1. A method, comprising: forming a processing image based on corresponding pixels in a plurality of frames of image data, the processing image comprising values corresponding to the pixels in the plurality of frames; identifying a set of lines in the plurality of frames that are candidate for boundaries of a screen; for each line in the set, comparing the processing image on one side of the line with the other side of the line; forming a set of screen candidates based on the comparisons; scoring screens in the screen candidates according to a criterion; and selecting a first of the screens in the set of screen candidates based on the scoring.