The following article describes a stereophotogrammetry based technique for 3D measurement of human faces. The method was developed for function orientated diagnostics and therapy in dentistry to provide prognoses for jaw-growth or surgical procedures. The main aim of our activities was to realize both -- a rapid measurement and a dense point cloud.
The setup consists of two digital cameras in a convergent arrangement and a digital projector. During the measurement a rapid sequence of about 20 statistical generated patterns were projected onto the face and synchronously captured by the two cameras. Therefore, every single pixel of the two cameras is encoded by a characteristically stack of intensity values. To find corresponding points into the image sequences a correlation technique is used. At least, the 3D
reconstruction is done by triangulation.
The advantages of the shown method are the possible short measurement time (< 1 second) and - in comparison to gray code and phase shift techniques - the low quality requirements of the projection unit. At present the reached accuracy is +/- 0.1mm (rms), which is sufficient for medical applications. But the demonstrated method is not restricted to evaluate the shape of human faces. Also technical objects could be measured.
At present several methods are adapted for the optical characterization of 3D surface profiles and forms, which are based on fringe projection, moire techniques, gray-code projection or photogrammetry [1-5]. According to principle and application the methods differ in accuracy of measurement as well as computation time or their technical complexity.
Photogrammetry is a well-adapted method for the measurement of 3D objects. The basic idea of the method is to get the whole 3D matrix of real objects by capturing a number of 2D images. In this work we show a possibility for a rapid measurement (< 1 second) of the shape of a human face for medical applications (e. g. jaw-measurement).
The surface structure of the human face is too homogenous to find homologous points by an ordinary illumination; therefore about 20 special statistical patterns are projected on the face and taken by cameras of a convergent stereo system. At present a digital projector is used but it is also possible to generate the statistical patterns by a classical one.
To find the corresponding points in the pictures we use an enhanced correlation technique, which takes into account the characteristic intensity sequence of every single sensor element - unlike other correlation techniques, which avail a pixel area as a template. The influence of distortion - caused by the surface profile - is kept to a minimum. Therefore at higher profile gradients a denser point cloud is generated.
At present the reachable accuracy is +/- 0.1mm (rms), which is sufficient for medical and other applications. But the demonstrated method is not restricted to evaluate the shape of human faces. Also technical objects could be measured.