public void applyWithAlpha(int alpha, OffscreenLayer.ComposeOp op) {
    op.shadow = new DropShadow(this);
    op.shadow.multiplyOpacity(alpha);
  }
Applies a shadow to the provided ComposeOp, mixing its alpha by the provided value.
lottie/src/main/java/com/airbnb/lottie/utils/DropShadow.java