IPD question

[atamasan]

Hello.
I'm trying to get the ipd information.
When I use this code, ipd is changed and printed when I push key '1' or '2'.

viz.go(viz.STEREO)
ipd=.06
def key_count(key):
global ipd
if key=='1':
ipd=ipd+.005
viz.ipd(ipd)
print viz.MainWindow.getIPD()
if key=='2':
ipd=ipd-.005
viz.ipd(ipd)
print viz.MainWindow.getIPD()
viz.callback(viz.KEYDOWN_EVENT,key_count)

But when I change viz.go(viz.STEREO)---->viz.go(viz.HMD), ipd isn't changed(same ipd value is printed everytime).

What is the reason for this, please let me know.
And how can I know the value of ipd when I use viz.go(viz.HMD)?

Thank you very much.