# The Method of Multi-Camera Layout in Motion Capture System for Diverse Small Animals
## 4 Discussion
### The Effect of Intersecting Volume and Depth of Field on the Motion Capture  
The comparison between intersection volume size of different layout patterns of multi-cameras and corresponding motion capture standard deviation rate, meanwhile through the comparison between the intersection volume size of three kinds of pitch angle (30\({}^{\circ}\), 45\({}^{\circ}\)and 60\({}^{\circ}\)) in a half-annular layout of multi-camera unit module and the corresponding average standard deviation rate (Table 1), both prove that the enlargement of vision intersection volume of multi-camera helps to reduce the standard deviation rate of motion capture (Figure 7a, Table S3). The maximum coverage theory [37] of cameras' layout used in human body motion capture and the object visibility maximization principle [19] applied in a large-scale environment, both have consistency with the theory of this paper that the maximization of vision intersection volume of multi-camera helps increase motion capture accuracy rate. Moreover, since currently the influence of the depth of field on motion capture is still not detailed. This paper explores the effect of the selection depth of field range on the accuracy of motion capture (shown in Figure 7b and Table S4) and preliminary validates the enlarged depth of field of multi-camera cameras which is beneficial to the reduction of the motion capture standard deviation rate, it is because the depth of field is a critical factor affecting image resolution and the image is the main source of kinematics parameters. Whether it is depended on computer identification or human recognition, the clearer the images, the greater the accuracy of marker selection.  
In addition, by keeping that the depth of field range is greater than the required motion area length (300 mm) in different angles (30\({}^{\circ}\), 45\({}^{\circ}\) and 60\({}^{\circ}\)) to ensure a better clarity of the images and the actual scope of depth of field and intersection volume (Table S1) were calculated. As we can see, the view intersection volume in the pitch angle of 30\({}^{\circ}\) and 45\({}^{\circ}\) are the same and it is about 43% larger than that of 60\({}^{\circ}\) and the actual scope of the depth of field increases with the increase of pitch angle and that of 60\({}^{\circ}\) is larger by about 54% than that of 30\({}^{\circ}\). The experiment result shows that the standard deviation rate of pitch angle 60\({}^{\circ}\) is significantly greater than that of 30\({}^{\circ}\) and 45\({}^{\circ}\) but the standard deviation rate under 30\({}^{\circ}\) is higher than when that of 45\({}^{\circ}\). Therefore, despite the actual scope of depth of field under 60\({}^{\circ}\) is larger, the intersection volume is the smallest, which leads to its standard deviation rate the largest. Intersection volume under 30\({}^{\circ}\) and 45\({}^{\circ}\) is the same but the actual scope of the depth of field under 45\({}^{\circ}\) is about 35% larger than that of 30\({}^{\circ}\). Such a wide range of the depth of field may be the reason for the motion capture standard deviation rate. Therefore, under the constant volume camera layout, increasing the depth of field range is also one way to reduce the standard deviation rate. To increase the view intersection volume of multi-camera or to increase the depth of field range are both beneficial to the improvement of capture accuracy. Furthermore, the experimental results also show that the view intersection volume of multi-camera has a larger influence on capture accuracy compared with the depth of field range.