java - 无法在 Android 中将位图修改为透明

Question

我正在尝试在 android 的 Bitmap 上绘制一个透明的圆圈。我有三个主要变量：

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

如果我执行以下操作，我会得到预期的结果：

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

但是，如果我先在画布上绘制其他东西，然后尝试用透明度清除它，旧数据仍然存在。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色方块。底部的两行应该“擦除”填充的红色以使其再次透明。最终，面具被绘制在另一个画布上，如下所示：

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

    super.onDraw(c);
}

标签： javaandroidbitmaptransparency

Answer 1

事实证明，它确实与Paint对象和设置 Xfermode ...

    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);