title: Make certain area of bitmap transparent on touch
body: <p>My idea is to overlap 2 images on top of each other and upon onTouch, the top image should be made transparent on that touched radius, thus exposing the bottom image.</p>
  
  <p>This is how I overlay the 2 images:</p>
  
  <pre><code>        Bitmap bmOverlay = Bitmap.createBitmap(bmp1.getWidth(), bmp1.getHeight(), bmp1.getConfig());
          Canvas canvas = new Canvas(bmOverlay);
          canvas.drawBitmap(bmp1, new Matrix(), null);
          canvas.drawBitmap(bmp2, new Matrix(), null);
  </code></pre>
  
  <p>I have looked into <a href="https://stackoverflow.com/questions/3021401/making-overlaid-image-transparent-on-touch-in-android">this post</a> and have a Paint like below to make it transparent:</p>
  
  <pre><code>        mPaint = new Paint();
          mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC));
          mPaint.setColor(Color.TRANSPARENT);
          mPaint.setAntiAlias(true); 
  
  public void onDraw(Canvas canvas) {
              canvas.drawCircle(40, 40, 30, mPaint); //hardcode to test
  }
  </code></pre>
  
  <p>Problem is, I think the circle straight away make the 2 images transparent on the defined radius, how can I make only the top bitmap transparent?        </p>