1. A non-transitory computer readable 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: forming a reduced image having a smaller size than the original image by reducing the size of the original image; forming a peripheral average luminance image having a smaller size than the original image from the reduced image; forming a Retinex image having a smaller size than the original image from the reduced image and the peripheral average luminance image by performing logarithmetic conversion for each pixel; enlarging the Retinex image to form an enlarged Retinex image; and forming an output image based on the enlarged Retinex image and the original image.