fix: Fix brighten and darken alpha issue

examples/lib/stories/image/brighten.dart

@@ -0,0 +1,40 @@
+import 'package:flame/components.dart';
+import 'package:flame/extensions.dart';
+import 'package:flame/game.dart';
+
+class ImageBrightnessExample extends FlameGame {
+  ImageBrightnessExample({
+    required this.brightness,
+  });
+
+  final double brightness;
+
+  static const String description = '''
+     Shows how a dart:ui `Image` can be brightened using Flame Image extensions.
+     Use the properties on the side to change the brightness of the image.
+  ''';
+
+  @override
+  Future<void> onLoad() async {
+    final image = await images.load('flame.png');
+    final brightenedImage = await image.brighten(brightness / 100);
+
+    add(
+      SpriteComponent(
+        sprite: Sprite(image),
+        position: (size / 2) - Vector2(0, image.height / 2),
+        size: image.size,
+        anchor: Anchor.center,
+      ),
+    );
+
+    add(
+      SpriteComponent(
+        sprite: Sprite(brightenedImage),
+        position: (size / 2) + Vector2(0, brightenedImage.height / 2),
+        size: image.size,
+        anchor: Anchor.center,
+      ),
+    );
+  }
+}

examples/lib/stories/image/darken.dart

@@ -0,0 +1,40 @@
+import 'package:flame/components.dart';
+import 'package:flame/extensions.dart';
+import 'package:flame/game.dart';
+
+class ImageDarknessExample extends FlameGame {
+  ImageDarknessExample({
+    required this.darkness,
+  });
+
+  final double darkness;
+
+  static const String description = '''
+     Shows how a dart:ui `Image` can be darkened using Flame Image extensions.
+     Use the properties on the side to change the darkness of the image.
+  ''';
+
+  @override
+  Future<void> onLoad() async {
+    final image = await images.load('flame.png');
+    final darkenedImage = await image.darken(darkness / 100);
+
+    add(
+      SpriteComponent(
+        sprite: Sprite(image),
+        position: (size / 2) - Vector2(0, image.height / 2),
+        size: image.size,
+        anchor: Anchor.center,
+      ),
+    );
+
+    add(
+      SpriteComponent(
+        sprite: Sprite(darkenedImage),
+        position: (size / 2) + Vector2(0, darkenedImage.height / 2),
+        size: image.size,
+        anchor: Anchor.center,
+      ),
+    );
+  }
+}

examples/lib/stories/image/image.dart

@@ -1,6 +1,8 @@
 import 'package:dashbook/dashbook.dart';
 
 import 'package:examples/commons/commons.dart';
+import 'package:examples/stories/image/brighten.dart';
+import 'package:examples/stories/image/darken.dart';
 import 'package:examples/stories/image/resize.dart';
 import 'package:flame/game.dart';
 
@@ -19,5 +21,25 @@ void addImageStories(Dashbook dashbook) {
       ),
       codeLink: baseLink('image/resize.dart'),
       info: ImageResizeExample.description,
+    )
+    ..add(
+      'brightness',
+      (context) => GameWidget(
+        game: ImageBrightnessExample(
+          brightness: context.numberProperty('brightness', 50),
+        ),
+      ),
+      codeLink: baseLink('image/brighten.dart'),
+      info: ImageBrightnessExample.description,
+    )
+    ..add(
+      'darkness',
+      (context) => GameWidget(
+        game: ImageDarknessExample(
+          darkness: context.numberProperty('darkness', 50),
+        ),
+      ),
+      codeLink: baseLink('image/darkness.dart'),
+      info: ImageDarknessExample.description,
     );
 }

packages/flame/lib/src/extensions/color.dart

@@ -1,6 +1,8 @@
 import 'dart:math';
 import 'dart:ui';
 
+import 'package:flutter/painting.dart' show HSLColor;
+
 export 'dart:ui' show Color;
 
 extension ColorExtension on Color {
@@ -12,13 +14,12 @@ extension ColorExtension on Color {
   Color darken(double amount) {
     assert(amount >= 0 && amount <= 1);
 
-    final f = 1 - amount;
-    return Color.fromARGB(
-      a ~/ 255,
-      (r * f).round(),
-      (g * f).round(),
-      (b * f).round(),
-    );
+    final hsl = HSLColor.fromColor(this);
+    return hsl
+        .withLightness(
+          clampDouble(hsl.lightness * amount, 0.0, 1.0),
+        )
+        .toColor();
   }
 
