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I have created two spheres:

  • A UV sphere with 48 segments and 24 rings
  • A NURBS sphere, which I converted to a mesh using Object -> Apply -> Visual Geometry to Mesh

I then enabled cloth simulation, with the default options, to both meshes. I placed a plane beneath them with collision enabled.

When I play the animation, only the UV sphere animates. In fact, the converted NURBS sphere doesn't even fall, as shown in the image below. Why is the cloth animation not working?

An image of the problem after 40 frames

To clarify, although the spheres clearly have different geometry, it doesn't appear that the NURBS sphere has anything preventing it from deforming:

The sphere in edit mode

In addition, I can force some very strange behaviour if I place a cube with collision enabled intersecting the spheres. The UV sphere simply pops up above the cube, while the converted NURBS sphere goes INSANE and explodes!

Exploding sphere

I would be very grateful to anyone who could explain this behaviour and how I can use a converted NURBS object with a cloth simulation.

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  • $\begingroup$ I can confirm that something is not working as should, while i got the cloth sim working, it seems to only work once i activate the "Self collision", and then it's like the top snaps loose and the sphere starts falling. $\endgroup$ – Xylvier Mar 24 at 14:10
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Found the problem:

Select the converted nurbs object, press Tab to go in edit mode, press A to select all elements. Now press Alt+M and select By Distance.

This will remove all the unnecessary vertices, that essentially made the sphere a patchwork become one closed object with no split faces.

The reason Cloth did not work is that it needs a structure, the nurbs sphere was though only patches of separate faces.

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  • $\begingroup$ Solved perfectly, thank you very much! $\endgroup$ – devrobf Mar 24 at 22:14
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The resulting mesh from a NURBS sphere has too many overlapping vertex, which apparently unsettles the physics engine and the cloth simulation doesn't like.

If you enter Edit Mode and remove doubles with A to select all then Alt + M > Merge > By Distance it will work as expected.

The culprit appears to be the poles where all the loops meet. In Wireframe shading mode if your switch to top view and erasing the two poles it will also make it work as expected.

Remeshing or any other tools that rework the topology also seem to have beneficial effects.

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  • $\begingroup$ Thanks for your detailed answer! The other one was just a bit quicker :D The remeshing tools certainly sound useful too, thanks for that $\endgroup$ – devrobf Mar 24 at 22:15

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