1. A storage medium storing a set of program instructions executable on a data processing device and usable for controlling the data processing device to execute Retinex processing, instructions comprising: a) forming a peripheral average luminance image of an original image; b) forming a Retinex image having a smaller size than the original image based on the peripheral average luminance image; c) enlarging the Retinex image to form an enlarged Retinex image; and d) forming an output image based on the enlarged Retinex image and the original image.