   /// Brighten the shade of the color by the [amount].
@@ -29,12 +30,12 @@ extension ColorExtension on Color {
   Color brighten(double amount) {
     assert(amount >= 0 && amount <= 1);
 
-    return Color.fromARGB(
-      a ~/ 255,
-      (r + ((1.0 - r) * amount)) ~/ 255,
-      (g + ((1.0 - g) * amount)) ~/ 255,
-      (b + ((1.0 - b) * amount)) ~/ 255,
-    );
+    final hsl = HSLColor.fromColor(this);
+    return hsl
+        .withLightness(
+          clampDouble(hsl.lightness + (1 - hsl.lightness) * amount, 0.0, 1.0),
+        )
+        .toColor();
   }
 
   // used as an example hex color code on the documentation below

packages/flame/lib/src/extensions/image.dart

@@ -37,6 +37,45 @@ extension ImageExtension on Image {
     return (await toByteData())!.buffer.asUint8List();
   }
 
+  Future<Image> transformPixels(
+    Color Function(Color) transform, {
+    bool reversePremultipliedAlpha = true,
+  }) async {
+    final pixelData = await pixelsInUint8();
+    final newPixelData = Uint8List(pixelData.length);
+
+    for (var i = 0; i < pixelData.length; i += 4) {
+      final r = pixelData[i + 0] / 255;
+      final g = pixelData[i + 1] / 255;
+      final b = pixelData[i + 2] / 255;
+      final a = pixelData[i + 3] / 255;
+
+      final d = a == 0 || !reversePremultipliedAlpha ? 1 : a;
+
+      // Reverse the pre-multiplied alpha.
+      final color = Color.from(
+        alpha: a,
+        red: r / d,
+        green: g / d,
+        blue: b / d,
+      );
+
+      final newColor = a == 0 ? color : transform(color);
+
+      final newR = newColor.r;
+      final newG = newColor.g;
+      final newB = newColor.b;
+
+      // Pre-multiply the alpha back into the new color.
+      newPixelData[i + 0] = (newR * d * 255).round();
+      newPixelData[i + 1] = (newG * d * 255).round();
+      newPixelData[i + 2] = (newB * d * 255).round();
+      newPixelData[i + 3] = pixelData[i + 3];
+    }
+
+    return fromPixels(newPixelData, width, height);
+  }
+
   /// Returns the bounding [Rect] of the image.
   Rect getBoundingRect() => Vector2.zero() & size;
 
@@ -46,51 +85,31 @@ extension ImageExtension on Image {
   /// Change each pixel's color to be darker and return a new [Image].
   ///
   /// The [amount] is a double value between 0 and 1.
-  Future<Image> darken(double amount) async {
+  Future<Image> darken(
+    double amount, {
+    bool reversePremultipliedAlpha = true,
+  }) async {
     assert(amount >= 0 && amount <= 1);
 
-    final pixelData = await pixelsInUint8();
-    final newPixelData = Uint8List(pixelData.length);
-
-    for (var i = 0; i < pixelData.length; i += 4) {
-      final color = Color.fromARGB(
-        pixelData[i + 3],
-        pixelData[i + 0],
-        pixelData[i + 1],
-        pixelData[i + 2],
-      ).darken(amount);
-
-      newPixelData[i] = color.r ~/ 255;
-      newPixelData[i + 1] = color.g ~/ 255;
-      newPixelData[i + 2] = color.b ~/ 255;
-      newPixelData[i + 3] = color.a ~/ 255;
-    }
-    return fromPixels(newPixelData, width, height);
+    return transformPixels(
+      (color) => color.darken(amount),
+      reversePremultipliedAlpha: reversePremultipliedAlpha,
+    );
   }
 
   /// Change each pixel's color to be brighter and return a new [Image].
   ///
   /// The [amount] is a double value between 0 and 1.
-  Future<Image> brighten(double amount) async {
+  Future<Image> brighten(
+    double amount, {
+    bool reversePremultipliedAlpha = false,
+  }) async {
     assert(amount >= 0 && amount <= 1);
 
