1. A method to process a proximity image, comprising: receiving a proximity image; generating a dispersion image, wherein the dispersion image is based on a high-pass filtered version of the proximity image; determining an irregularity measure value for the dispersion image, the irregularity measure value indicating a degree of irregularity in the shape of the dispersion image; and controlling an operation of a touch-surface device when the irregularity measure value is above a specified threshold.