Motion Capture - Computer Animation: Algorithms and Techniques - page 194

6.6 Output from motion capture systems
As the previous discussion has outlined, there is a lot of processing that must be done in order to convert
the raw video data from the cameras to joint angles for a figure. Fortunately, motion capture software is
available to enable the user to set up the marker set hierarchy, calibrate cameras, and automate as much
of the marker processing as possible as well as provide interactive tools when user input is required to
resolve ambiguities. The result can be used to animate a figure or saved to a file for later use.
Various de facto standard file formats exist for motion capture data. Two of the more common for-
mats are the Acclaim two-file format (.asf/.asm) and the Biovision (a group of motion capture studios)
file format (.bvh). While the various formats differ in their specifics, they all have the same basic infor-
mation: the static information describing the joint hierarchy and the time-varying information contain-
ing the values for each degree of freedom (DOF) for each joint.
The joint hierarchy is described by (1) defining the joints including the number and type of each DOF,
the order that DOF parameters are given, and joint limits, and (2) defining the hierarchical connectivity
including the location of joint relative to previous joint. Ignoring header information and other detail not
related to geometry, the static joint hierarchy information would look something like the following:
root
joint Chest offset 0 5.21 0
3 DOFs: Zrotation, Yrotation, Xrotation
Limits: ( 180,180) ( 90,90) (0, 270)
joint Neck
offset 0 5.45 0
3 DOFs: Zrotation, Yrotation, Xrotation
...
joint LeftUpperLeg
offset 3.91 0.0 0.0
3 DOFs: Xrotation, Yrotation, Zrotation
joint LeftLowerLeg
offset 0.0 8.1 0.0
1 DOF: Xrotation
...
In this simplified example, joint names are just for user identification, indentation indicates child-
parent relationship, the offset is the location of the current joint relative to its parent, and the DOF line
indicates the number of DOFs for that joint and the order they appear in the data. Optionally, joint limits
can be specified.
The time-varying data records the reconstructed joint angles over some number of frames and might
look something like the following:
total frames: 300
2.4 40.2 3.2 2.8 22.0 23.0 ... 12.4 40.2 3.2 2.8 ...
2.8 42.1 3.4 2.8 22.1 23.2 ... 12.2 42.3 3.3 3.8 ...
0.2 48.2 3.5 3.9 30.2 25.2 ... 14.4 45.8 3.1 3.0 ...
2.4 50.4 3.3 5.0 35.2 23.3 ... 20.2 44.9 3.3 2.8 ...
1.4 51.6 3.2 8.8 35.3 20.2 ... 22.4 45.2 3.7 2.8 ...
...