Craniofacial superimposition

One of the main goals of forensic anthropology [Burns 2007] is to determine the identity of a person from the study of some skeletal remains. In the last few decades, anthropologists have focused their attention on improving those techniques that allow a more accurate identification.
Before making a decision on the identification, it is necessary to follow different processes that let them assign a sex, age, human group, and height to the subject from the study of bones found. Different methodologies have been proposed, according to the features of the different human groups of each region.
Once the sample of candidates for identification is constrained by these preliminary studies, a forensic identification technique is applied. Among them, craniofacial superimposition [Iscan 1993] is a complex and uncertain forensic process where a photograph of a missing person is compared with the skull that is found. By projecting both photographs on top of each other (or, even better, matching a scanned three-dimensional skull model against the face photo/series of video shots), the forensic anthropologist can try to establish whether that is the same person [Krogman and Iscan 1986]. 

The said process is guided by a number of landmarks located in both the skull (craniometric landmarks) and the photograph (cephalometric landmarks) of the missing person. The selected landmarks are located in those parts where the thickness of the soft tissue is low 

 

In our view, the whole craniofacial superimposition process is composed of three stages:

1. The first stage involves achieving a digital model of the skull and the enhancement of the face image. This stage is not present in all the systems. Indeed, the oldest systems and most of the recent ones still acquire a photograph and/or a series of video shots of the skull, instead of building a 3D model of it. The use of a skull 3D model instead of a skull 2D image should be preferred because it is definitively a more accurate and informative representation. Concer ning the face image, the most recent systems use a 2D digital image. This stage aims to apply image processing techniques in order to enhance the quality of the face image that was typically provided when the person disappeared.
2. The second stage is the skull-face overlay. It consists of searching for the best overlay of either the skull and face 2D images or the skull 3D model and the face 2D image achieved during the first stage. This is usually done by bringing to match some corresponding landmarks on the skull and the face.
3. Finally, the third stage of the craniofacial superimposition process corresponds to the decision making. Based on the skull-face overlay achieved, the identification decision is made by either judging the matching between the corresponding landmarks in the skull and in the face, or by analyzing the respective profiles. Notice that, the use of computers in this stage aims to support the final identification decision that will be always made by the forensic anthropologist.

[Burns 2007] Burns, K. 2007. Forensic Anthropology Training Manual. Prentice-Hall.
[Iscan 1993] Iscan, M. Y. 1993. Introduction to techniques for photographic comparison. In Forensic Analysis of the Skull, M. Y. Iscan and R. Helmer, Eds. Wiley, New York, USA, 57-90.
[Krogman and Iscan 1986] Krogman, W. M. and Iscan, M. Y. 1986. The Human Skeleton in Forensic Medicine. Charles C. Thomas, Springfield, USA. 2nd edition.