flood-opacity="1" result="flood">'
-      '</feFlood>'
-      '<feComposite in="flood" in2="SourceGraphic" '
-      'operator="arithmetic" k1="$k1" k2="$k2" k3="$k3" k4="$k4" result="comp">'
-      '</feComposite>'
-      '</filter></svg>';
-}
-
-// Porter duff source * destination , keep source alpha.
-// First apply color filter to source to change it to [color], then
-// composite using multiplication.
-String _modulateColorFilterToSvg(ui.Color color) {
-  _filterIdCounter += 1;
-  final double r = color.red / 255.0;
-  final double b = color.blue / 255.0;
-  final double g = color.green / 255.0;
-  return '<svg width="0" height="0">'
-      '<filter id="_fcf$_filterIdCounter" '
-      'filterUnits="objectBoundingBox" x="0%" y="0%" width="100%" height="100%">'
-      '<feColorMatrix values="0 0 0 0 $r ' // Ignore input, set it to absolute.
-      '0 0 0 0 $g '
-      '0 0 0 0 $b '
-      '0 0 0 1 0" result="recolor"/>'
-      '<feComposite in="recolor" in2="SourceGraphic" '
-      'operator="arithmetic" k1="1" k2="0" k3="0" k4="0" result="comp">'
-      '</feComposite>'
-      '</filter></svg>';
-}
-
-// Uses feBlend element to blend source image with a color.
-String _blendColorFilterToSvg(ui.Color? color, String? feBlend,
-    {bool swapLayers = false}) {
-  _filterIdCounter += 1;
-  return '<svg width="0" height="0">'
-      '<filter id="_fcf$_filterIdCounter" filterUnits="objectBoundingBox" '
-      'x="0%" y="0%" width="100%" height="100%">'
-      '<feFlood flood-color="${colorToCssString(color)}" flood-opacity="1" result="flood">'
-      '</feFlood>' +
-      (swapLayers
-      ? '<feBlend in="SourceGraphic" in2="flood" mode="$feBlend"/>'
-      : '<feBlend in="flood" in2="SourceGraphic" mode="$feBlend"/>') +
-      '</filter></svg>';
-}

