Improve drop shadow effect accuracy (#2523)

These changes make several improvements to how drop shadow effects are displayed:
- Drop shadows now take parent alpha into account. This means that if a layer or fill has an animated opacity, the drop shadow will multiply that opacity with its own.
- Adds drop shadow support to Image layers. There are some visual bugs with how the shadows are rendered, however.
- Applies a constant scale factor to the distance and softness values from the Lottie file before passing them to `Paint.setShadowLayer()` to more closely match how they are displayed in After Effects. See airbnb/lottie-ios#2175 for similar changes on iOS.
- Adds three snapshot test files for distance, softness, and alpha validation.

Distance test file:
<img src="https://github.com/user-attachments/assets/aa559e60-0d8f-403b-acd0-fa571d6dcff5" width=400>

Softness test file:
<img src="https://github.com/user-attachments/assets/faa9819f-6584-49f5-91b7-7462213044f5" width=400>

Example of Image layer shadow bug, right side is how it should look (capture from After Effects):
<img width="1824" alt="image" src="https://github.com/user-attachments/assets/a3fb85c3-a5d1-4a6b-bbda-ff4e5ebd2fb5">

Co-authored-by: Gabriel Peal <gabriel@gpeal.com>

Author: allenchen1154

lottie/src/main/java/com/airbnb/lottie/animation/content/BaseStrokeContent.java

@@ -191,7 +191,7 @@ public abstract class BaseStrokeContent
       blurMaskFilterRadius = blurRadius;
     }
     if (dropShadowAnimation != null) {
-      dropShadowAnimation.applyTo(paint);
+      dropShadowAnimation.applyTo(paint, parentMatrix, Utils.mixOpacities(parentAlpha, alpha));
     }
 
     for (int i = 0; i < pathGroups.size(); i++) {

lottie/src/main/java/com/airbnb/lottie/animation/content/FillContent.java

@@ -24,6 +24,7 @@
 import com.airbnb.lottie.model.content.ShapeFill;
 import com.airbnb.lottie.model.layer.BaseLayer;
 import com.airbnb.lottie.utils.MiscUtils;
+import com.airbnb.lottie.utils.Utils;
 import com.airbnb.lottie.value.LottieValueCallback;
 
 import java.util.ArrayList;
@@ -120,7 +121,7 @@ public FillContent(final LottieDrawable lottieDrawable, BaseLayer layer, ShapeFi
       blurMaskFilterRadius = blurRadius;
     }
     if (dropShadowAnimation != null) {
-      dropShadowAnimation.applyTo(paint);
+      dropShadowAnimation.applyTo(paint, parentMatrix, Utils.mixOpacities(parentAlpha, alpha));
     }
 
     path.reset();

lottie/src/main/java/com/airbnb/lottie/animation/content/GradientFillContent.java

@@ -32,6 +32,7 @@
 import com.airbnb.lottie.model.content.GradientType;
 import com.airbnb.lottie.model.layer.BaseLayer;
 import com.airbnb.lottie.utils.MiscUtils;
+import com.airbnb.lottie.utils.Utils;
 import com.airbnb.lottie.value.LottieValueCallback;
 
 import java.util.ArrayList;
@@ -150,13 +151,14 @@ public GradientFillContent(final LottieDrawable lottieDrawable, LottieCompositio
       }
       blurMaskFilterRadius = blurRadius;
     }
-    if (dropShadowAnimation != null) {
-      dropShadowAnimation.applyTo(paint);
-    }
 
     int alpha = (int) ((parentAlpha / 255f * opacityAnimation.getValue() / 100f) * 255);
     paint.setAlpha(clamp(alpha, 0, 255));
 
+    if (dropShadowAnimation != null) {
+      dropShadowAnimation.applyTo(paint, parentMatrix, Utils.mixOpacities(parentAlpha, alpha));
+    }
+
     canvas.drawPath(path, paint);
     if (L.isTraceEnabled()) {
       L.endSection("GradientFillContent#draw");

lottie/src/main/java/com/airbnb/lottie/animation/keyframe/DropShadowKeyframeAnimation.java

@@ -1,29 +1,38 @@
 package com.airbnb.lottie.animation.keyframe;
 
 import android.graphics.Color;
+import android.graphics.Matrix;
 import android.graphics.Paint;
-
 import androidx.annotation.Nullable;
-
 import com.airbnb.lottie.model.layer.BaseLayer;
 import com.airbnb.lottie.parser.DropShadowEffect;
 import com.airbnb.lottie.value.LottieFrameInfo;
 import com.airbnb.lottie.value.LottieValueCallback;
 
