1. A computer-implemented method comprising: receiving infrared (IR) image data of an eye, wherein the infrared image data is captured by an imaging device that is associated with a head-mountable display (HMD), wherein three or more IR light sources are arranged to reflect light off the eye, wherein the IR light sources are switched on and off according to a predetermined pattern while the infrared image data is being recorded, and wherein the predetermined pattern is such that one of the IR light sources is switched off during the recording of any given frame of the infrared image data; subtracting a first frame of the IR image data from a second frame of the IR image data to determine a first frame difference; and analyzing the first frame difference to determine at least one location for a first glint corresponding to a first one of the IR light sources, wherein the first IR light source is switched off during the first frame and is switched on during the second frame.