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; for a first rectangular sub-region of the digital image, examining, by a hardware processor, only pixels bounding the first rectangular sub-region, wherein the first rectangular sub-region has a predetermined size and includes one or more pixels different from the bounding pixels, and determining, by the hardware processor based on said examining only the bounding pixels of the first rectangular sub-region, that a predetermined percentage of the bounding pixels satisfy a specified criterion; and selectively performing, by the hardware processor, a face detection algorithm on the digital image, wherein the face detection algorithm is performed on the first rectangular sub-region by treating, in response to said determining that the predetermined percentage of the bounding pixels satisfy the specified criterion, all pixels within the first rectangular sub-region as also satisfying the specified criterion.