Are meshes in a urdf limited to a number of faces?

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Theoretically: no.

Practically: yes.

There is no hard-coded limit to the size of meshes in RViz -- or more precisely: in Assimp, which is what RViz uses to load meshes.

Practically though, loading times will increase and framerates will start to drop for larger meshes, but it's hard to put an exact number of faces on that, as it's largely hardware dependent.

Perhaps also important: loading a urdf is (almost) separate from loading the meshes. Many consumers of .urdfs will load the XML, but will not bother with loading the meshes.