Motion Capture - Computer Animation: Algorithms and Techniques - page 186

CHAPTER

6

Motion Capture

Creating physically realistic motion using techniques involving key frames, forward kinematics, or
inverse kinematics is a daunting task. In the previous chapters, techniques to move articulated linkages
are covered, but they still require a large amount of animator talent in order to make an object move as
in the real world. There is a certain quality about physically correct motion that we are all aware of, and
it only takes a small deviation from that quality to make what is supposed to be realistic movement quite
distracting. As a result, it is often easier to record movement of a real object and map it onto a synthetic
object than it is for the animator to craft realistic-looking motion. The recording and mapping of real
motion to a synthetic object is called motion capture ,or mocap for short.
Motion capture involves sensing, digitizing, and recording of an object's motion [ 6 ] . Often, the term
specifically refers to capturing human motion, but some very creative animation can be accomplished
by capturing the motion of structures other than animal life. However, the discussion in this chapter
focuses on motion capture of a human figure.
In motion capture, a (human) subject's movement is recorded. It should be noted that locating and
extracting a figure'smotiondirectly fromraw (unadulterated) video is extremely difficult and is the subject
of current research. As a result, the figure whose motion is to be captured (sometimes referred to as the
talent ) is typically instrumented in some way so that positions of key feature points can be easily detected
and recorded. This chapter is concernedwith how this raw data is converted into data usable for animation.
There are various technologies that can be applied in this instrumentation and recording process. If
the recordings are multiple two-dimensional images, then camera calibration is necessary in order to
reconstruct the three-dimensional coordinates of joints and other strategic positions of the figure. Usu-
ally, joint angles are extracted from the position data and a synthetic figure, whose body dimensions
match those of the mocap talent, can then be fitted to the data and animated using the extracted joint
angles. In some cases, the mocap data must be edited or combined with other motion before it can be
used in the final animation. These topics are discussed in the following sections.

6.1 Motion capture technologies
There are primarily two approaches to this instrumentation 1 : electromagnetic sensors and optical
markers . Electromagnetic tracking, also simply called magnetic tracking , uses sensors placed at the
joints that transmit their positions and orientations back to a central processor to record their

1 There are several other technologies used to capture motion, including electromechanical suits, fiber-optic sensors, and dig-
ital armatures [ 6 ]. However, electromagnetic sensors and (passive) optical markers are by far the most commonly used tech-
nologies for capturing full-body motion.