The controller software moves the vcr the next video frame

332 15 Motion Tracking

Medicine. The main medical application, clinical gait analysis, is usu-ally done in a laboratory under controlled conditions. Markers can be applied to the patient, who is usually observed by two to four video cameras for 3-D measurements. The time requirements to get the results vary from close to real-time demands of up to several hours, if a complete clinical report is intended. Another medical application, the analysis of vocal cord vibrations, differs in the re-quired sampling frequency—here, usually several thousand frames/s is needed.

Advances in image processing have led to automatic approaches for motion analysis. The automatic methods can typically be separated into online (real-time) and offline (after recording) tracking. Both of them usually require markers on the objects to be tracked. Markers can be passive or active by reflecting or emitting light, they are usu-ally round, sometimes 3-D or even colored. In Fig. 15.2 several typical

15.2 Flexible automatic motion tracking 333

Although those methods are designed to deal with very complex scenes, they are only partially valuable for the typical motion analysis problem—which involves determining the exact movement of a certain object point.

15.2 Flexible automatic motion tracking