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Arbitrary Mesh - how to create collision shape?
Posted: Tue Apr 12, 2016 10:48 pm
by monty_hall
Suppose I have some closed blob mesh (non selft intersecting and possibly non convex) that I created in blender. How do I create a collision shape for it using the *exact* triangles that make it up? btGImpactShape?
Thanks,
Monty
Re: Arbitrary Mesh - how to create collision shape?
Posted: Wed Apr 13, 2016 11:28 am
by Flix
1) btGImpactShape
2) Convex decomposition: decompose the mesh into convex child shapes of a btCompoundShape (or of a btGImpactConvexDecompositionShape that's probably deprecated...). Known algorithms:
-> V-HACD:
https://github.com/kmammou/v-hacd (good for closed (=waterproof) meshes) -> includes a Blender plugin
-> HACD:
https://sourceforge.net/projects/hacd (good for other meshes)
See also the Bullet ConvexDecompositionDemo.
Please note that convex decomposition is used in every physic library (well, at least most of them).
Re: Arbitrary Mesh - how to create collision shape?
Posted: Wed Apr 13, 2016 3:18 pm
by monty_hall
Do you know where I can find ConvexDecompositionDemo.cpp?
I see it here "
https://bullet.googlecode.com/svn/trunk ... onDemo.cpp" But I cannot seem to find it in my git repo in "origin/master". Further it would appear that the web page is pointing to some subversion repo.
Thanks,
M
Re: Arbitrary Mesh - how to create collision shape?
Posted: Wed Apr 13, 2016 7:28 pm
by Flix
Yes, that demo has not been ported to the Bullet3 package...
However, there's still a libConvexDecomposition (that is built by default and it's based on John Ratcliff's work) and there are still the two files: btGImpactConvexDecompositionShape.h btGImpactConvexDecompositionShape.cpp: AFAIR they were used in the GImpact demo, but they needed a change in a preprocessor definition to use btGImpactConvexDecompositionShape instead of btGImpactMeshShape.
The problem is that I think that the GImpact demo is missing from Bullet3 too, but I'm sure you can retrieve it somewhere...
Alternatively, if you want to use HACD (not V-HACD), you may try and see if my OLD header-only helper file still works, but you need to clone the HACD library from the link I posted, the SVN repository, and you can't use the one that used to be bundled with Bullet2 (... hoping that it'is still compatible):
btHACDCompoundShape.h
Hope it helps.
Re: Arbitrary Mesh - how to create collision shape?
Posted: Thu Apr 14, 2016 2:46 pm
by Evan407
You have to make an IndexedMesh so you can make a TriangleIndexVertexArray to load in the geometry. Here is some code I use for jBullet
Code: Select all
BvhTriangleMeshShape createBvhTriangleMeshShape(){
ByteBuffer triangleIndexBase = BufferUtils.createByteBuffer(templateFaces.length*3*4);//number of triangles * 3 verticies * 4 bytes
ByteBuffer vertexBase = BufferUtils.createByteBuffer(points.length * 3 * 4);//vertecies * 3 dimensions * 4 bytes
IntBuffer triangleIndexBaseIB = triangleIndexBase.asIntBuffer();
FloatBuffer vertexBaseFB = vertexBase.asFloatBuffer();
for(TemplateFace f : templateFaces){
triangleIndexBaseIB.put(f.p1Number);//the index number
triangleIndexBaseIB.put(f.p2Number);//points.indexOf(f.p2)
triangleIndexBaseIB.put(f.p3Number);
}
for(Point p : points){
vertexBaseFB.put(p.x * GameLogic.DEVERSION); //"deversion" is the inverse conversion i know it's silly
vertexBaseFB.put(p.y * GameLogic.DEVERSION);
vertexBaseFB.put(-p.z * GameLogic.DEVERSION);
}
IndexedMesh indexedMesh = new IndexedMesh();
indexedMesh.numTriangles = templateFaces.length;
indexedMesh.numVertices = points.length;
indexedMesh.triangleIndexBase = triangleIndexBase;
indexedMesh.triangleIndexStride = 12;
indexedMesh.vertexBase = vertexBase;
indexedMesh.vertexStride = 12;
TriangleIndexVertexArray triangleIndexVertexArray = new TriangleIndexVertexArray();
triangleIndexVertexArray.addIndexedMesh(indexedMesh);
return new BvhTriangleMeshShape(triangleIndexVertexArray,true);
}
The BvhTriangleMeshShape is for static world objects. For making a GImpactMeshShape you'll still need the TriangleIndexVertexArray.
Code: Select all
new GImpactMeshShape(triangleIndexVertexArray)