1. A method performed by a software process executing on a computer system, the method comprising: accessing a digital image comprising a plurality of pixels; determining, by a hardware processor, 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 only the bounding pixels satisfy the specified criterion, assuming that all pixels within the first rectangular sub-region also satisfy the specified criterion; and selectively performing, by the hardware processor, an image analysis algorithm on the digital image using the assumption that all pixels within the first rectangular sub-region also satisfy the specified criterion.