Draw a transparent circle onto a filled android canvas

Question

I am trying to do something very simple (see above). I want all of the pixels of a canvas to be a solid color, except for the the pixels that fill a centered circle. I have read hundreds stack overflow post on this subject and have tried hundreds of things including setting the PorterDuff.Mode. Here is my current onDraw() of MyView extends View:

  protected void onDraw(Canvas canvas) {
    int w = canvas.getWidth();
    int h = canvas.getHeight();

    Paint framePaint = new Paint();
    framePaint.setStyle(Paint.Style.FILL);
    framePaint.setColor(Color.BLUE);
    canvas.drawRect(0, 0, w, h, framePaint);

    Paint transparentPaint = new Paint();
    transparentPaint.setColor(Color.TRANSPARENT);
    transparentPaint.setAntiAlias(true);
    canvas.drawCircle(w / 2, h / 2, (w + h) / 4, transparentPaint);
  }

Am I misunderstanding something, why cant I paint over an existing pixel with transparent paint. When I do this the pixel stays the same. When I use PorterDuff, the pixel turns black. Please help.

Hindsight

Answer 1

Try this:

public class TransparentCircle extends View {

    Bitmap bm;
    Canvas cv;
    Paint eraser;

    public TransparentCircle(Context context) {
        super(context);
        Init();
    }

    public TransparentCircle(Context context, AttributeSet attrs) {
        super(context, attrs);
        Init();
    }

    public TransparentCircle(Context context, AttributeSet attrs,
                             int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        Init();
    }

    private void Init(){

        eraser = new Paint();
        eraser.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
        eraser.setAntiAlias(true);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {

        if (w != oldw || h != oldh) {
            bm = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
            cv = new Canvas(bm);
        }
        super.onSizeChanged(w, h, oldw, oldh);
    }

    @Override
    protected void onDraw(Canvas canvas) {

        int w = getWidth();
        int h = getHeight();
        int radius = w > h ? h / 2 : w / 2;

        bm.eraseColor(Color.TRANSPARENT);
        cv.drawColor(Color.BLUE);
        cv.drawCircle(w / 2, h / 2, radius, eraser);
        canvas.drawBitmap(bm, 0, 0, null);
        super.onDraw(canvas);
    }
}

Osmometer

This worked! I removed the onSizeChanged() since these objects wont be changing size and did everything in onDraw(). Thanks! – Hindsight

The reason of onSizeChanged() is to avoid create bitmap and canvas too many times. onDraw() is call more times than onSizeChange(). – Osmometer

I just noticed that this code produces a circle with rough edges. Is there any way to make it more smooth by using some anti-aliase setting? – Hindsight

Just add this code eraser.setAntiAlias(true); after eraser.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR)); in init method. – Osmometer