engine/src/flutter/lib/web_ui/lib/src/engine/html/color_filter.dart

@@ -0,0 +1,292 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// @dart = 2.10
+part of engine;
+
+/// A surface that applies an [ColorFilter] to its children.
+class PersistedColorFilter extends PersistedContainerSurface
+    implements ui.ColorFilterEngineLayer {
+  PersistedColorFilter(PersistedColorFilter? oldLayer, this.filter)
+      : super(oldLayer);
+
+  @override
+  html.Element? get childContainer => _childContainer;
+
+  /// The dedicated child container element that's separate from the
+  /// [rootElement] is used to compensate for the coordinate system shift
+  /// introduced by the [rootElement] translation.
+  html.Element? _childContainer;
+
+  final ui.ColorFilter filter;
+  html.Element? _filterElement;
+  bool containerVisible = true;
+
+  @override
+  void adoptElements(PersistedColorFilter oldSurface) {
+    super.adoptElements(oldSurface);
+    _childContainer = oldSurface._childContainer;
+    oldSurface._childContainer = null;
+  }
+
+  @override
+  void discard() {
+    super.discard();
+    // Do not detach the child container from the root. It is permanently
+    // attached. The elements are reused together and are detached from the DOM
+    // together.
+    _childContainer = null;
+  }
+
+  @override
+  html.Element createElement() {
+    html.Element element = defaultCreateElement('flt-color-filter');
+    html.Element container = html.Element.tag('flt-filter-interior');
+    container.style.position = 'absolute';
+    _childContainer = container;
+    element.append(_childContainer!);
+    return element;
+  }
+
+  @override
+  void apply() {
+    if (_filterElement != null) {
+      _filterElement?.remove();
+    }
+    final EngineColorFilter? engineValue = filter as EngineColorFilter?;
+    if (engineValue == null) {
+      rootElement!.style.backgroundColor = '';
+      childContainer?.style.visibility = 'visible';
+      return;
+    }
+    if (engineValue._blendMode == null) {
+      rootElement!.style.backgroundColor =
+          colorToCssString(engineValue._color!);
+      childContainer?.style.visibility = 'visible';
+      return;
+    }
+
+    ui.Color filterColor = engineValue._color!;
+    ui.BlendMode? colorFilterBlendMode = engineValue._blendMode;
+    html.CssStyleDeclaration style = rootElement!.style;
+    if (colorFilterBlendMode != null) {
+      switch (colorFilterBlendMode) {
+        case ui.BlendMode.clear:
+        case ui.BlendMode.dstOut:
+        case ui.BlendMode.srcOut:
+          childContainer?.style.visibility = 'hidden';
+          return;
+        case ui.BlendMode.dst:
+        case ui.BlendMode.dstIn:
+          // Noop.
+          return;
+        case ui.BlendMode.src:
+        case ui.BlendMode.srcOver:
+          // Uses source filter color.
+          // Since we don't have a size, we can't use background color.
+          // Use svg filter srcIn instead.
+          colorFilterBlendMode = ui.BlendMode.srcIn;
+          break;
+      }
+
+      // Use SVG filter for blend mode.
+      String? svgFilter =
+          svgFilterFromBlendMode(filterColor, colorFilterBlendMode);
+      if (svgFilter != null) {
+        _filterElement =
+            html.Element.html(svgFilter, treeSanitizer: _NullTreeSanitizer());
+        rootElement!.append(_filterElement!);
+        rootElement!.style.filter = 'url(#_fcf${_filterIdCounter})';
+        if (colorFilterBlendMode == ui.BlendMode.saturation ||
+            colorFilterBlendMode == ui.BlendMode.multiply ||
+            colorFilterBlendMode == ui.BlendMode.modulate) {
+          style.backgroundColor = colorToCssString(filterColor);
+        }
+        return;
+      }
+    }
+  }
+
+  @override
+  void update(PersistedColorFilter oldSurface) {
+    super.update(oldSurface);
+
+    if (oldSurface.filter != filter) {
+      apply();
+    }
+  }
+}
+
+String? svgFilterFromBlendMode(
+    ui.Color? filterColor, ui.BlendMode colorFilterBlendMode) {
+  String? svgFilter;
+  switch (colorFilterBlendMode) {
+    case ui.BlendMode.srcIn:
+    case ui.BlendMode.srcATop:
+      svgFilter = _srcInColorFilterToSvg(filterColor);
+      break;
+    case ui.BlendMode.srcOut:
+      svgFilter = _srcOutColorFilterToSvg(filterColor);
+      break;
+    case ui.BlendMode.xor:
+      svgFilter = _xorColorFilterToSvg(filterColor);
+      break;
+    case ui.BlendMode.plus:
+      // Porter duff source + destination.
+      svgFilter = _compositeColorFilterToSvg(filterColor, 0, 1, 1, 0);
+      break;
+    case ui.BlendMode.modulate:
+      // Porter duff source * destination but preserves alpha.
+      svgFilter = _modulateColorFilterToSvg(filterColor!);
+      break;
+    case ui.BlendMode.overlay:
+      // Since overlay is the same as hard-light by swapping layers,
+      // pass hard-light blend function.
+      svgFilter =
+          _blendColorFilterToSvg(filterColor, 'hard-light', swapLayers: true);
+      break;
+    // Several of the filters below (although supported) do not render the
+    // same (close but not exact) as native flutter when used as blend mode
+    // for a background-image with a background color. They only look
+    // identical when feBlend is used within an svg filter definition.
+    //
+    // Saturation filter uses destination when source is transparent.
+    // cMax = math.max(r, math.max(b, g));
