Slow scan frame grabber help

Back in the 80's there was this video frame grabber made by Koala for the Mac. You plugged in a video camera, aimed it at something not moving, hit a button, and it slowly (10 plus seconds or more) grabbed the frame in black and white. The subject had to be still because of the slow frame grab. But if it moved it got real interesting. I want to recreate this effect and I have been looking around trying to figure out how I can sample each column of ARGB pixels, from left to right, in a matrix fed by live video, into another matrix to create a still image. I want to grab a column, say every 0.02 seconds (an adjustable amount), across the 720 columns of pixels. And suggestions on where to start? Thanks!