人體皮膚形變技術之研究

Abstract

在最近十年來，由於電腦角色動畫的發展，使得逼真的3D人體動態模型成為一個重要的研究方向。我們主要的模擬概念是以外形近似為主，進而加入人體解剖學的分析，以便產生一個更加真實的人體模型。以解剖學為基礎的人體模擬方式，包含骨骼、肌肉、脂肪和皮膚。其中人體運動過程中以肌肉形變最為明顯，進而影響人體的表皮外觀。故在模擬人體運動形變後的外型中，以肌肉扮演主要的控制角色。在本論文中，我們所提出的作法主要是以基本的幾何原件做為人體骨骼的建構，由於骨骼主要是用來控制人體的運動，故採用簡單的幾何原件可以簡化控制點的運作方式。而人體肌肉部分則以變形的圓柱體為主要的建構方式，至於較為複雜的肌肉外型則採用多個變形的圓柱體組合而成。而肌肉在形變的過程中，主要是利用體積不變的原則來做控制，由此我們導出一個簡化的數學函式，來加速肌肉形變過程中的運算。最後，我們建構皮膚及模擬皮膚因肌肉形變引起的形變。基本上脂肪與皮膚的建構則是採用內部肌肉組織的外型當作參考取樣出皮膚外型的參考點。同時我們將皮膚的參考點與下層組織建立一個對應的關係，如此在人體運動的過程中，就可以利用此對應關係，隨著肌肉的形變，動態的產生表皮的外觀控制點，進而利用這些控制點來當作NURBS曲面的控制點，而形成人體最後的表皮外形。

The modeling and animation of virtual human have become a very important topic in applications of character animation and virtual reality. Different from previous methods, anatomy-based approach provides us with a natural and effective way of modeling the human body and its deformation resulting from the motion. The approach usually consists of bone layer, muscle layer, fat layer, and skin layer, in which the most important layer is the muscle layer because the shape change is the direct result of the muscle deformation. In this project, bones are designed with simple geometry primitives, muscles are modeled by using the deformable cylinders formed by a set of cross-sections, and finally the fat and skin are approximated by offsetting the boundary of a group of the underlying bone and muscles. Such approximation is represented by NURBS surfaces instead of meshes for the sake of smooth shading. A simple mathematical formula has been proposed to model the muscle deformation based on the property of volume preserving. The skin deformation due to the muscle deformation is effectively computed by utilizing the correspondence between skin’s control points and the points on the underlying muscle or bone. Such a correspondence is built in preprocessing time and has proven its effectiveness in our experiments. Adjustment based on interpolation is also proposed to prevent the skin from self-penetration or holes, and hence provide a smooth NURBS skin patch.