How does player capture raw video for record?

[leftwing]

Pressing the red record button on the Windows player captures the un-encoded stream in a file in the user's Videos directory. We just started using the VLC library and I would like to do something similar. We analyze video so I need the decoded video but when "interesting" things occur I want to preserve encoded clips. I don't want to re-encode so I was hoping that I could do what the player can do which is display decoded and capture encoded video at the same time. I haven't found the way this is done either by googling or looking at vlc.py.