To prevent the erroneous ``colorization'' of 3D surface patches that are occluded or point away from the intensity image, it is necessary to compute the normals of each point in the 3D range data image and to perform exhaustive polygon occlusion checks or polygon rendering. However, real-time constraints prevent us from implementing such techniques in a fast face-recognition system. Instead, we shall using the symmetry of the human face to perform mirroring. This simple, efficient, though suboptimal technique is described below.
Furthermore, we choose to crop the 3D model so that only the front of the face will be utilized in texture mapping. The back of the head, the neck and the top of the head will not be useful for recognition, so there is no need to compute their projections onto the image or to worry about the validity of their colorization by tracking their surface normals.