Game Development Reference
In-Depth Information
determine the location of certain points in 3D space, mechanical motion
capture principally determines the orientation. When animating a charac-
ter using keyframe animation, an animator will spend most of the time
rotating joints. A mechanical suit works in much the same way. To record
the rotation in a single direction requires a potentiometer, a variable
resistor. This simple electronic device is used to control the volume of
your hi-fi. Varying the resistance has the effect of varying the voltage. We
can use an analogue to digital (A/D) converter to convert this changing
voltage into a numerical value. With a little simple scaling this can be
converted into an orientation value for this joint. To allow a joint to move
in all directions we need three potentiometers for each joint, one to record
the heading, pitch and bank. A full suit uses around 50 potentiometers,
which all have to be tracked around 25 times per second. The root of the
hierarchy of a character is usually the hips, and this single part of the
skeleton chain must be tracked for position and orientation. Unlike the
other parts of the skeleton, the position and orientation cannot be
calculated using a rigid outer skeleton and potentiometers. Instead, we
must use some form of sensor that calculates exact position and
orientation in world space. Absolute position and orientation of the hips is
often calculated using a digital inertial gyroscope. A major disadvantage
of mechanical capture is the weight of the suit, which can restrict the
motion of the actor. An important advantage is the range of capture
available. Mechanical suits usually have a
range more than ten times that of the pre-
viously described methods.
When deciding on an option it is a balance
between the cost of the capture and the current
requirements. A mechanical method over long
distances is likely to be the best option. If the
actor wants to feel the least encumbered then
optical will be the way to go. If fine accuracy
over short distances is required then magnetic
will provide the best option.
Making a simple mocap suit
The simplest and cheapest technique for
capturing data is to make your own mechan-
ical suit. Although a full body suit is compli-
cated to engineer, you can get some very
effective motion capture data using just upper
body movements.
Figure 12.5 Mechanical
motion capture.
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