Android Bitmap Masking (Xfermode) leaves opaque black background behind

Question

I have a custom view and in the onDraw(), I am trying to perform bitmap masking. I have a squareBitmap (red color) which fills the whole view, and I have a circleBitmap (blue color) that acts as the mask. I am using the mode: PorterDuff.Mode.DST_IN.

The result I am expecting is a RED filled circle. I get that, but I also get a BLACK opaque background. I don't want this opaque background, rather it should be transparent. Figure 1 is the result I got, and Figure 2 is the result that I am looking for.

My code: (I have moved everything inside onDraw() for the purpose of this question)

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, 0, width, height);
    squareCanvas.drawRect(squareRect, p);

    Paint q = new Paint(Paint.ANTI_ALIAS_FLAG);
    canvas.drawBitmap(squareBitmap, 0, 0, q);
    q.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
    canvas.drawBitmap(circleBitmap, 0, 0, q);
    q.setXfermode(null);
}

Where am I going wrong? How can I avoid this black opaque background?

Answer 1

It seems that to get the Xfermode done, I need to redirect the drawing to an offscreen bitmap. So adding the following fixed my problem.

canvas.saveLayer(0, 0, canvas.getWidth(), canvas.getHeight(), q);

But then again, the documentation of saveLayer says to avoid using this method since it is expensive. It is suggested to use a harware layer instead of this method.

Avoid using this method, especially if the bounds provided are large, or if the CLIP_TO_LAYER_SAVE_FLAG is omitted from the saveFlags parameter. It is recommended to use a hardware layer on a View to apply an xfermode, color filter, or alpha, as it will perform much better than this method.

Hence, intead of saveLayer method, I was able to fix it by adding the following:

setLayerType(LAYER_TYPE_HARDWARE, q);