Communicating a higher-level model of the image than pixels is an active area of research. The idea is to have the transmitter and receiver agree on the basic model for the image; the transmitter then sends parameters to manipulate this model in lieu of picture elements themselves. Model-based decoders are similar to computer graphics rendering programs. The model-based coder trades generality for extreme efficiency in its restricted domain. Better rendering and extending of the domain are research themes.