USD -> SDF: polygon triangulation

[koonpeng]

🎉 New feature

Summary

This implements the PolygonToTriangles function using the fan-triangulation algorithm. Initially tried to implement delaunay triangulation based on ignition::common's implementation, but the implementation is a wrapper around gts's implementation, which does not support 3D.

While researching a custom implementation, found out that there is a much simpler way using the fan-triangulation, it is not as robust as delaunay but it looks like a better fit for the use case of triangulating convex polygons. It seems like tools like blender also uses a form of fan-triangulation of these use cases and delaunay is more commonly used for more complex scenarios like generating terrains/heightmaps etc.

Test it

Currently tested with GearJoint.usd in isaac sim's examples (should be able to find it with a file system search). It seems to work somewhat, but I'm not sure if the current output is due to bad triangulation or other bugs.

usd:

sdf:

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[koonpeng]

It appears that gts already supports triangulating meshes, gts_surface_strip should do the trick. I will start that functionality to ignition common, after which, this PR will need the new version.

gts_surface_strip is to create triangle strips from a gts surface, which must already be a triangle mesh, there is gts_triangles_from_edges, I thought it can create a list of triangles from the edges of a polygon but looking at the source, it requires the edges to be already part of a triangle. I can't find any functions in gts to do polygon triangulation.

[ahcorde]

I think something is wrong here.

• I modified the gearjoint.usd inside IssacSim and I removed everything expect one of the cylinders and I saved this world
• Then I converted this saved world with the sdfconverter and I get the following shape:

If we forget for a moment the transformations, the mesh is not rendered properly.

[koonpeng]

Opening the dae in blender at least looks like a cylinder

Will try to find out more about what is going on, maybe the way ign performs smoothing causes the distortion.

[koonpeng]

I think the problem is caused by wrong scaling, changing the scales from

<scale>0.733134 0.1 0.1</scale>

to

<scale>0.1 0.1 0.733134</scale>

fixes it