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Real-time Motion Capture

The main goal is to capture real time human data and apply it to computer-generated characters. The characters are modeled in Maya and animated using incoming data from the motion capture system.

What is Motion Capture?

Motion Capture is the process of capturing the movement of a real object in 3D space and mapping it onto a computer generated object. The result is a computer character that moves exactly like the real object. Usually, Motion Capture is used to capture the motions of humans.

How does Motion Capture work?

For ìopticalî motion capture, a series of high resolution cameras with special strobe lights are set up around an area to be ìcaptured.î  Small spheres (markers) covered with a retro-reflective substance are placed in strategic locations on the person (or object). Specialized software will locate the markers seen through the cameras. The markers are then recorded as 3D coordinates (xyz). The collected marker data over time creates motion data.

What is real time Motion Capture?

There are two types of motion capture: offline and online (real time). In the case of the offline, the data is captured, stored, processed (cleaning data from noise is common) and then later applied to the computer character.Ý However, in real time capture the motion data is captured and applied to the character simultaneously. Depending on the capture system, there should be no noticeable delay in the characterís movement compared to the movement of the real subject.

A common problem with real time motion capture is the presence of noise. If you plot the motion data you will notice that the curves arenít always smooth, there are some unsettledness or jerkiness. There are many filtering systems and techniques to eliminate the ìbad dataî. During offline capture, one can go back to the data and ìclean it upî before applying it to the computer character. But how about real time capture?

. . . Stages of Work . . .

Ý Stage 1: Understanding how motion capture is done

The first step was to familiarize myself with the motion capture lab and learn about conducting a motion capture session. I attended one of the capture sessions and went through a basic tutorial of how to work the equipment and what are the necessary steps in capturing a motion. Here are the basic steps:

1. Marker placement: Using the marker placement guide, markers are placed on the subject(s). For humans there are either 40 or 41 markers total. Placing markers requires some practice, if some of the markers are not in their ìexpectedî positions then that will result in generating bad data later on.

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2. Calibrate the cameras: there are a total of 12 cameras in the lab. This is necessary to make sure that all the cameras are able to view the area used for capture. Basically a number of markers are placed on the floor for ìstatic calibrationî and then a stick with markers is waved around the area of capture, this is for ìdynamic calibrationî.

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Calibration AxisÝÝÝÝÝÝÝÝÝÝÝÝÝ ÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝCalibration Stick

3. Subject calibration: the subject of capture is calibrated. For human capture, the person would stand in a ìTî position with the arms straight out for ìstatic calibrationî and then the person would do a simple motion such as walking across the room for ìdynamic calibrationî.

4. Labeling: using the software ìWorkstationî, you can view the captured motion as a collection of markers moving over a period of time. Next step is to label the markers. Itís simply a matching game, you are given a list of default marker names and you are to match those names to the corresponding markers on the body.Ý Workstation will then connect the markers to create segments.

(photo from the Vicon website)Ý

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5. Ready to Capture: All the preprocessing is complete and you are ready to capture. Simply click on the start button and Vicon starts capturing the data.

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Ý Stage 2: Writing a plugin for Maya

ÝÝÝÝÝÝÝÝÝÝÝÝ The captured data from Vicon will be applied to a computer character generated in Maya. Maya is a 3D modeling and animation software. In order to apply the real time data to the character, there must be some type of a link or communication method between Maya and Vicon. This part of the project involves writing a plugin ( an additional program that enhances the features of an existing program) that will allow Maya to receive data from Vicon.

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ÝÝÝÝÝÝ Writing the plugin requires a good understanding of networking concepts, an in-depth knowledge of how data is handled by Tarsus (a Vicon software used for motion capture) and how Maya maps incoming data to objects. This project seemed to be heavily involved since I didnít know much about networking, Tarsus or Maya. Luckily, I was able to contact the people at Vicon and they told me that they had already written a plugin for Maya and that theyíll e-mail a copy to me.

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Ý Stage 3: Modifying the plugin

ÝÝÝÝÝÝÝÝÝÝÝÝ This is the general idea of how data is communicated between Tarsus and Maya:

1. Tarsus outputs data using named channels. Channels are strings of double values varying over time. Each channel corresponds to a specific source that could be either a marker or a body segment. In turn, the plugin creates channels (vectors) to store the incoming channel data. coming from the Tarsus channels.Ý

2. The data stored in the vector channels is parsed, modified and sent out to Maya using routines in the Maya server library.

Tarsus data consists of the position(x,y,z) and orientation (q_x, q_y, q_z ) of the body parts for the real time subject and the position (only) of the corresponding markers. However, Tarsus doesnít provide joint angle data, which is what we need. Therefore, the code must be modified to extract joint angle information from the available position/orientation data.

ÿ Calculating joint-angle information:

ÝÝÝÝÝÝÝÝÝÝÝÝÝ Given the position and orientation of two body parts we can calculate the angle for the joint joining the two bodies using quaternions. A quaternion is a 4-tuple of real numbers [s,x,y,z] or [s,v], s is a scalar and v is a three-dimensional vector.Ý First, represent the position/orientation data of both body parts as quaternions, find the difference of the two quaternions and the result is the angle for the joint

connecting the two body parts.

A , B = a 6-dimensional vector with the center at (x,y,z) and orientation at (qx, qy, qz ).

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Ý Stage 4: Modifying the Vicon files

ÝÝÝÝÝÝÝÝ Part of the Vicon system are files called .MKR and .KM files, they contain lists of: 1) marker labels, 2) body segments and their corresponding markers. By default, the marker file (.MKR) for a human contains 40-41 markers and the kinematic model file (.KM) contains 17 body segments. Ý

ÝÝÝÝÝÝÝÝÝÝ A typical human skeleton file contains around 30 body parts, which adds more degrees of freedom to the body and results in a better looking motion. To match with the current skeleton files being used, the marker and kinematic model files need to be modified to include more body segments. Both files can be viewed as text files and to change the number of segments requires typing in few additional text lines.