A method, performed by a software process executing on a computer system, includes accessing a digital image comprising a plurality of pixels. The method also includes determining whether one or more pixels bounding a first rectangular sub-region of a predetermined size within the digital image satisfy a specified criterion. If a predetermined percentage of bounding pixels satisfy the specified criterion, the method assumes that all pixels within the first rectangular sub-region also satisfy the specified criterion. The method further includes selectively executing an image analysis algorithm on the digital image using the assumption that all pixels within the rectangular sub-region also satisfy the specified criterion.