A probable explanation for the discrepancies that exist between the theoretical and observed performance of single model based image coding methods such as transform coding, is the poor applicability of the continuous Gauss Markov source generally used to model images in the spatial domain, caused by the presence of abrupt discontinuities associated with edge detail. Although using adaptive methods transform coders can to a certain extent overcome this problem, the fact remains that the transform coding approach in itself is not a particularly efficient way of representing discontinuous edge detail. As an alternative approach, two component coding methods are based upon the assumption that an image may be modelled better as the sum of two distinct sources than as a single source. One component is associated with discontinous edge detail and the other with continuous texture, thus enabling independent coding strategies to be designed to match the quite different character-istics of each component model and hence produce better performance overall.
R. J. Clarke,
"Coding of YUV Colour Images based upon a Two Dimensional Two Component Model", Proc. SPIE 0594, Image Coding, (1 May 1986); doi: 10.1117/12.952224; https://doi.org/10.1117/12.952224