+
 public class DropShadowKeyframeAnimation implements BaseKeyframeAnimation.AnimationListener {
-  private static final double DEG_TO_RAD = Math.PI / 180.0;
+  private static final float DEG_TO_RAD = (float) (Math.PI / 180.0);
 
+  private final BaseLayer layer;
   private final BaseKeyframeAnimation.AnimationListener listener;
   private final BaseKeyframeAnimation<Integer, Integer> color;
-  private final BaseKeyframeAnimation<Float, Float> opacity;
-  private final BaseKeyframeAnimation<Float, Float> direction;
-  private final BaseKeyframeAnimation<Float, Float> distance;
-  private final BaseKeyframeAnimation<Float, Float> radius;
+  private final FloatKeyframeAnimation opacity;
+  private final FloatKeyframeAnimation direction;
+  private final FloatKeyframeAnimation distance;
+  private final FloatKeyframeAnimation radius;
+
+  // Cached paint values.
+  private float paintRadius = Float.NaN;
+  private float paintX = Float.NaN;
+  private float paintY = Float.NaN;
+  // 0 is a valid color but it is transparent so it will not draw anything anyway.
+  private int paintColor = 0;
 
-  private boolean isDirty = true;
+  private final float[] matrixValues = new float[9];
 
   public DropShadowKeyframeAnimation(BaseKeyframeAnimation.AnimationListener listener, BaseLayer layer, DropShadowEffect dropShadowEffect) {
     this.listener = listener;
+    this.layer = layer;
     color = dropShadowEffect.getColor().createAnimation();
     color.addUpdateListener(this);
     layer.addAnimation(color);
@@ -42,24 +51,49 @@ public DropShadowKeyframeAnimation(BaseKeyframeAnimation.AnimationListener liste
   }
 
   @Override public void onValueChanged() {
-    isDirty = true;
     listener.onValueChanged();
   }
 
-  public void applyTo(Paint paint) {
-    if (!isDirty) {
-      return;
-    }
-    isDirty = false;
-
-    double directionRad = ((double) direction.getValue()) * DEG_TO_RAD;
+  /**
+   * Applies a shadow to the provided Paint object, which will be applied to the Canvas behind whatever is drawn
+   * (a shape, bitmap, path, etc.)
+   * @param parentAlpha A value between 0 and 255 representing the combined alpha of all parents of this drop shadow effect.
+   *                    E.g. The layer via transform, the fill/stroke via its opacity, etc.
+   */
+  public void applyTo(Paint paint, Matrix parentMatrix, int parentAlpha) {
+    float directionRad = this.direction.getFloatValue() * DEG_TO_RAD;
     float distance = this.distance.getValue();
-    float x = ((float) Math.sin(directionRad)) * distance;
-    float y = ((float) Math.cos(directionRad + Math.PI)) * distance;
+    float rawX = ((float) Math.sin(directionRad)) * distance;
+    float rawY = ((float) Math.cos(directionRad + Math.PI)) * distance;
+
+    // The x and y coordinates are relative to the shape that is being drawn.
+    // The distance in the animation is relative to the original size of the shape.
+    // If the shape will be drawn scaled, we need to scale the distance we draw the shadow.
+    layer.transform.getMatrix().getValues(matrixValues);
+    float layerScaleX = matrixValues[Matrix.MSCALE_X];
+    float layerScaleY = matrixValues[Matrix.MSCALE_Y];
+    parentMatrix.getValues(matrixValues);
+    float parentScaleX = matrixValues[Matrix.MSCALE_X];
+    float parentScaleY = matrixValues[Matrix.MSCALE_Y];
+    float scaleX = parentScaleX / layerScaleX;
+    float scaleY = parentScaleY / layerScaleY;
+    float x = rawX * scaleX;
+    float y = rawY * scaleY;
+
     int baseColor = color.getValue();
-    int opacity = Math.round(this.opacity.getValue());
+    int opacity = Math.round(this.opacity.getValue() * parentAlpha / 255f);
     int color = Color.argb(opacity, Color.red(baseColor), Color.green(baseColor), Color.blue(baseColor));
-    float radius = this.radius.getValue();
+
+    // Paint.setShadowLayer() removes the shadow if radius is 0, so we use a small nonzero value in that case
+    float radius = Math.max(this.radius.getValue() * scaleX, Float.MIN_VALUE);
+
+    if (paintRadius == radius && paintX == x && paintY == y && paintColor == color) {
+      return;
+    }
+    paintRadius = radius;
+    paintX = x;
+    paintY = y;
+    paintColor = color;
     paint.setShadowLayer(radius, x, y, color);
   }
 

lottie/src/main/java/com/airbnb/lottie/model/animatable/AnimatableFloatValue.java

