How to create a canvas with image of video frame

I'm hoping to use canvas' drawimage to create what is effectively开发者_如何学Python a screenshot of the video tag (currently playing). I know this is possible, but I'm not clear on how.

Source of inspiration: http://www.craftymind.com/2010/04/20/blowing-up-html5-video-and-mapping-it-into-3d-space/

Literally you just do drawImage(videoTagReference, 0, 0)

That will draw the <video> onto the canvas at the current frame.