[ODE] Trimesh Sphere Interactions - how to break 'test_trimesh.cpp'

[Megan Fox]

I dropped my trimesh heightmap in and it worked on the first try -
same code worked for arbitrary meshes as well, extremely solid.  I
would be curious what you consider a small stepsize (try 0.01), and I
would also be curious what scale your forces are and what scale your
objects are vs the trimesh triangles.

Winding Order - affects the generation of the normals from the trimesh
data.  So, you need the same winding order on all your triangles (I
believe you need counter-clockwise winding order, but if that doesn't
work, just flip the mesh or try it the other way).

Normals - I ended up letting ODE generate the normals for me.  As I
said, they're influenced by the winding order of the polygon.  If you
don't wind all the triangles in the same way, your normals will point
in different directions.


>From the results you're getting, it sounds as if your winding might be
problematic, but I really can't say going from just one email posting.

-Megan Fox

On 5/16/05, Alex Green <alexg at acfr.usyd.edu.au> wrote:
> My simulation has a vehicle with spherical wheels and a trimesh terrain.
> I have had a lot of trouble getting consistent results with the trimesh
> sphere collisions. My stepsize is small and my erp is fine. Trimesh
> sphere collisions only work when the trimesh data is upside down and it
> depends on the shape of the trimesh. I have scoured the mail archives to
> no avail. For testing I output a txt file of every triangle in the
> trimesh data structure and they are: valid triangles, wound in the
> clockwise fashion and the normal points correctly. No Surprises.
> 
> Today I ran some experiments with the trimesh example code. Try this:
> Take 'test_trimesh.cpp' turn the trimesh upside down by inserting the
> following lines after the trimesh is created:
>   dMatrix3 triRot;
>   dRFromAxisAndAngle(triRot, 0, 1, 0, M_PI);
>   dGeomSetRotation(TriMesh, triRot);
>   dGeomSetPosition(TriMesh, 0, 0, 1.0);
> 
> Now drop a sphere on the trimesh, it falls straight through!! Drop a box
> it may work. To start with the following questions need to be put
> straight for all the new users using trimesh:
> 
> What effect does the WINDING ORDER of the triMesh have on opcode/ode
> simulation?
> 
> What effect does the SHAPE of the trimesh (eg. convex, concave) have on
> the opcode/ode simulation?
> 
> What effect does including the NORMALS actually have in
> dGeomTriMeshDataBuildSingle1(, , , , , , ,normalsPTR)?
> 
> What else do you need to know to get reliable trimesh/shpere collisions?
> 
> When I sort this out I will be happy to write up a section in the WIKI
> on trimesh use. Many thanks -alex green
> _______________________________________________
> ODE mailing list
> ODE at q12.org
> http://q12.org/mailman/listinfo/ode
>