在Android中无法将位图修改为透明

Question

        mask = Bitmap.createBitmap(this.getWidth(),this.getHeight(), Bitmap.Config.ARGB_8888);
        Canvas can = new Canvas(mask);
        Paint clear = new Paint();

clear.setColor(Color.TRANSPARENT);
can.drawRect(new Rect(0,0,this.getWidth(),this.getHeight()),clear);

但是，如果我先在画布上画一些其他的东西，然后尝试用透明度清除它，旧的数据仍然存在。例如：

clear.setColor(Color.argb(255,255,0,0));
can.drawRect(new Rect(0,0,this.getWidth(),this.getHeight()),clear);
clear.setColor(Color.TRANSPARENT);
can.drawRect(new Rect(0,0,this.getWidth(),this.getHeight()),clear);

我只看到一个巨大的红场。底部的两条线应该是“抹去”填充的红色，使其再次透明。最终，面具被绘制在另一张画布上，如下所示：

@Override
public void onDraw(Canvas c)
{
    c.drawBitmap(mask,0,0,null);

    super.onDraw(c);
}

Answer 1

    mask = Bitmap.createBitmap(this.getWidth(),this.getHeight(), 
    Bitmap.Config.ARGB_8888);
    Canvas can = new Canvas(mask);

    Paint clear = new Paint();
    clear.setColor(Color.argb(255,255,0,0));
    can.drawRect(new Rect(0,0,this.getWidth(),this.getHeight()),clear);

    PorterDuffXfermode xfer = new PorterDuffXfermode(PorterDuff.Mode.CLEAR);
    clear.setXfermode(xfer);
    clear.setColor(Color.TRANSPARENT);
    can.drawCircle(this.getWidth()/2, this.getHeight()/2, this.getHeight()/2, clear);