Second, discrepancies between the shapes or dimensions of the subject and the graphical character can lead to problems. If, for example, the subject was recorded touching a real table, the hands of a shorter graphical character might appear sunk into the table. Finally, the current technology makes it difficult to record certain movements. Magnetic systems often require the subject to be connected to a computer by cables, restricting the range of motion. These systems also produce noisy data when metal objects or equipment such as treadmills, which might be used to capture the motion of the subject running, are close by.
Optical systems have problems with occlusion caused by one body part blocking another from view.
In spite of these drawbacks, much of the motion in commercial animation is generated by modifying captured data to match the size and desired behavior of a virtual character. Unlike keyframing and motion capture, simulation uses the laws of physics to generate motion of figures and other objects. Virtual humans are usually represented as a collection of rigid body parts. The lower body, for instance, might consist of a torso, upper and lower legs, and feet connected by rotary joints for the hips, knees and ankles. My students and I have used biomechanical data from various studies, including measurements from cadavers, to build accurate models.