Abstract: In 1964 Edwin H. Land formulated the Retinex theory, the first attempt to simulate and explain how the human visual system perceives color. His theory and an extension, the ``reset Retinex'' were further formalized by Land and McCann. Several Retinex algorithms have been developed ever since. These color constancy algorithms modify the RGB values at each pixel to give an estimate of the physical color independent of the shading. the Retinex original algorithm is complex and not fully described. Indeed, it computes at each pixel an average of a very large and unspecified set of paths on the image. For this reason, Retinex has received several interpretations and implementations which, among other aims, attempt to tune down its excessive complexity. I will show that Retinex solutions satisfy a discrete linear partial differential equation in the Poisson form. This yields an exact and fast implementation of the Land-McCann theory using only two FFT's. Variants of the PDE permitting color enhancement will also be discussed.