Deformable appearance models of shape and intensity are described
in a reasonable level of detail by Cootes et al. in [7]
and in [8] although they were first introduced by Edwards
et al. in [9]. The basic idea was that a measure
of similarity between image intensities guided a progressive search.
Minimisation of the difference between a couple of images using the
sum of a pixel-wise comparison brought the statistical appearance
model and its target to convergence1. During this process, parameters were repetitively being re-evaluated
so that they better described the target object - that is the object
in the image that resembled the model (see Figure 1). Likewise, free-hand
manipulation of these parameters allowed synthesis of new realistic
images2, meaning that a set of assignments for a collection of parameters
would describe a legal (earthly from
an anatomical perspective) type of variation for the model. The following
figure shows an appearance model hovering over some target. The model
is highlighted in red and labelled 'M'.
The following segment expresses in more detail the full process involved. It comprises: