# Pose Measurement Method Based on Machine Vision and Novel Directional Target
## 3 Methods  
### Target Topology Design and Image Matching  
Existing cooperative targets are mainly divided into checkerboard grid targets and circular targets. To further improve the accuracy of target feature point coordinate extraction and avoid the ambiguity caused by rotation, this paper combines the advantages of the two kinds of target markers and designs a new cooperative target marker, as shown in Figure 2a. It only consists of circles and rings as the target's design pattern. The target's approximate spatial pose is determined according to the special positional relationship of the five rings while avoiding the ambiguity caused by the target after 180-degree rotation. According to the application requirements, the target is easy to retrofit for use, the relative pose of the rings remains unchanged, and the corresponding sorting logic can be combined to be compatible with target feature points of different dimensions, as shown in Figure 2b, which can be changed to a \(7\times 7\) feature point arrangement. To further improve the accuracy and increase the number of circles based on the limited space design, the experiments in this paper, all shown in Figure 2a, are used \(21\times 21\). The new type of cooperative target has the size of 220 mm \(\times\) 220 mm. In the overall measurement system, feature point matching with its image is significant. According to the characteristics of the new type of target marker feature point sorting, this paper proposes an algorithm that can quickly and accurately match feature points with their images. The algorithm can accurately number the feature points on the target and the feature points in the picture and then correspond to them individually, providing an adequate data basis for the subsequent coordinate solving. As in Figure 3, the feature points on the target mark are sorted and then connected two by two in order:  
The sorting logic implemented is as follows:  
1. The computer processes the image to obtain the coordinates of the feature points in the image plane \(p_{i}=(x_{i},y_{i})\), and the corresponding grey values of each feature point in the binarised image \(C_{i}\), \(i=[1,441]\). 2. Based on the new target's unique circular feature point design, the grey value 255 is five particular circular coordinate points. As in Figure 3, the five particular coordinate points are recorded as Point 1, Point 2, Point 21, Point 421, and Point 441, and at this time, the ordering of the coordinates of the five particular feature points is unknown. 3. The points are processesed two by two to obtain the Euclidean distance; to meet the distance nearest condition of the two characteristics of the point--that is, Point 1  
Figure 3: Matching of feature points with corresponding image point images. Figure 2: Novel cooperative targets. (**a**) \(21\times 21\) new co-operative targets, (**b**) \(7\times 7\) new co-operative targets. and Point 2, at this time only two relative relationships--the specific correspondence is unknown. 4. Points 1 and 2 define a straight line, and the closest point of the remaining 3 points to this line is Point 21. 5. Point 1 and 2 are each distanced from Point 21, the point closest to Point 21 is Point 2, and the point furthest away is Point 1. 6. The remaining two points are made as a line segment, and this segment's midpoint is noted as Point Z. A vector is made from Point 1 to Point Z. Since the positive direction of the y-axis is downward in the pixel coordinate system if the point lies to the left of the vector, it is Point 441. Otherwise, the point is Point 421. At this point, the order of the coordinates of the feature points of the five particular circles is determined. 7. Point 1 and Point 421 are used to find a straight line, according to the perpendicular distance from the point to the straight line and the size of the space from Point 1 to obtain the correct order of the Point 1 and Point 421 columns of all the points as well as the correct order of Point 21 and Point 441 between the columns of all the facts on the order.