eir channels have the same type; otherwise they are called <em>heterogeneous</em>. The channels of a pixel may be addressed using semantic or physical indexing, or by color; all color base algorithms work on pixels as well. Two pixels are <em>compatible</em> if their color spaces are the same and their channels, paired semantically, are compatible. Note that constness, memory organization and reference/value are ignored. For example, an 8-bit RGB planar reference is compatible to a constant 8-bit BGR interl