CASIA OpenIR  > 毕业生  > 博士学位论文
真实感布料的计算模型及其应用
马光辉
学位类型工学博士
导师张晓鹏 ; 叶军涛
2017-05
学位授予单位中国科学院研究生院
学位授予地点北京
关键词过度拉伸 各项异性 应变极限 不等式优化 离散碰撞检测 服装建模 穿透修复 交互式服装设计
其他摘要

       三维布料仿真及服装建模一直是计算机图形学中的一个难点问题,经过近三十年的发展仍然存在着多个亟待解决的技术问题。本论文围绕真实感布料的各向异性的拉伸属性,以及复杂场景下的碰撞响应等问题进行了研究。论文的主要内容包括:
       提出了一种符合真实布料属性的各向异性的应变极限约束算法。对多边形网格表示的衣服模型进行仿真时一直存在过度拉伸的问题,对应变极限进行约束是常用的解决方案。根据拓扑连接关系,布料网格通常可以分为四边形额三角形网格两类不同的表示,因此也需要采用不同的应变极限约束。对于三角形网格表示的布料模型,我们提出了基于各向异性的应变极限的不等式约束的优化算法。跟已有的进行各向同性的约束方法不同,我们的方法有效地保持了剪切方向的形变自由。对于四边形构成的衣服网格模型,我们尝试使用不等式约束算法取代常用的等式约束算法,在保证仿真效果的同时,获得了更快的收敛速度。
       在不对曲面的朝向做任何假设的情况下,提出了无向曲面穿透修复的最小位移量算法。主流的碰撞解决方法是基于连续碰撞检测的,它本质上也是依赖历史信息的。我们认为绝大多数情况下,如果通过借助历史信息,通过离散碰撞检测就能够解决穿透问题。对于离散检测到的相交单元,结合物相交的历史状态,可以推断出发生了哪个碰撞事件,从而推断出如何移动相关单元中的元素使得相交状态得到化解。对于同时检测到的多个相交单元,我们会根据临近关系进行归并,然后形式化为一个最优化问题,最后统一求解修复后的顶点新位置。对顶点位置进行移动往往会造成网格局部的过度形变,对此我们在最优化的公式中增加了对边长的约束,使得顶点的移动更平滑均匀。
       针对多种碰撞类型同时存在的复杂场景,提出了基于离散碰撞检测的碰撞修复统一框架。当实体模型和软体模型之间发生了穿透,可以利用实体模型的法线来确定非法点以及形成相交模板。当无向曲面自身或者无向曲面之间发生了穿透,此时无法直接利用法线信息来确定相交模板,但如果有无相交状态的历史信息可以信赖,仍然能够确定相交模板从而进行碰撞修复。当模型法线和历史信息都无法使用的时候,可以使用全局相交分析算法形成相交模板。上述三种形成相交模板的机制由离散碰撞检测统一到一个框架中。当仿真中实体模型之间或者自身发生夹挤的时候,处于夹挤区域的布料没有合适的移动目的地,会导致仿真系统的出现不稳定和失真现象。为了保证动画运行的稳定性,将处于实体模型相交区域的非法点对应的约束函数从系统优化方程中去除;为了保证仿真过程的连贯性,将处于实体模型夹挤区域的相交模板中的非法点与夹挤区域进行绑定实现更新。
       交互式服装系统的原型实现。对交互式服装系统中的服装设计领域常用的DXF文件格式进行解析,根据解析的数据内容提取出关键点信息,将关键点信息进行整合来确定出缝合边。利用建立好的缝合边和交互缝合系统的数据接口,对从DXF提取出的二维裁片的重新进行网格化和交互缝合操作,最终生成符合物理仿真系统需要的三维服装模型。

;       3D clothing modeling has been a major research topic in computer graphics. Even after the development of nearly three decades, various technical obstacles exist and creating realistic clothing effects is still not easy. This thesis concentrates on two aspects of cloth simulation: anisotropic strain limiting, an collision handling with discrete collision detection, partcularly in a complex scene. The main constributions of this thesis include:
      An anisotropic strain-limiting algorithm with inequality constraints along the warp and weft directions. The cloth simulation systems often suffer from excessive extension on the polygonal mesh, so an additional strain-limiting process is typically used as a remedy in the simulation pipeline. A cloth model can be discretize as either a quadrilateral mesh or a triangular mesh, and their strains are measured differently. For a triangular mesh, the state-of-the-art technique measures and constrains the strains along the two principal directions in a trianglue, resulting in an isotropic behavior which prohibits shearing. Based on the framework of inequality-constrained optimization, we proposed a warp and weft strain-limiting formulation. This anisotropic model is more appropriate for textile materials that do not exhibit isotropic strain behavior. The edge-based strain-limiting method for a quadrilateral mesh creates anisotropic behavior by nature, as discretization usually aligns the edges along the warp and weft directions. We improve this anisotropic technique by replacing the traditionally used equality constraints with inequality ones in the mathematical optimization, and achieve faster convergence.
    An untangling strategy based on minimum positional displacements, for the intersections of unoriented surfaces. The popular collision response methods rely on continuous collision detection, which is history-based. If the same hard constraint is assumed as in CCD, we found the majority of collisions can be resolved by discrete detection with the help of history information. Instead of detecting all collision events happening during interal, ours MPD detects all edge-face intersections at the end of interval via DCD algorithm, and inters the collisions events that cause the intersections. At the same time, collision normals are determined and stencils are identified. Minimum displacements are computed so that stencil vertices are relocated to a collision-free status. To overcome the excessive elongation due to vertex adjustment, edge strain limits are integrated into the optimization process as constraints, so that the displacement are propagated smoothly.
    A framework unified under discrete collision detection, for resolving various types of penetrations within compltex scenes. For the intersections between a soft and solid model, we use the predefined normal of the solid model to decide the illegal vertices and the direction for their movement. For the collision and self-collision between soft models, we use a history-dependent method to form penetration stencils. When neither the predefined normal nor history information exist, we use global intersection analysis to form stencils. The pinching happens when the solid model intersect each other during animation. To get a stable animation, we use a robust skinning-like method to bind the garment vertices on the pinched area of the solid model.
    Interactive garment design system. First, we parse DXF file format, which is widely used in the fashion industry, to extract key points on the outline of the panels. Then, we use the key points to form sewing edges. By using our interactive sewing system, we can convert 2D panels to 3D models.
语种中文
文献类型学位论文
条目标识符http://ir.ia.ac.cn/handle/173211/14696
专题毕业生_博士学位论文
作者单位中国科学院自动化研究所
推荐引用方式
GB/T 7714
马光辉. 真实感布料的计算模型及其应用[D]. 北京. 中国科学院研究生院,2017.
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