-    final pixelData = await pixelsInUint8();
-    final newPixelData = Uint8List(pixelData.length);
-
-    for (var i = 0; i < pixelData.length; i += 4) {
-      final color = Color.fromARGB(
-        pixelData[i + 3],
-        pixelData[i + 0],
-        pixelData[i + 1],
-        pixelData[i + 2],
-      ).brighten(amount);
-
-      newPixelData[i] = color.r ~/ 255;
-      newPixelData[i + 1] = color.g ~/ 255;
-      newPixelData[i + 2] = color.b ~/ 255;
-      newPixelData[i + 3] = color.a ~/ 255;
-    }
-    return fromPixels(newPixelData, width, height);
+    return transformPixels(
+      (color) => color.brighten(amount),
+      reversePremultipliedAlpha: reversePremultipliedAlpha,
+    );
   }
 
   /// Resizes this image to the given [newSize].

packages/flame/test/extensions/image_extension_test.dart

@@ -54,53 +54,66 @@ void main() {
     });
 
     test('darken colors each pixel darker', () async {
-      const originalColor = Color.fromARGB(193, 135, 73, 73);
+      const transparentColor = Color.fromARGB(0, 255, 0, 255);
+      const originalColor = Color.fromARGB(255, 135, 73, 73);
       final pixels = Uint8List.fromList(
         List<int>.generate(
           100 * 4,
-          (index) => _colorBit(index, originalColor),
+          (index) => _colorBit(
+            index,
+            index < 200 ? transparentColor : originalColor,
+          ),
         ),
       );
       final image = await ImageExtension.fromPixels(pixels, 10, 10);
 
       const darkenAmount = 0.5;
-      final originalDarkenImage = await image.darken(darkenAmount);
-      final originalDarkenPixelsList =
-          await originalDarkenImage.pixelsInUint8();
+      final actualDarkenedImage = await image.darken(darkenAmount);
+      final actualDarkenedPixels = await actualDarkenedImage.pixelsInUint8();
 
-      final darkenColor = originalColor.darken(darkenAmount);
+      final darkenedColor = originalColor.darken(darkenAmount);
       final expectedDarkenPixels = Uint8List.fromList(
         List<int>.generate(
           100 * 4,
-          (index) => _colorBit(index, darkenColor),
+          (index) => _colorBit(
+            index,
+            index < 200 ? transparentColor : darkenedColor,
+          ),
         ),
       );
-      expect(originalDarkenPixelsList, expectedDarkenPixels);
+      expect(actualDarkenedPixels, expectedDarkenPixels);
     });
 
     test('brighten colors each pixel brighter', () async {
-      const originalColor = Color.fromARGB(193, 135, 73, 73);
+      const transparentColor = Color.fromARGB(0, 255, 0, 255);
+      const originalColor = Color.fromARGB(255, 255, 0, 0);
+
       final pixels = Uint8List.fromList(
         List<int>.generate(
           100 * 4,
-          (index) => _colorBit(index, originalColor),
+          (index) => _colorBit(
+            index,
+            index < 200 ? transparentColor : originalColor,
+          ),
         ),
       );
       final image = await ImageExtension.fromPixels(pixels, 10, 10);
 
       const brightenAmount = 0.5;
-      final originalBrightenImage = await image.brighten(brightenAmount);
-      final originalBrightenPixelsList =
-          await originalBrightenImage.pixelsInUint8();
+      final brightenedImage = await image.brighten(brightenAmount);
+      final actualBrightenedPixels = await brightenedImage.pixelsInUint8();
 
       final brightenColor = originalColor.brighten(brightenAmount);
       final expectedBrightenPixels = Uint8List.fromList(
         List<int>.generate(
           100 * 4,
-          (index) => _colorBit(index, brightenColor),
+          (index) => _colorBit(
+            index,
+            index < 200 ? transparentColor : brightenColor,
+          ),
         ),
       );
-      expect(originalBrightenPixelsList, expectedBrightenPixels);
+      expect(actualBrightenedPixels, expectedBrightenPixels);
     });
 
     test('resize resizes the image', () async {
@@ -121,10 +134,10 @@ void main() {
 
 int _colorBit(int index, Color color) {
   return switch (index % 4) {
-    0 => color.r ~/ 255,
-    1 => color.g ~/ 255,
-    2 => color.b ~/ 255,
-    3 => color.a ~/ 255,
+    0 => (color.r * 255).round(),
+    1 => (color.g * 255).round(),
+    2 => (color.b * 255).round(),
+    3 => (color.a * 255).round(),
     _ => throw UnimplementedError(),
   };
 }

packages/flame/test/extensions/paint_test.dart

@@ -32,7 +32,7 @@ void main() {
 
       paint.brighten(brightenAmount);
 
-      expect(
+      expectColor(
         paint.color,
         brightenBaseColor,
         reason: "Paint's color does not match brighten color",