If you are overdrawn or if your bank account is overdrawn, you have spent more money than you have in your account, and so you are in debt to the bank. Nick's bank sent him a letter saying he was £100 overdrawn. if you or your bank account is overdrawn, you have spent more than you had in your account and so you owe the bank money
A value determining/describing “Overdraw” or “Overdraw factor” is commonly the number of times each pixel would have been overwritten in the course of rendering averaged over a given frame or series of frames
Commonly described in graphics technical terms as the process by which during the rendering of a scene, a pixel at a given X,Y location in the final image is replaced by one which is closer to the view point than the existing pixel as determined by their corresponding Z values