1. An image processing apparatus for quantizing an input image, comprising: a first image generation unit configured to generate, from the input image, a reflectance image that represents a reflectance distribution, and an illuminance component image that represents an illuminance distribution; a region specifying unit configured to specify a plurality of small regions divided based on illuminance components of the illuminance component image generated by the first image generation unit; a second image generation unit configured to generate a quantized image from the reflectance image generated by the first image generation unit for respective small regions specified by the region specifying unit; a region acquisition unit configured to acquire regions having equal quantized pixel values in the quantized image generated by the second image generation unit; a pixel value acquisition unit configured to acquire representative pixel values for respective regions corresponding to the regions acquired by the region acquisition unit based on the illuminance component image generated by the first image generation unit; and a correction unit configured to correct quantized pixel values of the respective regions acquired by the region acquisition unit using the representative pixel values acquired by the pixel value acquisition unit.