protected int setPaintAlpha(Paint paint, int alpha) {
        final int prevAlpha = paint.getAlpha();
        paint.setAlpha(Ui.modulateAlpha(prevAlpha, alpha));
        return prevAlpha;
    }
Set the draw paint alpha by modulateAlpha

@param paint Paint
@param alpha Alpha
@return PrevAlpha
caprice/ui/src/main/java/net/qiujuer/genius/ui/drawable/effect/Effect.java