title: Android Alpha Masking question
body: <p>in my overridden onDraw() function i have a canvas with two gauges in it (gauges are PNG's). The gauges are stacked on top of eachother. I would like only half of the gauge on the top to show while revealing the bottom gauge on the other half. Here's what i have so far:</p>
  
  <pre><code>@Override
  public void onDraw(Canvas c){
  
      int targetWidth=200;
      int targetHeight=200;
  
      Paint p = new Paint();
  
      Bitmap bottom = BitmapFactory.decodeResource(getResources(), R.drawable.dashboard_rpm_bottom);
      Bitmap top = BitmapFactory.decodeResource(getResources(), R.drawable.dashboard_rpm_top);
  
      p.setFilterBitmap(false);
  
      c.translate(55,320);
  
      c.drawBitmap(
              bottom,
              new Rect(0, 0, bottom.getWidth(), bottom.getHeight()),
              new Rect(0, 0, targetWidth, targetHeight),
              p);
  
  
      p.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_OUT));
  
      c.drawBitmap(
              top,
              new Rect(0, 0, top.getWidth(), top.getHeight()),
              new Rect(0, 0, targetWidth, targetHeight),
              p);
  
  
  
      }
  </code></pre>
  
  <p>doesn't seem to work, anyone have any ideas?</p>