Abstract: Bit-mapped and compressed image data are converted one another through an intermediate image data or code form wherein each image is represented by a list of sublists, each sublist being itself a list of numerical values representing run lengths transistions from one type of picture element (pel) to a second and opposite type of picture element (pel) (e.g., a black pel and a white pel). Hence the intermediate code data structure according to the invention is termed a transition list or TL code or data structure. The data structure and coding rules are defined by rules relating to code type, code word length and code word interdependence. A specific conversion process specifies that the transition list be composed of sublists wherein each represents a scan line.