Add ability to control dithering on Paint (flutter/engine#13868)

engine/src/flutter/lib/ui/painting.dart

@@ -1066,6 +1066,7 @@ class Paint {
   static const int _kMaskFilterBlurStyleIndex = 10;
   static const int _kMaskFilterSigmaIndex = 11;
   static const int _kInvertColorIndex = 12;
+  static const int _kDitherIndex = 13;
 
   static const int _kIsAntiAliasOffset = _kIsAntiAliasIndex << 2;
   static const int _kColorOffset = _kColorIndex << 2;
@@ -1080,8 +1081,9 @@ class Paint {
   static const int _kMaskFilterBlurStyleOffset = _kMaskFilterBlurStyleIndex << 2;
   static const int _kMaskFilterSigmaOffset = _kMaskFilterSigmaIndex << 2;
   static const int _kInvertColorOffset = _kInvertColorIndex << 2;
+  static const int _kDitherOffset = _kDitherIndex << 2;
   // If you add more fields, remember to update _kDataByteCount.
-  static const int _kDataByteCount = 52;
+  static const int _kDataByteCount = 56;
 
   // Binary format must match the deserialization code in paint.cc.
   List<dynamic> _objects;
@@ -1090,6 +1092,14 @@ class Paint {
   static const int _kImageFilterIndex = 2;
   static const int _kObjectCount = 3; // Must be one larger than the largest index.
 
+  /// Constructs an empty [Paint] object with all fields initialized to
+  /// their defaults.
+  Paint() {
+    if (enableDithering) {
+      _dither = true;
+    }
+  }
+
   /// Whether to apply anti-aliasing to lines and images drawn on the
   /// canvas.
   ///
@@ -1417,6 +1427,30 @@ class Paint {
     _data.setInt32(_kInvertColorOffset, value ? 1 : 0, _kFakeHostEndian);
   }
 
+  bool get _dither {
+    return _data.getInt32(_kDitherOffset, _kFakeHostEndian) == 1;
+  }
+  set _dither(bool value) {
+    _data.setInt32(_kDitherOffset, value ? 1 : 0, _kFakeHostEndian);
+  }
+
+  /// Whether to dither the output when drawing images.
+  ///
+  /// If false, the default value, dithering will be enabled when the input
+  /// color depth is higher than the output color depth. For example,
+  /// drawing an RGB8 image onto an RGB565 canvas.
+  ///
+  /// This value also controls dithering of [shader]s, which can make
+  /// gradients appear smoother.
+  ///
+  /// Whether or not dithering affects the output is implementation defined.
+  /// Some implementations may choose to ignore this completely, if they're
+  /// unable to control dithering.
+  ///
+  /// To ensure that dithering is consistently enabled for your entire
+  /// application, set this to true before invoking any drawing related code.
+  static bool enableDithering = false;
+
   @override
   String toString() {
     final StringBuffer result = StringBuffer();
@@ -1475,6 +1509,8 @@ class Paint {
     }
     if (invertColors)
       result.write('${semicolon}invert: $invertColors');
+    if (_dither)
+      result.write('${semicolon}dither: $_dither');
     result.write(')');
     return result.toString();
   }

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

@@ -32,7 +32,8 @@ constexpr int kMaskFilterIndex = 9;
 constexpr int kMaskFilterBlurStyleIndex = 10;
 constexpr int kMaskFilterSigmaIndex = 11;
 constexpr int kInvertColorIndex = 12;
-constexpr size_t kDataByteCount = 52;  // 4 * (last index + 1)
+constexpr int kDitherIndex = 13;
+constexpr size_t kDataByteCount = 56;  // 4 * (last index + 1)
 
 // Indices for objects.
 constexpr int kShaderIndex = 0;
@@ -154,6 +155,10 @@ Paint::Paint(Dart_Handle paint_objects, Dart_Handle paint_data) {
     paint_.setColorFilter(invert_filter);
   }
 
+  if (uint_data[kDitherIndex]) {
+    paint_.setDither(true);
+  }
+
   switch (uint_data[kMaskFilterIndex]) {
     case Null:
       break;

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

@@ -961,6 +961,10 @@ class PaintData {
 class Paint {
   PaintData _paintData = PaintData();
 
