Erase bitmap parts using PorterDuff mode

Question

I try to erase parts of a bitmap in my Android application by using Porter-Duff Xfermodes.

I have a green background which is overlayed by a blue bitmap. When I touch the screen a "hole" in the overlaying bitmap is supposed to be created making the green background visible. Instead of a hole my current code produces a black dot.

Below is my code. Any ideas, what I am doing wrong here?

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
            WindowManager.LayoutParams.FLAG_FULLSCREEN);
    setContentView(new DrawView(this));
}

public class DrawView extends View implements OnTouchListener {

    private int x = 0;
    private int y = 0;

    Bitmap bitmap;
    Canvas bitmapCanvas;

    private final Paint paint = new Paint();
    private final Paint eraserPaint = new Paint();

    public DrawView(Context context) {
        super(context);
        setFocusable(true);
        setFocusableInTouchMode(true);

        this.setOnTouchListener(this);

        // Set background
        this.setBackgroundColor(Color.GREEN);

        // Set bitmap
        bitmap = Bitmap.createBitmap(320, 480, Bitmap.Config.RGB_565);
        bitmapCanvas = new Canvas();
        bitmapCanvas.setBitmap(bitmap);
        bitmapCanvas.drawColor(Color.BLUE);

        // Set eraser paint properties
        eraserPaint.setAlpha(0);
        eraserPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
        eraserPaint.setAntiAlias(true);
    }

    @Override
    public void onDraw(Canvas canvas) {
        bitmapCanvas.drawColor(Color.BLUE);
        bitmapCanvas.drawCircle(x, y, 10, eraserPaint);

        canvas.drawBitmap(bitmap, 0, 0, paint);
    }

    public boolean onTouch(View view, MotionEvent event) {
        x = (int) event.getX();
        y = (int) event.getY();

        invalidate();
        return true;
    }

}

Answer 1

Here is working code... may help somebody

public class ImageDemo extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(new Panel(this));   
    }

    class Panel extends View {

        private Bitmap  mBitmap;
        private Canvas  mCanvas;
        private Path    mPath;
        private Paint   mPaint;

        Bitmap bitmap;
        Canvas pcanvas;

        int x = 0;
        int y =0;
        int r =0;

        public Panel(Context context) {
            super(context);

            Log.v("Panel", ">>>>>>");

            setFocusable(true);
            setBackgroundColor(Color.GREEN);

            // setting paint 
            mPaint = new Paint();
            mPaint.setAlpha(0);
            mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
            mPaint.setAntiAlias(true);

            // getting image from resources
            Resources r = this.getContext().getResources();

            Bitmap bm = BitmapFactory.decodeResource(getResources(),  R.drawable.mickey);

            // converting image bitmap into mutable bitmap  
            bitmap =  bm.createBitmap(295, 260, Config.ARGB_8888);
            pcanvas = new Canvas();
            pcanvas.setBitmap(bitmap);    // drawXY will result on that Bitmap
            pcanvas.drawBitmap(bm, 0, 0, null);           
        }

        @Override
        protected void onDraw(Canvas canvas) {  
            // draw a circle that is  erasing bitmap            
            pcanvas.drawCircle(x, y, r, mPaint);

            canvas.drawBitmap(bitmap, 0, 0,null);

            super.onDraw(canvas);
        }       

        @Override
        public boolean onTouchEvent(MotionEvent event) {        
             // set parameter to draw circle on touch event
             x = (int) event.getX();
             y = (int) event.getY();

             r =20;
             // At last invalidate canvas
             invalidate();
             return true;
        }
    }
}