1. An optical navigation device comprising: an image sensor comprising a pixel array to generate a plurality of tracking images corresponding to incident light at the pixel array; and a tracking engine coupled to the image sensor, the tracking engine configured to determine a sub-pixel displacement value of a tracking surface based on a comparison of at least two of the tracking images, wherein the tracking engine comprises: a sub-pixel approximation engine to generate an intermediate sub-pixel displacement value based on a sub-pixel approximation according to a non-linear sub-pixel distribution; and a linear approximation engine to generate a final sub-pixel displacement value from the intermediate sub-pixel displacement value.