Questions Answered At a Glance
Q. How is the data motion captured in your Lab?
In the Graphics and Virtual Reality Lab we have several motion capture systems, including a 24-cameras PhaseSpace Impulse X2 motion capture system, a 12-cameras Optitrack Flex motion capture system, XSens suits, several Kinect sensors (including the new Azure), as well as other depth sensors, and high quality RGB-video cameras.
For the purposes of this project, we used a passive motion capture system of twelve Optitrack Flex 3 cameras (resolution of 640x480) to capture the 3D motion of our articulated subjects. The cameras operate at high frequency (in our experiments, we capture movements at 100Hz), each of which can capture the position of any number of bright spots from the reflective markers. Motions were captured in a working volume of approximately 2m x 3m.
In addition to that, we used the AXY-4 triaxial accelerometers of Technosmat to record the accelerations of the reptiles, that have been strategically positioned on the reptiles’ body. These accelerometers can record at high frame rates, and in our experiment we record at 100 frames per second.
Q. Which formats do you use in your Lab?
- Video format: Movement recording using an HD camera. Data saved in .MP4 (MPEG-4 part 10 H.264) or .FLV (flash) format.
- C3D format: stored 3D coordinate information, analog data and associated information as it is recorded from the motion capture system.
- Character (.FBX) format: A rigged and skinned virtual character baked with the animation. This format is compatible with Adobe MotionBuilder, 3Ds Max, Maya, Unity3D, Blender etc.
- BVH format: BioVision Hierarchy file format. Exported BVH files do not include individual marker data. Instead, a selected skeleton is exported using hierarchical segment relationships.
- CSV format: This format stores the xyz-acceleration readings taken from the AXY-4 triaxial accelerometers of Technosma.
Q. What kind of RGB cameras did you use?
Q. How did you sync all optical cameras together?
Q. How did you sync cameras with the accelerometers?
Q. What were the main challenges you faced when using mo-cap for these animals?
- For small animals like the Stellagama stellio lizard we had to use small face markers (3mm diameter) not to interfere with animals’ motion. This small reflectance surface limited the working distance from cameras forcing us to monitor the movements of the lizard in an area of 1,5m X 1,5m. In the case of snakes where larger markers were used, this area was increased to approximately another 1 meter from each site.
- Adding natural obstacles although will provide a more hospitable environment for the animal and will enrich its motions will highly interfere with line of sight of the markers. In any point in time at least three cameras must “see” each marker. When the animal hides behind an object (e.g., a brick) the system will not be able to see all markers and the reconstruction of animal movement will be practically impossible. Although adding more cameras (we used 12 cameras) might assist it will not provide a solid solution.
- Since each recording equipment (i.e., video cameras, mo-cap cameras, accelerometers) works independently, proper timestamping is of absolute importance for the post-process synchronization between the recording methods. We recommend using a neutral handheld digital chronometer as a reference instrument for this timestamping. In addition, we stress the importance of talking during filming and initiating each record by pronouncing the time and making a clear fast knock for timestamping the initiation second.
Q. Were animal stressed under this procedure?
All individuals were collected from the field using time survey and funnel traps and were released back in the field after the end of the behavioural trials. In all cases handling and human interaction was kept as limited as possible to minimize stress to the animals.
A total of 20 recording efforts (8 for lizard and 12 for snake), approximately 10 minute each, took place over a period of four days to further minimize stress to the animals. Between recordings animals were left to relax either within the monitoring arena, or within their terrarium. All appropriate licenses had been acquired from the Department of Environment, the Veterinary Services and Cyprus Bioethics Committee prior to collection.