Motion capture technology is a good example of how digital techniques are being applied to the movie (and related) industries to allow more convincing visualizations of imaginary or composite images. For motion capture you use human actors who are dressed in a leotard with integral reflective or magnetic markers. The actor performs the actions that are required, and the digital cameras - or array of cameras - capture the motion of the reflective markers
You subject the data to a computer process that converts this motion into a composite figure. You then modify this composite figure by normal computer animation software The end product gives the effect of an animated character acting directly with human actors. Gollum, in the Lord of the Rings, was shot in this fashion, giving an absolutely life-like image of a composite character. That guy on the left here? That would be Gollum, or at least the movements of Gollum.
Motion capturing techniques are very effective, but the computer processing needs much human intervention, and if there is any error in the data, you can find it more effective to re-shoot the whole scene rather than correct the data. However, motion capture technology is so much more effective and realistic than traditional techniques, and ultimately less time consuming, that its future looks assured in movies and in video games.
The capture is based on optical shooting several synchronized cameras, the synthesis of coordinates (x, y) of the same object from different angles, allows to deduce the coordinates (x, y, z). This method involves the consideration of complex problems such as optical parallax, distortion lens used, etc.. The signal thus undergoes many interpolations. However, a correct calibration of these parameters allows a high accuracy of data collected.
All references from Google and freemotionfiles.blogspot.com