+  /// Constructs an empty [Paint] object with all fields initialized to
+  /// their defaults.
+  Paint();
+
   /// A blend mode to apply when a shape is drawn or a layer is composited.
   ///
   /// The source colors are from the shape being drawn (e.g. from
@@ -1182,6 +1186,23 @@ class Paint {
     // TODO(flutter/flutter#35156): Implement ImageFilter.
   }
 
+  /// Whether to dither the output when drawing images.
+  ///
+  /// If false, the default value, dithering will be enabled when the input
+  /// color depth is higher than the output color depth. For example,
+  /// drawing an RGB8 image onto an RGB565 canvas.
+  ///
+  /// This value also controls dithering of [shader]s, which can make
+  /// gradients appear smoother.
+  ///
+  /// Whether or not dithering affects the output is implementation defined.
+  /// Some implementations may choose to ignore this completely, if they're
+  /// unable to control dithering.
+  ///
+  /// To ensure that dithering is consistently enabled for your entire
+  /// application, set this to true before invoking any drawing related code.
+  static bool enableDithering = false;
+
   // True if Paint instance has used in RecordingCanvas.
   bool _frozen = false;
 

engine/src/flutter/testing/dart/canvas_test.dart

@@ -109,14 +109,20 @@ Future<bool> fuzzyGoldenImageCompare(
     Image image, String goldenImageName) async {
   final String imagesPath = path.join('flutter', 'testing', 'resources');
   final File file = File(path.join(imagesPath, goldenImageName));
-  final Uint8List goldenData = await file.readAsBytes();
 
-  final Codec codec = await instantiateImageCodec(goldenData);
-  final FrameInfo frame = await codec.getNextFrame();
-  expect(frame.image.height, equals(image.width));
-  expect(frame.image.width, equals(image.height));
+  bool areEqual = false;
+
+  if (file.existsSync()) {
+    final Uint8List goldenData = await file.readAsBytes();
+
+    final Codec codec = await instantiateImageCodec(goldenData);
+    final FrameInfo frame = await codec.getNextFrame();
+    expect(frame.image.height, equals(image.width));
+    expect(frame.image.width, equals(image.height));
+
+    areEqual = await fuzzyCompareImages(frame.image, image);
+  }
 
-  final bool areEqual = await fuzzyCompareImages(frame.image, image);
   if (!areEqual) {
     final ByteData pngData = await image.toByteData();
     final ByteBuffer buffer = pngData.buffer;
@@ -151,4 +157,44 @@ void main() {
         await fuzzyGoldenImageCompare(image, 'canvas_test_toImage.png');
     expect(areEqual, true);
   });
+
+  Gradient makeGradient() {
+    return Gradient.linear(
+      Offset.zero,
+      const Offset(100, 100),
+      const <Color>[Color(0xFF4C4D52), Color(0xFF202124)],
+    );
+  }
+
+  test('Simple gradient', () async {
+    Paint.enableDithering = false;
+    final PictureRecorder recorder = PictureRecorder();
+    final Canvas canvas = Canvas(recorder);
+    final Paint paint = Paint()..shader = makeGradient();
+    canvas.drawPaint(paint);
+    final Picture picture = recorder.endRecording();
+    final Image image = await picture.toImage(100, 100);
+    expect(image.width, equals(100));
+    expect(image.height, equals(100));
+
+    final bool areEqual =
+        await fuzzyGoldenImageCompare(image, 'canvas_test_gradient.png');
+    expect(areEqual, true);
+  });
+
+  test('Simple dithered gradient', () async {
+    Paint.enableDithering = true;
+    final PictureRecorder recorder = PictureRecorder();
+    final Canvas canvas = Canvas(recorder);
+    final Paint paint = Paint()..shader = makeGradient();
+    canvas.drawPaint(paint);
+    final Picture picture = recorder.endRecording();
+    final Image image = await picture.toImage(100, 100);
+    expect(image.width, equals(100));
+    expect(image.height, equals(100));
+
+    final bool areEqual =
+        await fuzzyGoldenImageCompare(image, 'canvas_test_dithered_gradient.png');
+    expect(areEqual, true);
+  });
 }