我有一个自定义视图,在onDraw()中,我正在尝试执行位图屏蔽.我有一个squareBitmap(红色)填充整个视图,我有一个circleBitmap(蓝色)作为掩码.我使用的模式：PorterDuff.Mode.DST_IN.

我期待的结果是一个红色的圆圈.我明白了,但我也得到了一个黑色的不透明背景.我不想要这种不透明的背景,而应该是透明的.图1是我得到的结果,图2是我要找的结果.

我的代码:(为了这个问题的目的,我已经将所有内容都移到了onDraw()中)

protected void onDraw(Canvas canvas) {

final int width = canvas.getWidth();

final int height = canvas.getHeight();

Bitmap circleBitmap = Bitmap.createBitmap(width,height,Bitmap.Config.ARGB_8888);

Canvas circleCanvas = new Canvas(circleBitmap);

Paint p = new Paint(Paint.ANTI_ALIAS_FLAG);

p.setStyle(Paint.Style.FILL_AND_STROKE);

p.setColor(Color.BLUE);

circleCanvas.drawCircle(width / 2,height / 2,width / 2,p);

p.setColor(Color.RED);

Bitmap squareBitmap = Bitmap.createBitmap(canvas.getWidth(),canvas.getHeight(),Bitmap.Config.ARGB_8888);

Canvas squareCanvas = new Canvas(squareBitmap);

final Rect squareRect = new Rect(0,width,height);

squareCanvas.drawRect(squareRect,p);

Paint q = new Paint(Paint.ANTI_ALIAS_FLAG);

canvas.drawBitmap(squareBitmap,q);

q.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));

canvas.drawBitmap(circleBitmap,q);

q.setXfermode(null);

}

我哪里错了？如何避免这种黑色不透明背景？