+    // cMin = math.min(r, math.min(b, g));
+    // delta = cMax - cMin;
+    // lightness = (cMax + cMin) / 2.0;
+    // saturation = delta / (1.0 - (2 * lightness - 1.0).abs());
+    case ui.BlendMode.saturation:
+    case ui.BlendMode.colorDodge:
+    case ui.BlendMode.colorBurn:
+    case ui.BlendMode.hue:
+    case ui.BlendMode.color:
+    case ui.BlendMode.luminosity:
+    case ui.BlendMode.multiply:
+    case ui.BlendMode.screen:
+    case ui.BlendMode.overlay:
+    case ui.BlendMode.darken:
+    case ui.BlendMode.lighten:
+    case ui.BlendMode.colorDodge:
+    case ui.BlendMode.colorBurn:
+    case ui.BlendMode.hardLight:
+    case ui.BlendMode.softLight:
+    case ui.BlendMode.difference:
+    case ui.BlendMode.exclusion:
+      svgFilter = _blendColorFilterToSvg(
+          filterColor, _stringForBlendMode(colorFilterBlendMode));
+      break;
+    default:
+      break;
+  }
+  return svgFilter;
+}
+
+int _filterIdCounter = 0;
+
+// The color matrix for feColorMatrix element changes colors based on
+// the following:
+//
+// | R' |     | r1 r2 r3 r4 r5 |   | R |
+// | G' |     | g1 g2 g3 g4 g5 |   | G |
+// | B' |  =  | b1 b2 b3 b4 b5 | * | B |
+// | A' |     | a1 a2 a3 a4 a5 |   | A |
+// | 1  |     | 0  0  0  0  1  |   | 1 |
+//
+// R' = r1*R + r2*G + r3*B + r4*A + r5
+// G' = g1*R + g2*G + g3*B + g4*A + g5
+// B' = b1*R + b2*G + b3*B + b4*A + b5
+// A' = a1*R + a2*G + a3*B + a4*A + a5
+String _srcInColorFilterToSvg(ui.Color? color) {
+  _filterIdCounter += 1;
+  return '<svg width="0" height="0">'
+      '<filter id="_fcf$_filterIdCounter" '
+      'filterUnits="objectBoundingBox" x="0%" y="0%" width="100%" height="100%">'
+      '<feColorMatrix values="0 0 0 0 1 ' // Ignore input, set it to absolute.
+      '0 0 0 0 1 '
+      '0 0 0 0 1 '
+      '0 0 0 1 0" result="destalpha"/>' // Just take alpha channel of destination
+      '<feFlood flood-color="${colorToCssString(color)}" flood-opacity="1" result="flood">'
+      '</feFlood>'
+      '<feComposite in="flood" in2="destalpha" '
+      'operator="arithmetic" k1="1" k2="0" k3="0" k4="0" result="comp">'
+      '</feComposite>'
+      '</filter></svg>';
+}
+
+String _srcOutColorFilterToSvg(ui.Color? color) {
+  _filterIdCounter += 1;
+  return '<svg width="0" height="0">'
+      '<filter id="_fcf$_filterIdCounter" '
+      'filterUnits="objectBoundingBox" x="0%" y="0%" width="100%" height="100%">'
+      '<feFlood flood-color="${colorToCssString(color)}" flood-opacity="1" result="flood">'
+      '</feFlood>'
+      '<feComposite in="flood" in2="SourceGraphic" operator="out" result="comp">'
+      '</feComposite>'
+      '</filter></svg>';
+}
+
+String _xorColorFilterToSvg(ui.Color? color) {
+  _filterIdCounter += 1;
+  return '<svg width="0" height="0">'
+      '<filter id="_fcf$_filterIdCounter" '
+      'filterUnits="objectBoundingBox" x="0%" y="0%" width="100%" height="100%">'
+      '<feFlood flood-color="${colorToCssString(color)}" flood-opacity="1" result="flood">'
+      '</feFlood>'
+      '<feComposite in="flood" in2="SourceGraphic" operator="xor" result="comp">'
+      '</feComposite>'
+      '</filter></svg>';
+}
+
+// The source image and color are composited using :
+// result = k1 *in*in2 + k2*in + k3*in2 + k4.
+String _compositeColorFilterToSvg(
+    ui.Color? color, double k1, double k2, double k3, double k4) {
+  _filterIdCounter += 1;
+  return '<svg width="0" height="0">'
+      '<filter id="_fcf$_filterIdCounter" '
+      'filterUnits="objectBoundingBox" x="0%" y="0%" width="100%" height="100%">'
+      '<feFlood flood-color="${colorToCssString(color)}" flood-opacity="1" result="flood">'
+      '</feFlood>'
+      '<feComposite in="flood" in2="SourceGraphic" '
+      'operator="arithmetic" k1="$k1" k2="$k2" k3="$k3" k4="$k4" result="comp">'
+      '</feComposite>'
+      '</filter></svg>';
+}
+
+// Porter duff source * destination , keep source alpha.
+// First apply color filter to source to change it to [color], then
+// composite using multiplication.
+String _modulateColorFilterToSvg(ui.Color color) {
+  _filterIdCounter += 1;
+  final double r = color.red / 255.0;
+  final double b = color.blue / 255.0;
+  final double g = color.green / 255.0;
+  return '<svg width="0" height="0">'
+      '<filter id="_fcf$_filterIdCounter" '
+      'filterUnits="objectBoundingBox" x="0%" y="0%" width="100%" height="100%">'
+      '<feColorMatrix values="0 0 0 0 $r ' // Ignore input, set it to absolute.
+      '0 0 0 0 $g '
+      '0 0 0 0 $b '
+      '0 0 0 1 0" result="recolor"/>'
+      '<feComposite in="recolor" in2="SourceGraphic" '
+      'operator="arithmetic" k1="1" k2="0" k3="0" k4="0" result="comp">'
+      '</feComposite>'
+      '</filter></svg>';
+}
+
+// Uses feBlend element to blend source image with a color.
+String _blendColorFilterToSvg(ui.Color? color, String? feBlend,
+    {bool swapLayers = false}) {
+  _filterIdCounter += 1;
+  return '<svg width="0" height="0">'
+          '<filter id="_fcf$_filterIdCounter" filterUnits="objectBoundingBox" '
+          'x="0%" y="0%" width="100%" height="100%">'
+          '<feFlood flood-color="${colorToCssString(color)}" flood-opacity="1" result="flood">'
+          '</feFlood>' +
+      (swapLayers
+          ? '<feBlend in="SourceGraphic" in2="flood" mode="$feBlend"/>'
+          : '<feBlend in="flood" in2="SourceGraphic" mode="$feBlend"/>') +
+      '</filter></svg>';
+}

