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 values in the processing image along one side of the line with the values in the processing image along the other side of the line, including forming a first running total of the values in the processing image along one side of at least a portion of the line and a second running total of the values in the processing image along the other side of the at least a portion of the line, and comparing a value in the first running total with the corresponding value in the second running total; forming a set of screen candidates based on the comparison of the first running total with the second running total for each line in the set; scoring screens in the set of screen candidates according to a criterion; and selecting a first of the screens in the set of screen candidates based on the scoring as a potential screen.