@@ -1,6 +1,5 @@
 package com.airbnb.lottie.model.animatable;
 
-import com.airbnb.lottie.animation.keyframe.BaseKeyframeAnimation;
 import com.airbnb.lottie.animation.keyframe.FloatKeyframeAnimation;
 import com.airbnb.lottie.value.Keyframe;
 
@@ -12,7 +11,7 @@ public AnimatableFloatValue(List<Keyframe<Float>> keyframes) {
     super(keyframes);
   }
 
-  @Override public BaseKeyframeAnimation<Float, Float> createAnimation() {
+  @Override public FloatKeyframeAnimation createAnimation() {
     return new FloatKeyframeAnimation(keyframes);
   }
 }

lottie/src/main/java/com/airbnb/lottie/model/layer/BaseLayer.java

@@ -109,7 +109,7 @@ static BaseLayer forModel(
   private List<BaseLayer> parentLayers;
 
   private final List<BaseKeyframeAnimation<?, ?>> animations = new ArrayList<>();
-  final TransformKeyframeAnimation transform;
+  public final TransformKeyframeAnimation transform;
   private boolean visible = true;
 
   private boolean outlineMasksAndMattes;

lottie/src/main/java/com/airbnb/lottie/model/layer/ImageLayer.java

@@ -16,6 +16,7 @@
 import com.airbnb.lottie.LottieProperty;
 import com.airbnb.lottie.animation.LPaint;
 import com.airbnb.lottie.animation.keyframe.BaseKeyframeAnimation;
+import com.airbnb.lottie.animation.keyframe.DropShadowKeyframeAnimation;
 import com.airbnb.lottie.animation.keyframe.ValueCallbackKeyframeAnimation;
 import com.airbnb.lottie.utils.Utils;
 import com.airbnb.lottie.value.LottieValueCallback;
@@ -28,10 +29,15 @@ public class ImageLayer extends BaseLayer {
   @Nullable private final LottieImageAsset lottieImageAsset;
   @Nullable private BaseKeyframeAnimation<ColorFilter, ColorFilter> colorFilterAnimation;
   @Nullable private BaseKeyframeAnimation<Bitmap, Bitmap> imageAnimation;
+  @Nullable private DropShadowKeyframeAnimation dropShadowAnimation;
 
   ImageLayer(LottieDrawable lottieDrawable, Layer layerModel) {
     super(lottieDrawable, layerModel);
     lottieImageAsset = lottieDrawable.getLottieImageAssetForId(layerModel.getRefId());
+
+    if (getDropShadowEffect() != null) {
+      dropShadowAnimation = new DropShadowKeyframeAnimation(this, this, getDropShadowEffect());
+    }
   }
 
   @Override public void drawLayer(@NonNull Canvas canvas, Matrix parentMatrix, int parentAlpha) {
@@ -54,6 +60,10 @@ public class ImageLayer extends BaseLayer {
       dst.set(0, 0, (int) (bitmap.getWidth() * density), (int) (bitmap.getHeight() * density));
     }
 
+    if (dropShadowAnimation != null) {
+      dropShadowAnimation.applyTo(paint, parentMatrix, parentAlpha);
+    }
+
     canvas.drawBitmap(bitmap, src, dst, paint);
     canvas.restore();
   }

lottie/src/main/java/com/airbnb/lottie/utils/Utils.java

@@ -316,6 +316,13 @@ public static void saveLayerCompat(Canvas canvas, RectF rect, Paint paint, int f
     }
   }
 
+  /**
+   * Multiplies 2 opacities that are 0-255.
+   */
+  public static int mixOpacities(int opacity1, int opacity2) {
+    return (int) ((opacity1 / 255f * opacity2 / 255f) * 255f);
+  }
+
   /**
    * For testing purposes only. DO NOT USE IN PRODUCTION.
    */