1. A system comprising: a computer-readable memory storing executable instructions; and one or more computer processors in communication with the computer-readable memory, wherein the one or more computer processors are configured to execute the executable instructions to at least: generate a normalized spatial distance between a structural reference of a subject in an image and a pixel in the image based at least in part on: (i) a spatial distance between the structural reference and the pixel, (ii) a first distance between a first pair of structural references of the subject, and (iii) a second distance between a second pair of structural references of the subject; and adjust a parameter included in a pixel classifier based at least in part on a category of content for the pixel, the normalized spatial distance, and the structural reference, wherein the pixel classifier, including the parameter as adjusted, receives the normalized spatial distance and the structural reference as input values and provides an output value indicating the category of content.