An image data sequencing method for a memory unit inside an optical scanning device. The image data sequencing method is particularly suitable for scanning a line of pixels with each pixel comprising a plurality of primary colors. The image data sequencing method involves scanning a line of pixels to obtain the data for a primary or secondary color. The pixels within the scan line are subdivided into groups. A storage space is reserved both before and after the address space inside the memory unit for holding the scanned primary or secondary color data so that all the primary or secondary color data constituting a pixel are in a fixed sequence next to each other inside the memory unit. When all the primary or secondary color data of pixels within a group are secured, the group of data is released from the memory unit.