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Physx之解决三角形顶点相近卡住的问题

 5 years ago
source link: http://pkxpp.github.io/2019/11/13/PhysX之解决三角形顶点相近卡住的问题/
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今天遇到一个问题，角色卡在一个模型边上，在PVD看模型也比较正常，一直调试跟踪，最终找到了问题所在。

原因

1.问题出在最后一个sweepCapsuleTriangles函数里面，代码如下：

#ifdef __SPU__
				Vec3V dummy1, dummy2;
				FloatV result = distanceSegmentTriangleSquared(
					V3LoadU(capsule.p0), V3LoadU(capsule.p1), V3LoadU(p0), V3LoadU(p1), V3LoadU(p2), dummy1, dummy2);
				PxReal dist2 = FStore(result);
	#else
				// AP: switching to SIMD version produced -25% regression in Sweep*SphereHfld
				PxReal dist2 = distanceSegmentTriangleSquared(capsule.p0, segmentExtent, p0, p1 - p0, p2 - p0);
	#endif
				if (dist2<=r2)
				{
					hitIndex    = i;
					t            = 0.0f;
					normal        = -unitDir;
					outFlags    = PxHitFlag::eDISTANCE|PxHitFlag::eNORMAL;
					_triNormal    = denormalizedNormal.getNormalized();
					return true;
				}

当p0、p1和p2有有两个点相同的时候，会导致p1-p0和p2-p0其中一个向量为0，得到的dist2为负无穷大，所以就会执行代码中的条件语句。最终结果是move函数移动成功了，但是只移动了0的位移，表现上就是一直卡在那个地方。

  1. 至于顶点一样的情况，其实并不是完全一样，而是有很小的差异。再加上浮点数运算会丢失精度，这个很小的差异被抹掉了，所以到上面代码的时候就出现相同的点了。

计算的代码位置(没看太明白)：

PX_FORCE_INLINE void Gu::TriangleMesh::computeWorldTriangle(PxTriangle& worldTri, PxTriangleID triangleIndex, const Cm::Matrix34& worldMatrix, PxU32* PX_RESTRICT vertexIndices, PxU32* PX_RESTRICT adjacencyIndices) const
	{
		PxU32 vref0, vref1, vref2;
		if(mMesh.has16BitIndices())
		{
			const Gu::TriangleT<PxU16>& T = ((const Gu::TriangleT<PxU16>*)getTrianglesFast())[triangleIndex];
			vref0 = T.v[0];
			vref1 = T.v[1];
			vref2 = T.v[2];
		}
		else
		{
			const Gu::TriangleT<PxU32>& T = ((const Gu::TriangleT<PxU32>*)getTrianglesFast())[triangleIndex];
			vref0 = T.v[0];
			vref1 = T.v[1];
			vref2 = T.v[2];
		}
		const PxVec3* PX_RESTRICT vertices = getVerticesFast();
		worldTri.verts[0] = worldMatrix.transform(vertices[vref0]);
		worldTri.verts[1] = worldMatrix.transform(vertices[vref1]);
		worldTri.verts[2] = worldMatrix.transform(vertices[vref2]);
	
		if(vertexIndices)
		{
			vertexIndices[0] = vref0;
			vertexIndices[1] = vref1;
			vertexIndices[2] = vref2;
		}

		if(adjacencyIndices)
		{
			if(mMesh.getAdjacencies())
			{
				adjacencyIndices[0] = mMesh.getAdjacencies()[triangleIndex*3 + 0];
				adjacencyIndices[1] = mMesh.getAdjacencies()[triangleIndex*3 + 1];
				adjacencyIndices[2] = mMesh.getAdjacencies()[triangleIndex*3 + 2];
			}
			else
			{
				adjacencyIndices[0] = 0xffffffff;
				adjacencyIndices[1] = 0xffffffff;
				adjacencyIndices[2] = 0xffffffff;
			}
		}
	}
  • 问题数据

出现问题的时候，断点的数据如下：

vertices[vref0] = {x = -4.10000086, y = -0.200000167, z = -3.56512594}
	vertices[vref1] = {x = -4.10000086, y = -0.200000077, z = -3.56512594}
	vertices[vref2] = {x = -4.10000086, y = -0.200000212, z = -4.10000229}

	worldMatrix.transform.base3 = {x=72.3379974 y=4.76143503 z=18.5843773 }

	// 得到这个三角形前两个点完全一样
	worldTri.verts[0] = { x = 68.237990, y = 4.56143475, z = 15.0192509}
	worldTri.verts[1] = { x = 68.237990, y = 4.56143475, z = 15.0192509}
	worldTri.verts[2] = { x = 68.237990, y = 4.56143475, z = 14.4843750}

简单来说就是：

4.76143503 + -0.200000167 = 4.56143475
	4.76143503 + -0.200000077 = 4.56143475

至于为什么请参考另外一篇文章[1] 为什单精度浮点数的有效位时7位

解决

  • 1.把顶点传到PhysX之前，先检查一遍。可以删掉重新组织Vertex和Index，我简单处理的方法是把两个顶点很近的时候，标记为同一个索引，这样的三角形比较容易标识 
    if (fMinDist <= 1e-5)
              {
                  // set the same index, and then cooking will process it. eREMOVE_DUPLICATED_TRIANGLES
                  pIndices[3 * i + nIndexIndex1] = pIndices[3 * i + nIndexIndex2];
              }
  • 2.PhysxCooking的参数增加去重的Flag，这样上面的三角形就会被PhysX剔除，从而就解决了这样的问题：如下
physx::PxCookingParams cookingParams(scale);
		cookingParams.buildTriangleAdjacencies = true;
		cookingParams.meshPreprocessParams = PxMeshPreprocessingFlags(PxMeshPreprocessingFlag::eWELD_VERTICES |
			PxMeshPreprocessingFlag::eREMOVE_UNREFERENCED_VERTICES | PxMeshPreprocessingFlag::eREMOVE_DUPLICATED_TRIANGLES);

参考

[1] 为什么单精度浮点数的精度是7位


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