# The Method of Multi-Camera Layout in Motion Capture System for Diverse Small Animals
## 4 Discussion
### The Performances of the Motion Capture System for Different Objects  
In spite of the researches on motion capture systems for human motion, large/medium animal, for example, [PERSON] et al. have compared different algorithms of motion capture for horses and obtained the capture deviation rate of the horse during continuous walking (fore limb: 4.1% and hind limb: 4.8%) [38] and [PERSON] et al. have measured the average capture deviation about \(-\)0.23 \(\pm\) 0.35 mm(\(-\)0.24 \(\pm\) 0.36%) of the human motion capture system (Cameras: Vicon Bonita, 200 Hz, Vicon Motion Systems, Ltd., Oxford, UK; Measurement volume of 5.5 \(\times\) 1.2 \(\times\) 2.0 m\({}^{3}\)) [39], there are few studies in the motion capture system for diverse small animals whose structure and movement characteristics are quite different from those of the human or large/medium animals, resulting in the relatively small experimental space and the requirement for higher accuracy and precision of the capture system. The performance of the motion capture system depends on various influencing factors. It is revealed that the layout pattern of the multi-camera has an important effect on the \(ACDR\) (\(p<\) 0.001) as well as the \(ASDR\) (\(p<\) 0.001). The results of calibration experiments on different multi-camera layout patterns show that the \(ACD\) of the rigid object is \(-\)0.10 \(\pm\) 0.015 mm (\(ACDR\): 0.23 \(\pm\) 0.03%) and the \(ASDR\) is 0.34 \(\pm\) 0.13% (The optimal system configuration: half-annular layout, \(\alpha\) = 45\({}^{\circ}\), \(\theta\) = 30\({}^{\circ}\); \(V\) = 0.061 m\({}^{3}\)). This is of great value for accurate motion capture for specific small animals in small experimental set. The \(ASDR\) of the arched layout and the half-annular layout is significantly smaller than that of the annular layout and the \(ASDR\) of the arch layout is 1.2 times larger than that of the half-annular (Figure 7). Body shape and locomotion characteristics of the geckos and spiders have also been taken into account. Therefore, the half-annular layout is selected to perform the motion capture experiments. Results show that the \(ACDR\) of geckos and spiders are 1-2 mm and 0.32-1.22 mm, which are both smaller than that of rigid body (1-4 mm, OptiTrack motion capture system) [40] as well as human motion (0.82-3.47 mm, Vicon8 motion capture system) [41]. The \(ASDR\) between geckos and spiders are close to 3.7% which are slightly larger than 2.28% of the rigid body [38]. It is resulted from the much more complicated body shape and movement characteristics of gecko and spider, which also lead to the great challenge for stable and accurate motion capture. There are studies on the kinematics and kinetics of spiders by introducing the virtual hip and knee joints for modeling analysis [26], it is because that the motion capture for spiders with eight small multi-joints legs attached a large number of markers is too difficult to capture all of the markers completely. Through the matching mechanism between the multi-camera layout patterns with diverse animals, the accuracy and reliability of the experimental data could be improved by increasing the capturing volume and avoiding occlusion, improving the success rate and accuracy of the capture, more can be introduced in the legs. In addition, the \(ASDR\) of gecko and spider are very similar but the \(ACDR\) of gecko is larger than that of spiders (Tables S5 and S6). The \(ACDR\) of the head distance of gecko is 2.1 \(\pm\) 0.28% and that of the leg length is 4.77 \(\pm\) 0.40%, so the \(ACDR\) of the leg is more than twice that of the head (Figure 8a). The \(ACDR\) between the tibia and the metatarsus of spiders are small (Figure 8b), since they have a typical exoskeleton structure. Therefore, researchers prefer to study the legs' movement of spider relative to its body. The metatarsi of spider are very close to the ground, which is similar to the leg movement of gecko. Locomotion characteristics of tibia is analogous to the metatarsus, so there is no significant difference between them.