FEMFX 仿真流程之 FmTaskFuncStepVelocityRebuildBvh

FmTaskFuncStepVelocityRebuildBvh 在 FEMFX/amd_femfx/src/Simulation/FEMFXSimulate.cpp 中定义，主要完成了仿真中（位置）速度的计算、bvh的更新等工作。

感觉该部分的主要内容为 FmTaskFuncTetMeshStepVelocityRebuildBvh 和 FmTaskFuncRbStepVelocityRebuildBvh 两部分。

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其中，FmTaskFuncTetMeshStepVelocityRebuildBvh 中的内容为软体的计算。

注：软体也区分软体对象是否为 KINEMATIC 对象。（KINEMATIC 对象意味着该对象的运动是由外界驱动的，也就是说，根据外界输入，直接设定该对象的运动状态、轨迹）

主要内容为：

FmStepVelocityImplicitEuler(scene, tetMesh, solverData, gravityVector, kRayleighMassDamping, kRayleighStiffnessDamping, tetMesh->extForceSpeedLimit, timestep, scene->params.epsilonCg);

FmBuildHierarchy(tetMesh, timestep, aabbPadding);

FmAddDeformationConstraints(tetMesh, constraintsBuffer, timestep);

具体为：
FmStepVelocityImplicitEuler 为求解步骤：

// Take a simulation step using current dynamic state, external forces, constraints set in tetMeshState.
// Also resets vertex flag FM_VERT_FLAG_FRACTURED and tetQuatSum

FmBuildHierarchy 为建立等级：

// For CCD, must build after integration computes new velocities.

FmAddDeformationConstraints 为添加形变约束：

// Add constraints to prevent excessive deformation.
// Expects that unconstrained end-of-step velocities have been updated.
// There are up to three constraints per tetrahedron, and using the same 3D constraints and 3x3 Jacobian submatrices as for contacts and glue.
// This allows some benefit from SIMD but wastes space when fewer dimensions are needed.
// Reference: Perez et al., "Strain Limiting for Soft Finger Contact Simulation"

具体的内容，还要进一步分析。

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另外，FmTaskFuncRbStepVelocityRebuildBvh 中基本就是计算刚体在无约束（重力作用）下的运动，计算速度、位置之类的，顺便把碰撞对象 aabb 等更新一下。