android 怎样获取图像触摸位置的rgb值

@Override
 public boolean onTouchEvent(MotionEvent event)
 {
    // TODO Auto-generated method stub
    x=(int)event.getX();
    y=(int)event.getY();
    if(MotionEvent.ACTION_UP == event.getAction()){
    i=x;
    j=y-200;


// 加以下4行代码 ,就不能显示坐标值了,bug
    pixel = bmp.getPixel(i,j);
    red= (pixel & 0xff0000) >> 16;
    green= (pixel & 0xff00) >> 8;
    blue= (pixel &0xff);


//

 // showInfo("RGB"+pixel); 
 showInfo("你点击的坐标为：("+x+","+y+")"+red+","+green+","+blue+"\n"+"再次点击返回界面");
 // notice.setText("您点击的位置是：\nx:"+x+"\n y:"+y);
 }
 return super.onTouchEvent(event);
 }