Fix html gradient rendering (flutter#97762) (flutter#31355)

lib/web_ui/lib/src/engine/html/render_vertices.dart

@@ -317,7 +317,7 @@ class _WebGlRenderer implements GlRenderer {
       NormalizedGradient gradient, int widthInPixels, int heightInPixels) {
     drawRectToGl(
         targetRect, gl, glProgram, gradient, widthInPixels, heightInPixels);
-    final Object? image = gl.readPatternData();
+    final Object? image = gl.readPatternData(gradient.isOpaque);
     gl.bindArrayBuffer(null);
     gl.bindElementArrayBuffer(null);
     return image;

lib/web_ui/lib/src/engine/html/shaders/image_shader.dart

@@ -260,7 +260,7 @@ class EngineImageShader implements ui.ImageShader {
       gl.unbindVertexArray();
     }
 
-    final Object? bitmapImage = gl.readPatternData();
+    final Object? bitmapImage = gl.readPatternData(false);
     gl.bindArrayBuffer(null);
     gl.bindElementArrayBuffer(null);
     return context!.createPattern(bitmapImage!, 'no-repeat')!;

lib/web_ui/lib/src/engine/html/shaders/normalized_gradient.dart

@@ -26,6 +26,7 @@ class NormalizedGradient {
   final Float32List _bias;
   final Float32List _scale;
   final int thresholdCount;
+  final bool isOpaque;
 
   factory NormalizedGradient(List<ui.Color> colors, {List<double>? stops}) {
     // If colorStops is not provided, then only two stops, at 0.0 and 1.0,
@@ -34,6 +35,7 @@ class NormalizedGradient {
     stops ??= const <double>[0.0, 1.0];
     final int colorCount = colors.length;
     int normalizedCount = colorCount;
+    final bool isOpaque = !colors.any((ui.Color c) => c.alpha < 1.0);
     final bool addFirst = stops[0] != 0.0;
     final bool addLast = stops.last != 1.0;
     if (addFirst) {
@@ -94,11 +96,11 @@ class NormalizedGradient {
       bias[colorIndex + 2] -= t * scale[colorIndex + 2];
       bias[colorIndex + 3] -= t * scale[colorIndex + 3];
     }
-    return NormalizedGradient._(normalizedCount, thresholds, scale, bias);
+    return NormalizedGradient._(normalizedCount, thresholds, scale, bias, isOpaque);
   }
 
   NormalizedGradient._(
-      this.thresholdCount, this._thresholds, this._scale, this._bias);
+      this.thresholdCount, this._thresholds, this._scale, this._bias, this.isOpaque);
 
   /// Sets uniforms for threshold, bias and scale for program.
   void setupUniforms(GlContext gl, GlProgram glProgram) {

lib/web_ui/lib/src/engine/html/shaders/shader.dart

@@ -79,11 +79,9 @@ class GradientSweep extends EngineGradient {
     final Object angleRange = gl.getUniformLocation(glProgram.program, 'angle_range');
     gl.setUniform2f(angleRange, startAngle, endAngle);
     normalizedGradient.setupUniforms(gl, glProgram);
-    if (matrix4 != null) {
-      final Object gradientMatrix =
+    final Object gradientMatrix =
           gl.getUniformLocation(glProgram.program, 'm_gradient');
-      gl.setUniformMatrix4fv(gradientMatrix, false, matrix4!);
-    }
+    gl.setUniformMatrix4fv(gradientMatrix, false, matrix4 ?? Matrix4.identity().storage);
     if (createDataUrl) {
       return glRenderer!.drawRectToImageUrl(
           ui.Rect.fromLTWH(0, 0, shaderBounds.width, shaderBounds.height),
@@ -293,9 +291,7 @@ class GradientLinear extends EngineGradient {
     // We compute location based on gl_FragCoord to center distance which
     // returns 0.0 at center. To make sure we align center of gradient to this
     // point, we shift by 0.5 to get st value for center of gradient.
-    if (tileMode != ui.TileMode.repeated) {
-      gradientTransform.translate(0.5, 0);
-    }
+    gradientTransform.translate(0.5, 0);
     if (length > kFltEpsilon) {
       gradientTransform.scale(1.0 / length);
     }

lib/web_ui/lib/src/engine/safe_browser_api.dart

@@ -814,12 +814,14 @@ class GlContext {
 
   /// Returns image data in a form that can be used to create Canvas
   /// context patterns.
-  Object? readPatternData() {
+  Object? readPatternData(bool isOpaque) {
     // When using OffscreenCanvas and transferToImageBitmap is supported by
     // browser create ImageBitmap otherwise use more expensive canvas
-    // allocation.
+    // allocation. However, transferToImageBitmap does not properly preserve
+    // the alpha channel, so only use it if the pattern is opaque.
     if (_canvas != null &&
-        js_util.hasProperty(_canvas!, 'transferToImageBitmap')) {
+        js_util.hasProperty(_canvas!, 'transferToImageBitmap') &&
+        isOpaque) {
       // TODO(yjbanov): find out why we need to call getContext and ignore the return value.
       js_util.callMethod<void>(_canvas!, 'getContext', <dynamic>['webgl2']);
       final Object? imageBitmap = js_util.callMethod(_canvas!, 'transferToImageBitmap',

lib/web_ui/test/html/shaders/gradient_golden_test.dart

@@ -80,7 +80,7 @@ Future<void> testMain() async {
     GradientSweep sweepGradient = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.clamp,
         0, 360.0 / 180.0 * math.pi,
-        Matrix4.rotationZ(math.pi / 6.0).storage);
+        null);
 
