The paper evaluates three categories of similarity measures: ordering-based (census), gradient-based, and illumination-based cost functions. The performance of those functions is evaluated especially with respect to illumination changes using two different sets of data, also including real world driving sequences of hundreds of stereo frames with strong illumination differences. The overall result is that there are cost functions in all three categories that can perform well on a quantitative and qualitative level. This leads to the assumption that those cost functions are in fact closely related at a qualitative level, and we provide our explanation.