setZOrderOnTop(true);
SurfaceHolder holder = this.getHolder();
holder.setFormat(PixelFormat.TRANSLUCENT);

Canvas canvas = null;
try {
　　synchronized (holder) {
　　　　canvas = holder.lockCanvas(null);
　　　　if (canvas == null) {
　　　　　　return;
　　　　}
　　　　canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR); 
　　}
} catch (Exception e) {
　　e.printStackTrace();
} finally {
　　if (canvas != null) {
　　　　holder.unlockCanvasAndPost(canvas);
　　}
}

 Paint paint = new Paint();  
 paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));  
 canvas.drawPaint(paint);  
 paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC));