Sag-Free Initialization for Strand-Based Hybrid Hair Simulation
Sag-Free Initialization for Strand-Based Hybrid Hair Simulation
复制标题
DOI：
10.1145/3592143
复制
发表时间：
2023-07
期刊：
影响因子：
--
通讯作者：
J. Hsu;Tongtong Wang;Zherong Pan;Xifeng Gao;Cem Yuksel;Kui Wu
中科院分区：
文献类型：
--
作者：
J. Hsu;Tongtong Wang;Zherong Pan;Xifeng Gao;Cem Yuksel;Kui Wu
基于拉格朗日/欧拉混合链的头发模拟技术已迅速成为VFX和实时图形应用中的流行方法。利用拉格朗日毛发动力学，使用连续体方法在欧拉网格中解决毛发间接触，即，MPM方案与颗粒Drucker-Prager流变学，以避免昂贵的碰撞检测和处理。这种模糊的碰撞处理使创作过程变得非常容易。然而，尽管当前的头发梳理工具为这种模拟方法提供了广泛的基于股的建模工具，但关键的无下垂初始化功能仍然经常被忽视。因此，当模拟开始时，重力会导致任何艺术发型下垂并变形为非预期和不期望的形状。本文提出了一种新的四阶段无下垂初始化框架，以解决稳定的准静态配置的混合股为基础的头发动力系统。这四个阶段分为两个全局-局部对。第一种方法通过附加的不等式约束确保每个欧拉网格节点处的静力平衡，以防止应力离开屈服面。然后，我们推导出几个相关的封闭形式的解决方案，在本地阶段计算段静止长度，方向和粒子变形梯度并行。第二个全局-局部步骤沿沿着每个发束求解，以确保所有弯曲和扭曲约束在每个头发片段上产生零净扭矩，然后执行局部步骤，将其余达布向量调整为单位四元数。我们还介绍了一个必要的修改达布矢量，以消除在初始化和模拟的Cosserat棒休息姿势的模糊性。我们评估了我们的方法在广泛的发型，我们的方法可以只需要几秒钟到几分钟，以获得其余的准静态配置数百头发束。我们的结果表明，我们的方法成功地防止下垂，并在模拟过程中对头发运动的影响最小。
Lagrangian/Eulerian hybrid strand-based hair simulation techniques have quickly become a popular approach in VFX and real-time graphics applications. With Lagrangian hair dynamics, the inter-hair contacts are resolved in the Eulerian grid using the continuum method, i.e., the MPM scheme with the granular Drucker-Prager rheology, to avoid expensive collision detection and handling. This fuzzy collision handling makes the authoring process significantly easier. However, although current hair grooming tools provide a wide range of strand-based modeling tools for this simulation approach, the crucial sag-free initialization functionality remains often ignored. Thus, when the simulation starts, gravity would cause any artistic hairstyle to sag and deform into unintended and undesirable shapes. This paper proposes a novel four-stage sag-free initialization framework to solve stable quasistatic configurations for hybrid strand-based hair dynamic systems. These four stages are split into two global-local pairs. The first one ensures static equilibrium at every Eulerian grid node with additional inequality constraints to prevent stress from exiting the yielding surface. We then derive several associated closed-form solutions in the local stage to compute segment rest lengths, orientations, and particle deformation gradients in parallel. The second global-local step solves along each hair strand to ensure all the bend and twist constraints produce zero net torque on every hair segment, followed by a local step to adjust the rest Darboux vectors to a unit quaternion. We also introduce an essential modification for the Darboux vector to eliminate the ambiguity of the Cosserat rod rest pose in both initialization and simulation. We evaluate our method on a wide range of hairstyles, and our approach can only take a few seconds to minutes to get the rest quasistatic configurations for hundreds of hair strands. Our results show that our method successfully prevents sagging and has minimal impact on the hair motion during simulation.