# An Initial Dot Encoding Scheme with Significantly Improved Robustness and Numbers
## 1 Introduction  
Vision measurement with its advantages of non-contact, high precision, high efficiency and good stability has a wide range of applications in large field-of-view measurements [1; 2]. Mark points, as important parts of machine vision, play an important role in visual measurement. For a large field of view, encoding the marker points can effectively distinguish each other and can complete automatic matching [3; 4; 5]. However, most existing coded mark points schemes have two problems such as insufficient coding and low robustness in a large field of view. Therefore, it is necessary to design a coding scheme which can satisfy the coding number and satisfy the decoding stability to meet the measurement requirements under a large field of view [6]. At this stage, the coded mark points have a good application prospect in measurement [7; 8]. [PERSON] applied the coded mark points to the automatic parking system to match the related information [9]. In building up 3D measurement model for large scale object, the using of coded mark points not only solves the cumulative error of the stitching, but also greatly reduces the difficulty of matching the non-coded points [10; 11]. Based on the stability of the marker point recognition, some people use the coded marker point on the measurement of the attitude angle of the drone [12]. At the same time, some people used a special coding pattern to design a target for underwater [13]. The coded marks are divided into three groups according to the features of the graph: dotted distribution, concentric ring and digital symbol. The most typical dotted distribution coded mark point is Hattori coded scheme which adopted by V-STARS system(Video-Simultaneous Triangulation and Resection System) [13]. In this scheme, the three outer circles are used to confirm the coordinatesystem of the coded mark points, and the three small circles in the middle are used to encode the mark points. Accordingly, the advantage of this scheme is that the mark points are simple, and only the circular patterns are identified as features, and it has been applied to industrial production widely. In the case of large field of view, due to the influence of affine transformation, the central point and encoded points of Hattori coded scheme eventually affect the stability of decoding and reconstruction results. Based on this plan, [PERSON] designed an error correcting coded point scheme [14]. With the support of error correcting points, the system has higher stability in recognition. While, the location of the image is affected by the method of edge detection [15]. At the same time, the decoding number is affected by the angle, and the code error rate is prone to occur when the number of coded points are different [16]. [PERSON] designed a point coordinate encoding which based on cross-ratio invariance [17]. Based on the basis of dotted distribution coded mark, through the characteristic of cross ratio, this scheme provides robustness of decoding. But, this scheme is still limited by the number of codes. In [PERSON]'s scheme, when the number of bits is 20, the maximum possible number of codes can only reach 900. As a classical method of the concentric ring, the Schneider coded mark is widely spread by GOM company from German [18], which consists of a dot and surrounding rings. Therefore, the advantage of the Schneider coded scheme is simple in construction and easy to recognize. Due to the limitation of structure, the number of coding points is much less than the point distribution under the same number of code bits, so it is difficult to afford the needs of a large field of view. [PERSON] and [PERSON] designed a square coded mark which was applied in underwater calibration [13]. Each coded target consists of a central dot, surrounded by a different combination of sides that may be any of the six segments, and then each coded target must be rotationally unique to all other coded targets. Therefore, 1296 kinds of coded values can be obtained at most.