Wavelet compression techniques are used to reduce the troublesome space requirements (especially in 3-D ) and make active appearance models far more compact. There are also some application-specific extensions such as the implementation of view-based models  and coupled-view models  for faces. The principal idea is that 5 different models can express full appearance irrespective of the wide range of viewing angles around the head. Due to the symmetry of a human head, only 3 models are used in practice. This idea can surely be exploited in some other applications, but it appears to have a limited demand in industry and it has not been pursued much lately. It is the switching between different models in real-time and the selection of the most suitable model that makes this study challenging. The effect that different facial expressions and aging factors have on statistical model was another intriguing aspect that was mainly pursued by Lanitis et al.[20,21] who now has another implementation of appearance models .