Color constancy is a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions, and therefore closer to the physical reflectance. This perceptual effect, discovered by Helmholtz, was formalized by Land and McCann in 1971, who formulated the Retinex theory. Several theories have ever since been developed, known as Retinex or color constancy algorithms. In particular an important historic variant was proposed by Horn in 1974 and another by Blake in 1985. These algorithms modify the RGB values at each pixel in an attempt to give an estimate of the physical color. Land's original algorithm is both complex and not fully specified. It computes at each pixel a stochastic integral on an unspecified set of paths on the image. For this reason, Land's algorithm has received many recent interpretations and implementations that attempt to tune down the excessive complexity. In this paper, a fast and exact FFT implementation of Land's, Horn and Blake theories is described. It permits for the first time a rigorous comparison of these algorithms. A slight variant of these three algorithms will be proposed, that makes them into contrast enhancing algorithms. Several comparative experiments on color images illustrate the superiority of Land's model to manipulate image contrast.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.