Make ScaleEffect.by multiplicative (#1174)

* Make ScaleEffect.by multiplicative

* formatting

Co-authored-by: Lukas Klingsbo <lukas.klingsbo@gmail.com>

packages/flame/CHANGELOG.md

@@ -11,6 +11,7 @@
  - Fix render order of components and add tests
  - `isHud` renamed to `respectCamera`
  - Fix `HitboxCircle` when component is flipped
+ - `ScaleEffect.by` now applies multiplicatively instead of additively
 
 ## [1.0.0-releasecandidate.18]
  - Forcing portrait and landscape mode is now supported on web

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

@@ -7,27 +7,36 @@ import 'transform2d_effect.dart';
 ///
 /// The following constructors are provided:
 ///
-///   - [ScaleEffect.by] will scale the target in relation to it's current
-///     scale;
-///   - [ScaleEffect.to] will scale the target to the specified scale
+///   - [ScaleEffect.by] will scale the target by the given factor, relative to
+///     its current scale;
+///   - [ScaleEffect.to] will scale the target to the specified scale.
 ///
 /// This effect applies incremental changes to the component's scale, and
 /// requires that any other effect or update logic applied to the same component
 /// also used incremental updates.
 class ScaleEffect extends Transform2DEffect {
-  ScaleEffect.by(Vector2 offset, EffectController controller)
-      : _offset = offset.clone(),
+  ScaleEffect.by(Vector2 scaleFactor, EffectController controller)
+      : _scaleFactor = scaleFactor.clone(),
         super(controller);
 
   factory ScaleEffect.to(Vector2 targetScale, EffectController controller) =>
       _ScaleToEffect(targetScale, controller);
 
-  Vector2 _offset;
+  final Vector2 _scaleFactor;
+  late Vector2 _scaleDelta;
+
+  @override
+  void onStart() {
+    _scaleDelta = Vector2(
+      target.scale.x * (_scaleFactor.x - 1),
+      target.scale.y * (_scaleFactor.y - 1),
+    );
+  }
 
   @override
   void apply(double progress) {
     final dProgress = progress - previousProgress;
-    target.scale += _offset * dProgress;
+    target.scale += _scaleDelta * dProgress;
     super.apply(progress);
   }
 }
@@ -42,6 +51,6 @@ class _ScaleToEffect extends ScaleEffect {
 
   @override
   void onStart() {
-    _offset = _targetScale - target.scale;
+    _scaleDelta = _targetScale - target.scale;
   }
 }

packages/flame/test/effects/scale_effect_test.dart

@@ -13,9 +13,8 @@ void main() {
       final component = PositionComponent();
       game.ensureAdd(component);
 
-      component.scale = Vector2.all(1.0);
       component.add(
-        ScaleEffect.by(Vector2.all(1.0), EffectController(duration: 1)),
+        ScaleEffect.by(Vector2.all(2.0), EffectController(duration: 1)),
       );
       game.update(0);
       expectVector2(component.scale, Vector2.all(1.0));
@@ -58,18 +57,18 @@ void main() {
       game.ensureAdd(component);
 
       final effect = ScaleEffect.by(
-        Vector2.all(1.0),
+        Vector2.all(2.0),
         EffectController(duration: 1),
       );
       component.add(effect..removeOnFinish = false);
       final expectedScale = Vector2.all(1.0);
       for (var i = 0; i < 5; i++) {
         expectVector2(component.scale, expectedScale);
-        // After each reset the object will be scaled up by Vector2(1, 1)
+        // After each reset the object will be scaled up twice
         // relative to its scale at the start of the effect.
         effect.reset();
         game.update(1);
-        expectedScale.add(Vector2.all(1.0));
+        expectedScale.multiply(Vector2.all(2));
         expectVector2(component.scale, expectedScale);
       }
     });
@@ -94,7 +93,7 @@ void main() {
     });
 
     flameGame.test('scale composition', (game) {
-      final component = PositionComponent();
+      final component = PositionComponent()..flipVertically();
       game.ensureAdd(component);
 
       component.add(
@@ -112,21 +111,17 @@ void main() {
       );
 
       game.update(1);
-      expectVector2(
-        component.scale,
-        Vector2.all(2),
-        epsilon: 1e-15,
-      ); // 5*1/10 + 0.5*1
+      expect(component.scale.x, closeTo(0.7, 1e-15)); // (1 + 0.4) * 0.5
+      expect(component.scale.y, closeTo(-0.7, 1e-15));
       game.update(1);
-      expectVector2(
-        component.scale,
-        Vector2.all(2),
-        epsilon: 1e-15,
-      ); // 5*2/10 + 0.5*1 - 0.5*1
-      for (var i = 0; i < 10; i++) {
+      expect(component.scale.x, closeTo(1.8, 1e-15)); // (1 + 2*0.4) * 1
+      expect(component.scale.y, closeTo(-1.8, 1e-15));
+      for (var i = 0; i < 8; i++) {
         game.update(1);
       }
-      expectVector2(component.scale, Vector2.all(6), epsilon: 1e-15);
+      expect(component.scale.x, closeTo(5, 1e-15));
+      expect(component.scale.y, closeTo(-5, 1e-15));
+      game.update(0);
       expect(component.children.length, 0);
     });