本研究致力於結構參數檔(Structure file)之研究，其主要為配合本實驗室開發之智慧型人體運動處理系統(iBMPS, intelligent Body Motion Processing System)所規範定義之輸入檔案。而如何配合本實驗室所使用之人體幾何模型(Body Geometric Model)及人體運動模型(Body Kinematic Model)規範定義出其鏈狀結構、桿件位置及方向、桿件之間自由度之關係以及關節運動範圍限制即為本研究所要探討之重點。
除了結構參數檔之架構規範外，本文亦有針對不同鏈狀結構之人體、非人體及人體使用工具之情形，配合相對應的幾何模型及運動模型或針對該生物之運動特性以及運動之合理性以撰寫出不同的結構參數檔，並且讀入iBMPS驗證結構參數檔之可行性及通用性。
本文亦有針對人體模型之外觀上作美化之處理，包含了人體桿件之材質貼圖以還原人體真實顏色之面貌，並且嘗試各種布料材質之貼圖以呈現出人體著衣之效果。而除此之外，人體連桿之平滑著色亦是本研究所要探討之重點，配合著Phong著色法之原理、不同連桿結構之頂點法向量演算法以及OpenGL向量線性插補之功能，即可完成人體模型表面之平滑化處理。

關鍵詞：運動捕捉、人體、檔案格式、鏈狀結構

A Study of Structure File for Motion Chains in iBMPS

Wei-An Lin
Ming-June Tsai
National Cheng Kung University Department of Mechanical Engineering

SUMMARY
We are committed to the research of Structure file, which is the input file of iBMPS(intelligent Body Motion Processing System) our laboratory develop. And how to define the chain structure, position and direction of links, degrees of freedom between links and joint motion range of our Body Geometric Model and Body Kinematic Model is the main point of our research.
In addition to the specification of Structure file, in this article, we also discuss different chain structure of human body, non-human body and human body with tools. We can write different Structure file of these different chain structure according to the corresponding body geometric model and body kinematic model or the motion characteristics or rationality of the subjects, and also load Structure file into iBMPS to verify its feasibility and versatility.
In this article, we also beautify the appearance of human model, including texture mapping of each human body link to return the true facial color of human body and trying various fabric material to render the effect of human body in clothes. Besides, smooth shading of each human body link is also the main point of our research. We can accomplish smooth shading of the appearance of human model by principle of Phong shading, algorithm of vertices normal computation of different link structure and linear interpolation of vector in OpenGL.

Keywords：Motion capture, Human body, File format, Hierarchical structure

INTRODUCTION

Nowaday there are many motion capture file formats developed, both of simulating 3D human body animation and motion analysis rely on Structure file for the adjustment of human body skeleton and motion parameter. And iBMPS which our laboratory develop has the function of motion tracking, motion analysis and motion replication. And how to develop Structure file for iBMPS is the main point of our research, containing the following items, definition of chain structure, degree of freedom between links and joint motion range, all of them are for the replication of human’s various postures.

MATERIALS AND METHODS

Body model in the iBMPS is created in according to the read in structural file specification. The hierarchical structure with links and joints of the body are specified in this parametric file. For body model with ideal joint axes, every joint axis should lie on its Z-axis of the local geometrical frame. Every link should have its own geometric files. The read in parametric files have two types: robot structure file and the geometry files. The structure file’s name is the same as the robot’s name, and with “stc” as the postfix to identify the file type; It describes the tree structure of the robot, its first line contains the name of the robot, the number of chain (variable name: Nchain), and the number of joint freedom in each chain respectively. The 2nd line (and so on) contains the links and joint freedoms in the first (and so on) chain starting from the base to the distal link. If a link name appears more than once, this link should be the base link of other chains. Each link name appears in the .stc file should have its corresponding .stl file to describe its body geometry. The joint freedom of a multiple degree-of-freedom link is specified after the link name. Notice that every physical link should have its name appear in the .stc file no matter how many joint freedom it has.
The link’s transformation data should contain the link name and the transformation parameters, written in order as: (Link_name, yaw, pitch, roll, x, y, z). Angle in degree, translation in mm. Starting from line (Nchain+6), the orders of the link should be the same as the orders appear in the “.stc” file (from line 2-6, excepting the chain name and duplicated link name). After read in the transformation parameters, the transformation is conversed to dual quaternion format and stored as L2O. For multi-freedoms link, the real link geometry (BDP) is attached on the last joint freedom of the link. Then the virtual link name (joint axis only) is given by the following lines (line #chain+#link+7) in the .stc file.
The last portion of .stc file contains joint information. This part of file has 9 records in each line, which are joint name, motion range (θMin, θMax), servo amplifier (PWMMin, PWMMax), and D&H parameters (twist angle, link length, joint angle, offset). If the axis is assigned on the z-axis of geometry frame, the D&H parameter can be computed by iBMPS automatically, the last 4 parameters can be leaving blank. So that usually for a human body, the joints are computed automatically, only the first 3 records are given. The name of joint should be the same as that appears in the “.stc” file.

RESULTS AND DISCUSSION

We compare Structure file with other file formats, such as BVH, the definition of Structure file is more complete than BVH, but Structure file doesn’t contain motion data. For this part, we should consider how to combine Structure file with BMP which is the motion file our laboratory develop.

CONCLUSION

The most important achievement of our research is specification, format and writing rules of Structure file, and also load Structure file into iBMPS to verify its feasibility and versatility. Once the structure, format and writing rules of Structure file have been clearly specified, no matter what chain structure of creature or non-creature, we can establish their geometric file, get the transformation matrix of each link’s coordinate system relative to world coordinate system by CAD software, and specify degree of freedom of links and joint motion range according to reasonable motion limit of creature or uses’ needs, and then we can read in Structure file into iBMPS normally to do the teaching of postures.

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