Real-Time Physics Simulation Forum

Re: Using btIndexedMesh with btTriangleIndexVertexArray+btBvhTriangleMeshShape

2023-11-20T00:37:53+00:00

Thank you!

That makes sense... sneaky. This language is no nuanced.

Statistics: Posted by goosesensor — Mon Nov 20, 2023 12:37 am

Re: Using btIndexedMesh with btTriangleIndexVertexArray+btBvhTriangleMeshShape

2023-11-12T19:43:47+00:00

The following code makes verts a copy of the data:

Code:

auto verts = element->vertices();

and is equivalent to:

Code:

std::vector verts = elements->vertices();

Note, the vertices() method is declared const (it does not modify the class instance's data members) which is why it returns a const reference, and since the local variable being assigned is not a const reference... a copy happens. And you would be able to modify the contents of verts without offending the compiler.

This will make verts a const reference to the data:

Code:

const auto& verts = elements->vertices();

and is equivalent to:

Code:

const std::vector& verts = elements->vertices();

Under the hood verts is effectively a pointer, but the syntactic sugar of the "const reference" feature of C++ obscures that fact.

Statistics: Posted by drleviathan — Sun Nov 12, 2023 7:43 pm

Re: Using btIndexedMesh with btTriangleIndexVertexArray+btBvhTriangleMeshShape

2023-10-31T19:19:50+00:00

Here is what I have found.

Recall my face/triangle and vertex data are declared in the GeometryElement class interface as:

Code:

std::vector_vertices;std::vector_faces;const std::vector& vertices() const;const std::vector&faces() const;

I find that if I reference the base array addresses in either of these ways, it works:

Code:

auto vertsBase = element->vertices().data();auto facesBase = element->faces().data();// - OR --auto vertsBase = &element->vertices()[0];auto facesBase = &element->faces()[0];auto indexedMesh = make_shared();indexedMesh->m_numTriangles = (int)element->faces().size();indexedMesh->m_triangleIndexBase = (const unsigned char *)facesBase;indexedMesh->m_triangleIndexStride = sizeof(Face);indexedMesh->m_numVertices = (int)element->vertices().size();indexedMesh->m_vertexBase = (const unsigned char *)vertsBase;indexedMesh->m_vertexStride = sizeof(Vertex);indexedMesh->m_vertexType = PHY_FLOAT;

Yet if I reference them like this, the vector contents appears to be copied and disappears off the stack once scope of the iteration is lost, causing btBvhTriangleMeshShape's constructor to fail.

Code:

auto verts = element->vertices();auto faces = element->faces();auto vertsBase = verts.data();auto facesBase = faces.data();

Sorry to turn this topic into my own personal C++ tutoring session, but clearly there is something about C++ references I am not understanding. Can someone point out what is going on?

Statistics: Posted by goosesensor — Tue Oct 31, 2023 7:19 pm

Using btIndexedMesh with btTriangleIndexVertexArray+btBvhTriangleMeshShape

2023-10-29T17:19:54+00:00

I am trying to create static physics bodies using btBvhTriangleMeshShape.

This works fine if I copy vertex data into btTriangleMesh, and use that to create a btBvhTriangleMeshShape. However, copying is obviously not desirable.

The approach I am currently trying is:

Describe vertex layout using btIndexedMesh referencing existing visual data
Create btTriangleIndexVertexArray using btIndexedMesh
Create btBvhTriangleMeshShape using btTriangleIndexVertexArray

My Face and Vertex structures:

Code:

typedef struct {int a;int b;int c;} Face;typedef struct {glm::vec3 position;glm::vec3 normal;glm::vec2 textureCoordinate;} Vertex;

In my mesh class ("GeometryElement"):

Code:

const std::vector& vertices() const;const std::vector&faces() const;

Code for creating the btIndexedMesh + btTriangleIndexVertexArray + btBvhTriangleMeshShape:

Code:

auto triangleIndexVertexArray = btTriangleIndexVertexArray();for (auto& element : geometry->elements()) {auto verts = element->vertices();auto faces = element->faces();auto indexedMesh = btIndexedMesh();indexedMesh.m_numTriangles = (int)faces.size();indexedMesh.m_triangleIndexBase = (const unsigned char *)&faces[0];indexedMesh.m_triangleIndexStride = sizeof(Face); // sizeof(Face), sizeof(int)?indexedMesh.m_indexType = PHY_INTEGER;indexedMesh.m_numVertices = (int)verts.size();indexedMesh.m_vertexBase = (const unsigned char *)&verts[0];indexedMesh.m_vertexStride = sizeof(Vertex); // sizeof(Vertex), sizeof(glm::vec3)?indexedMesh.m_vertexType = PHY_FLOAT;triangleIndexVertexArray.addIndexedMesh(indexedMesh);}auto shape = btBvhTriangleMeshShape(&triangleIndexVertexArray, true);

I am getting a segfault in btStridingMeshInterface::InternalProcessAllTriangles() when initializing the btBvhTriangleMeshShape. From the debugger gfxindextype is PHY_INTEGER, which leads me to believe it's getting hung up processing the vertex faces array.

Can anybody shed some light on this? Am I using btIndexedMesh incorrectly? Much thanks.

Statistics: Posted by goosesensor — Sun Oct 29, 2023 5:19 pm