How can I capture maya 3d view with QPixmap?

Question

I make a screen capture script for maya. So, I deal the job with QPixmap class.

I write the script like that.

from PyQt4 import QtCore, QtGui
import sip
import maya.cmds as cmds
import maya.OpenMayaUI as mui

def getMayaWindow():
    ptr = mui.MQtUtil.mainWindow()
    return sip.wrapinstance(long(ptr), QtCore.QObject)

pm = QtGui.QPixmap.grabWindow(getMayaWindow().winId())
pm.save('c:/test.png')

but, it can't grab the 3Dview, so it can't work.