# The Method of Multi-Camera Layout in Motion Capture System for Diverse Small Animals
## 2 The Parameters of Multi-Camera System  
The cuboid is one of the most basic geometric shapes. Small animals of different shapes can be abstractly simplified by the smallest outer enveloping cuboids (length \(L\), width \(W\), height \(H\)) which can completely capture the spatial distribution of the animal (Figure 1), using different ratios of width to length and height to length. It can be determined from this visual principle that when the object is simultaneously photographed by at least two cameras, the position information of the object in the defined space can be obtained. The motion information of the object can be acquired when a sufficient number of position images are recorded in a time sequence [30]. The number of cameras used is one of the key variable parameters in a motion capture system. Different camera numbers are associated with different requirements for accuracy and precision of the motion capture. From the perspective of stereoscopic vision, the ordinary monocular camera generally needs to move to provide depth information and the binocular vision system are often obscured. A trinocular camera system, or a multi-camera system consisting of more than three cameras, can effectively address the occlusion problem based on the third camera and the parallax of a single pixel can be calculated many times to make the error small, since the parallax of a sub-pixel is more reliable [31]. From the point of view of a biological motion capture system, when \(N\) is 1, the researchers generally use 2D image matching with mirror imaging to obtain three-dimensional positions, although the accuracy is not high and the visual acuity is poor. In the case when \(N\) is 2, a binocular stereoscopic vision system is generally used to obtain 2D images on two planes. When \(N\) is 3, motion capture is very difficult because it is necessary to ensure that the captured objects are in  
Figure 1: The smallest outer enveloping cuboids (length—\(l\), width—\(w\), height—\(h\)) that completely envelop the animals’ body (the ratio of the width to the length and the ratio of the height to the length of the smallest outer enveloping cuboids of the small animal are in the northwestern corner of the figure). the intersecting area of a certain common field of view, arbitrarily formed by at least two cameras. In addition, according to a previous study, three high-speed cameras within an orthogonal position were used to successfully capture the flight of mosquitoes in a glass room with a total volume of 0.37 m\({}^{3}\)[15]. The layout of these three cameras is orthogonal and the layout pattern of the trinocular camera system is relatively simplified and does not have good scalability. When \(N\) is 4, any three of the cameras can be selected to undertake the task of capturing and the other camera is used as the supplementary unit. The capturing task cameras capture the object and the supplementary camera records the multi-dimensional video. As such, this system is able to not only improve the success rate but also increase the interest by visual images. In summary, four cameras can form a self-complementary basic unit module. When \(N\) is greater than 4, this can be regarded as the expansion of the four-camera unit module. In a multi-camera motion capture system, the union of the intersecting volume of any two cameras can be used as a sampling area for motion capture. With the aim of motion capture for different types of small animals, the success rate and the accuracy rate are prime considerations. Secondly, the load capacity of the hardware system and the economy of the equipment system should also be taken into consideration. As such, we chose the minimum number of cameras to capture the motion of small animals, which can greatly improve the efficiency and cost performance of the motion capture system. Therefore, the theoretical calculations and experimental verification were carried out on a unit module consisting of four cameras.