Benson and Perrett (1991, 1993) have shown that a greater number of selected points on the face allows the reshaping of each face into an averaged facial structure and results in sharply defined facial composite images. 208 points were predefined in order to study the perception of gender differences, attractiveness, and age in faces (Burt & Perrett, 1995; Perrett et al., 1994). Based on these works a similar procedure was adopted for the production of facial expression composite images (Hoenlinger et al., 1994). In order to overcome the limitations that physiognomic variability poses, the "face stencil method" was developed in order to devise a standard facial structure which would be uniform for all facial images. At first 29 reference points were defined on each face. Five points lie on the central axis of the face; twelve points lie almost symmetrically on each side of the face. These are:
| Lower face | Upper face | ||
| 1 | tip of the chin | 10 | nose bridge (left, right) |
| 2 | between the lips | 11 | inner eye corner (left, right) |
| 3 | middle of upper lip | 12 | outer eye corner (left, right) |
| 4 | upper lip (left, right) | 13 | eye pupil (left, right) |
| 5 | under the tip of the nose | 14 | middle of upper lid (left, right) |
| 6 | middle of lower lip | 15 | middle of lower lid (left, right) |
| 7 | lip corner (left, right) | 16 | middle of eye brow (left, right) |
| 8 | nostril (left, right) | 17 | tail of eye brow (left, right) |
| 9 | under the ear lobe (left, right) |
For each of the 29 reference points an average point was calculated over the corresponding points of the ten original faces for each facial expression. In this manner a standard stencil consisting of 29 points was obtained for each expression.
The next step in the construction of the prototypes was the adjustment of each individual face to the standard stencil of the corresponding emotion. This was done by matching the individual reference points to the standard ones and interpolating all other pixels of the image. In other words each individual facial image was slightly distorted in order to fit the standard dimensions of the stencil. In this way each individual face takes on identical physiognomical dimensions, e.g., length of nose, width of jaws, distance between the eyes. Similarly appearance changes on the faces exhibiting the same expression are defined by identical reference points, e.g., points 4, 5, 7, and 8 (left and right correspondingly) for the lip corners position, points 16, 17 (left and right correspondingly) for the eye brow position etc. The face structure is marked by the following points: 1, 9 (L, R), and 17 (L, R) defining the outer shape of the head; 5, 10 (L, R) defining the shape of the nose; 11, 12, 13 (L, R) defining the shape of the eyes; 2 defining the center of the mouth.
Figure 1: Processing stages of facial images: a) point referencing of a
subject's face
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Figure 1 (a)
Figure 1 (b)
Figure 1 (c)