engine/src/flutter/lib/web_ui/lib/src/engine/html/scene_builder.dart

@@ -179,7 +179,7 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
     ui.ColorFilterEngineLayer? oldLayer,
   }) {
     assert(filter != null); // ignore: unnecessary_null_comparison
-    throw UnimplementedError();
+    return _pushSurface(PersistedColorFilter(oldLayer as PersistedColorFilter?, filter)) as ui.ColorFilterEngineLayer;
   }
 
   /// Pushes an image filter operation onto the operation stack.

engine/src/flutter/lib/web_ui/test/golden_tests/engine/color_filter_golden_test.dart

@@ -0,0 +1,89 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// @dart = 2.6
+import 'dart:html' as html;
+
+import 'package:test/bootstrap/browser.dart';
+import 'package:test/test.dart';
+import 'package:ui/ui.dart';
+import 'package:ui/src/engine.dart';
+
+import 'package:web_engine_tester/golden_tester.dart';
+
+final Rect region = Rect.fromLTWH(0, 0, 500, 500);
+
+void main() {
+  internalBootstrapBrowserTest(() => testMain);
+}
+
+void testMain() async {
+  setUp(() async {
+    debugShowClipLayers = true;
+    SurfaceSceneBuilder.debugForgetFrameScene();
+    for (html.Node scene in html.document.querySelectorAll('flt-scene')) {
+      scene.remove();
+    }
+
+    await webOnlyInitializePlatform();
+    webOnlyFontCollection.debugRegisterTestFonts();
+    await webOnlyFontCollection.ensureFontsLoaded();
+  });
+
+  test('Should apply color filter to image', () async {
+    final SurfaceSceneBuilder builder = SurfaceSceneBuilder();
+    final Picture backgroundPicture = _drawBackground();
+    builder.addPicture(Offset.zero, backgroundPicture);
+    builder.pushColorFilter(EngineColorFilter.mode(Color(0xF0000080),
+        BlendMode.color));
+    final Picture circles1 = _drawTestPictureWithCircles(30, 30);
+    builder.addPicture(Offset.zero, circles1);
+    builder.pop();
+    html.document.body.append(builder
+        .build()
+        .webOnlyRootElement);
+
+    await matchGoldenFile('color_filter_blendMode_color.png', region: region);
+  });
+}
+
+Picture _drawTestPictureWithCircles(double offsetX, double offsetY) {
+  final EnginePictureRecorder recorder = PictureRecorder();
+  final RecordingCanvas canvas =
+  recorder.beginRecording(const Rect.fromLTRB(0, 0, 400, 400));
+  canvas.drawCircle(
+      Offset(offsetX + 10, offsetY + 10), 10, Paint()..style = PaintingStyle.fill);
+  canvas.drawCircle(
+      Offset(offsetX + 60, offsetY + 10),
+      10,
+      Paint()
+        ..style = PaintingStyle.fill
+        ..color = const Color.fromRGBO(255, 0, 0, 1));
+  canvas.drawCircle(
+      Offset(offsetX + 10, offsetY + 60),
+      10,
+      Paint()
+        ..style = PaintingStyle.fill
+        ..color = const Color.fromRGBO(0, 255, 0, 1));
+  canvas.drawCircle(
+      Offset(offsetX + 60, offsetY + 60),
+      10,
+      Paint()
+        ..style = PaintingStyle.fill
+        ..color = const Color.fromRGBO(0, 0, 255, 1));
+  return recorder.endRecording();
+}
+
+Picture _drawBackground() {
+  final EnginePictureRecorder recorder = PictureRecorder();
+  final RecordingCanvas canvas =
+  recorder.beginRecording(const Rect.fromLTRB(0, 0, 400, 400));
+  canvas.drawRect(
+      Rect.fromLTWH(8, 8, 400.0 - 16, 400.0 - 16),
+      Paint()
+        ..style = PaintingStyle.fill
+        ..color = Color(0xFFE0FFE0)
+  );
+  return recorder.endRecording();
+}