Even though you need 640x480, it may make sense trying to set the graphics size bigger than the image in an attempt to find the cause of this bug. One of the loop goes from 0..479 and the other one from 1..479. The first loop causes GrabImage to grab an image at (0, -1). I did run the code and it works (very nice). To run it, I pasted the function into the main loop. I did that to avoid defining all variables globally. One other thing you could try is using CopyRect instead of GrabImage.