Following the above procedure six facial prototypes of emotions and one neutral were obtained, as shown in Fig. 2.
Figure 2: Computer generated prototypes of emotional expression.
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In order to verify the accuracy of the facial expression of the acquired artificial images the prototypes were coded according to FACS. During the posing of the expressions subjects varied in their ability to produce accurately the required combination of Action Units. Although training was offered in order to improve the performance some subjects still had difficulty with certain AUs. As a result of that the composite image of the final prototype diverged from the originally intended combination with regard to certain AUs. In the image of disgust, additionally to the original AU combination 10+17+4 the AUs 6 and 7 were also coded. This was due to the fact that most subjects activated the facial muscles responsible for AUs 6 and 7 when producing the "upper lip raise" (AU 10). This results in a more intensified disgust expression but does not change the emotional quality of the expression. In the anger image, AUs 4 and 24 were coded but the AUs 5 and 7 failed to reach the minimal requirements. This renders the anger expression less intense since the "upper lid raise" (AU 5) and the "lids tight" (AU 7) are absent but again it does not affect the essential quality of the expression. The fear image was coded as 5+20+25+6. The AUs 1+2+4 that mark movements of the eyebrows did not fulfill the minimal requirements for coding. This partial discrepancy between the originally intended facial expression and the resulted prototypical images is again accounted for by individual variations in the original photographic material. Nevertheless the overall expression of fear is also in the upper face recognizable - albeit in a lower intensity - mainly due to the presence of AU 5 ("upper lid raise"). On the contrary the prototypes of joy, sadness, and surprise were identically coded to the original photographic material. In fact for these expressions there was no variation in the posed combination of AUs across subjects. This suggests that the employed referencing points seem to be adequate for the purpose of producing composite images of facial expression and the employed warping procedure does not distort the facial expression of the final prototype.
These 7 digitalised images of prototypical expressions of emotion may be further processed with the computer in order to yield other facial expressions of less clear emotional meaning. By dividing the face in two autonomous areas of upper and lower parts a number of combined emotion expressions is possible. In this way 49 different prototypes of upper and lower face combinations emerged.
Figure 3: Combinations of upper and lower facial parts of prototypical expressions of emotion.
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