1. A method comprising: projecting a display onto a surface; illuminating a region near the surface during a plurality of different time periods; capturing a plurality of images of the surface during the plurality of different time periods; analyzing a variation of intensities of pixels in a same area across the plurality of images; determining a change in polarity between the plurality of images the polarity indicating whether the area is associated with a predetermined intensity value; and determining whether an object is touching the surface based at least partly on the analysis of the variation and on the determined change in polarity.