refactor: OpacityEffect now uses opacity instead of alpha internally (#…

…2064)

Using opacity instead of integer alpha simplifies the code (and also reduces round-off errors which may accumulate for longer-running effects).

packages/flame/lib/src/effects/opacity_effect.dart

@@ -8,18 +8,14 @@ import 'package:flame/src/effects/controllers/effect_controller.dart';
 /// requires that any other effect or update logic applied to the same component
 /// also used incremental updates.
 class OpacityEffect extends ComponentEffect<HasPaint> {
-  int _alphaOffset;
-  double _roundingError = 0.0;
-  final String? paintId;
-
   /// This constructor will set the opacity in relation to it's current opacity
   /// over time.
   OpacityEffect.by(
     double offset,
     super.controller, {
     this.paintId,
     super.onComplete,
-  }) : _alphaOffset = (255 * offset).round();
+  }) : _opacityOffset = offset;
 
   /// This constructor will set the opacity to the specified opacity over time.
   factory OpacityEffect.to(
@@ -62,16 +58,18 @@ class OpacityEffect extends ComponentEffect<HasPaint> {
     );
   }
 
+  double _opacityOffset;
+  double _roundingError = 0.0;
+  final String? paintId;
+
   @override
   void apply(double progress) {
     final deltaProgress = progress - previousProgress;
-    final currentAlpha = target.getAlpha(paintId: paintId);
-    final deltaAlpha =
-        (_alphaOffset * deltaProgress) + _roundingError * deltaProgress.sign;
-    var nextAlpha = (currentAlpha + deltaAlpha).round();
-    _roundingError = (currentAlpha + deltaAlpha) - nextAlpha;
-    nextAlpha = nextAlpha.clamp(0, 255);
-    target.setAlpha(nextAlpha, paintId: paintId);
+    final currentOpacity = target.getOpacity(paintId: paintId) + _roundingError;
+    final deltaOpacity = _opacityOffset * deltaProgress;
+    final newOpacity = (currentOpacity + deltaOpacity).clamp(0, 1).toDouble();
+    target.setOpacity(newOpacity, paintId: paintId);
+    _roundingError = newOpacity - target.getOpacity(paintId: paintId);
   }
 
   @override
@@ -101,7 +99,6 @@ class _OpacityToEffect extends OpacityEffect {
 
   @override
   void onStart() {
-    _alphaOffset =
-        (_targetOpacity * 255 - target.getAlpha(paintId: paintId)).round();
+    _opacityOffset = _targetOpacity - target.getOpacity(paintId: paintId);
   }
 }

packages/flame/test/effects/opacity_effect_test.dart

@@ -22,7 +22,7 @@ void main() {
   const _epsilon = 0.004; // 1/255, since alpha only holds 8 bits
 
   group('OpacityEffect', () {
-    flameGame.test('relative', (game) async {
+    testWithFlameGame('relative', (game) async {
       final component = _PaintComponent();
       await game.ensureAdd(component);
 
@@ -44,7 +44,7 @@ void main() {
       expectDouble(component.getOpacity(), 0.6, epsilon: _epsilon);
     });
 
-    flameGame.test('absolute', (game) async {
+    testWithFlameGame('absolute', (game) async {
       final component = _PaintComponent();
       await game.ensureAdd(component);
 
@@ -66,7 +66,7 @@ void main() {
       expectDouble(component.getOpacity(), 0.4, epsilon: _epsilon);
     });
 
-    flameGame.test('reset relative', (game) async {
+    testWithFlameGame('reset relative', (game) async {
       final component = _PaintComponent();
       await game.ensureAdd(component);
 
@@ -92,7 +92,7 @@ void main() {
       }
     });
 
-    flameGame.test('reset absolute', (game) async {
+    testWithFlameGame('reset absolute', (game) async {
       final component = _PaintComponent();
       await game.ensureAdd(component);
 
@@ -107,13 +107,11 @@ void main() {
         // regardless of its initial opacity.
         effect.reset();
         game.update(1);
-        // TODO(spydon): This is not good, since it sometimes won't hit the
-        // minima.
-        expectDouble(component.getOpacity(), 0.0, epsilon: _epsilon);
+        expect(component.getOpacity(), 0.0);
       }
     });
 
-    flameGame.test('opacity composition', (game) async {
+    testWithFlameGame('opacity composition', (game) async {
       final component = _PaintComponent();
       component.setOpacity(0.0);
       await game.ensureAdd(component);
@@ -151,7 +149,7 @@ void main() {
       expect(component.children.length, 0);
     });
 
-    flameGame.test(
+    testWithFlameGame(
       'fade out',
       (game) async {
         final rng = Random();
@@ -176,7 +174,7 @@ void main() {
       },
     );
 
-    flameGame.test(
+    testWithFlameGame(
       'infinite fade out',
       (game) async {
         final component = _PaintComponent();
@@ -198,7 +196,7 @@ void main() {
       },
     );
 
-    flameGame.test(
+    testWithFlameGame(
       'on custom paint',
       (game) async {
         final component =
@@ -242,11 +240,7 @@ void main() {
         game.update(dt);
       }
       game.update(1000 - totalTime);
-      // TODO(spydon): The loop above has an average of 100fps.
-      // It should change from 0-255 in 1s so it will change alpha with an
-      // average of 255/100=2.5 per tick, which should not result in a need of
-      // an epsilon value this high.
-      expectDouble(component.getOpacity(), 1.0, epsilon: 100 * _epsilon);
+      expectDouble(component.getOpacity(), 1.0, epsilon: _epsilon);
     });
   });
 }