     final GradientSweep sweepGradientRotated = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.clamp,
@@ -105,7 +105,7 @@ Future<void> testMain() async {
     sweepGradient = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.clamp,
         math.pi / 6, 3 * math.pi / 4,
-        Matrix4.rotationZ(math.pi / 6.0).storage);
+        null);
 
     rectBounds = rectBounds.translate(kBoxWidth + 10, 0);
     canvas.drawRect(rectBounds,
@@ -117,7 +117,7 @@ Future<void> testMain() async {
     sweepGradient = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.repeated,
         math.pi / 6, 3 * math.pi / 4,
-        Matrix4.rotationZ(math.pi / 6.0).storage);
+        null);
 
     canvas.drawRect(rectBounds,
         SurfacePaint()..shader = engineGradientToShader(sweepGradient, rectBounds));
@@ -128,7 +128,7 @@ Future<void> testMain() async {
     sweepGradient = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.mirror,
         math.pi / 6, 3 * math.pi / 4,
-        Matrix4.rotationZ(math.pi / 6.0).storage);
+        null);
     canvas.drawRect(rectBounds,
         SurfacePaint()..shader = engineGradientToShader(sweepGradient, rectBounds));
     canvas.drawRect(rectBounds, borderPaint);
@@ -159,7 +159,7 @@ Future<void> testMain() async {
     GradientSweep sweepGradient = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.clamp,
         0, 360.0 / 180.0 * math.pi,
-        Matrix4.rotationZ(math.pi / 6.0).storage);
+        null);
 
     final GradientSweep sweepGradientRotated = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.clamp,
@@ -184,7 +184,7 @@ Future<void> testMain() async {
     sweepGradient = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.clamp,
         math.pi / 6, 3 * math.pi / 4,
-        Matrix4.rotationZ(math.pi / 6.0).storage);
+        null);
 
     rectBounds = rectBounds.translate(kBoxWidth + 10, 0);
     canvas.drawOval(rectBounds,
@@ -196,7 +196,7 @@ Future<void> testMain() async {
     sweepGradient = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.repeated,
         math.pi / 6, 3 * math.pi / 4,
-        Matrix4.rotationZ(math.pi / 6.0).storage);
+        null);
 
     canvas.drawOval(rectBounds,
         SurfacePaint()..shader = engineGradientToShader(sweepGradient, rectBounds));
@@ -207,7 +207,7 @@ Future<void> testMain() async {
     sweepGradient = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.mirror,
         math.pi / 6, 3 * math.pi / 4,
-        Matrix4.rotationZ(math.pi / 6.0).storage);
+        null);
     canvas.drawOval(rectBounds,
         SurfacePaint()..shader = engineGradientToShader(sweepGradient, rectBounds));
     canvas.drawRect(rectBounds, borderPaint);
@@ -238,7 +238,7 @@ Future<void> testMain() async {
     GradientSweep sweepGradient = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.clamp,
         0, 360.0 / 180.0 * math.pi,
-        Matrix4.rotationZ(math.pi / 6.0).storage);
+        null);
 
     final GradientSweep sweepGradientRotated = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.clamp,
@@ -265,7 +265,7 @@ Future<void> testMain() async {
     sweepGradient = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.clamp,
         math.pi / 6, 3 * math.pi / 4,
-        Matrix4.rotationZ(math.pi / 6.0).storage);
+        null);
 
     rectBounds = rectBounds.translate(kBoxWidth + 10, 0);
     path = samplePathFromRect(rectBounds);
@@ -278,7 +278,7 @@ Future<void> testMain() async {
     sweepGradient = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.repeated,
         math.pi / 6, 3 * math.pi / 4,
-        Matrix4.rotationZ(math.pi / 6.0).storage);
+        null);
 
     path = samplePathFromRect(rectBounds);
     canvas.drawPath(path,
@@ -290,7 +290,7 @@ Future<void> testMain() async {
     sweepGradient = GradientSweep(const Offset(0.5, 0.5),
         colors, stops, TileMode.mirror,
         math.pi / 6, 3 * math.pi / 4,
-        Matrix4.rotationZ(math.pi / 6.0).storage);
+        null);
     path = samplePathFromRect(rectBounds);
     canvas.drawPath(path,
         SurfacePaint()..shader = engineGradientToShader(sweepGradient, rectBounds));

lib/web_ui/test/html/shaders/linear_gradient_golden_test.dart

@@ -45,6 +45,27 @@ Future<void> testMain() async {
         maxDiffRatePercent: 0.01);
   });
 
+  test('Should blend linear gradient with alpha channel correctly.', () async {
+    const Rect canvasRect = Rect.fromLTRB(0, 0, 500, 500);
+    final RecordingCanvas rc =
+        RecordingCanvas(canvasRect);
+    final SurfacePaint backgroundPaint = SurfacePaint()
+      ..style = PaintingStyle.fill
+      ..color = const Color(0xFFFF0000);
+    rc.drawRect(canvasRect, backgroundPaint);
+
+    const Rect shaderRect = Rect.fromLTRB(50, 50, 300, 300);
+    final SurfacePaint paint = SurfacePaint()..shader = Gradient.linear(
+        Offset(shaderRect.left, shaderRect.top),
+        Offset(shaderRect.right, shaderRect.bottom),
+        const <Color>[Color(0x00000000), Color(0xFF0000FF)]);
+    rc.drawRect(shaderRect, paint);
+    expect(rc.renderStrategy.hasArbitraryPaint, isTrue);
+    await canvasScreenshot(rc, 'linear_gradient_rect_alpha',
+        region: screenRect,
+        maxDiffRatePercent: 0.01);
+  });
+
   test('Should draw linear gradient with transform.', () async {
     final RecordingCanvas rc =
         RecordingCanvas(const Rect.fromLTRB(0, 0, 500, 500));