In an image processing apparatus for dividing image data into blocks having a predetermined number of pixels and processing each of the divided blocks in sequence, color data of pixels in a block to be processed is compared to identify a pattern flag that corresponds to a placement pattern of the color data included in the block. The identified pattern flag and the color data for the number of colors included in the block are output and, assuming that output color data that corresponds to a pixel at a predefined position in the block is first color data, the pattern flag, the first color data, and other color data that are output for each of the divided blocks are collectively stored in respectively different memory areas.