Using Groom binding on meshes modified through Mutable

Development Character & Animation
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1

Hi!

Our technical artists are currently facing issues using Mutable along with Groom, and I’d like to know if there’s an easy way to help them.

Here’s the situation :

- Hairstyles are produced using Groom

- Heads are customized with Mutable modifiers

- When using groom binding as an attempt to have the hair match the shape of the head, the hair geometry becomes chaotic.

According to this documentation, groom binding isn’t supported on meshes that use Mutable modifiers :

https://dev.epicgames.com/documentation/unreal\-engine/using\-grooms\-with\-mutable\-in\-unreal\-engine

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Yet our team would really like to make this work, even if it requires modding these systems.

So, is there any way to circumvent the issue? And what would be needed for Mutable-modified meshes to support groom binding?

Thanks in advance!

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2

Hey there,

The main issue here is that groom bindings are built for a source mesh, and the target meshes need to share the same topology and vertex order to work. When you do modifications that affect that, the groom would need to be rebuilt to support that. For Mutables built at runtime, this is pretty infeasible.

If the operations you perform in your Mutable setup don’t modify the mesh vertex order, you should be alright. Regarding modifications, you would need to consider two paths.

  1. Rebuilding meshes that have been modified to have the same vertex order once modified.
  2. Rebuilding grooms to match new meshes.

Both are possible, but I do not have any definitive code or pseudocode for you to do that.

Dustin

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3

Thanks a lot for your answer!

Ok, so if I understood what you’re saying, Mutable modifiers change the mesh topology by adding/removing/reordering vertices. And this throws off Groom binding which expects to be able to map 1:1 each vertex of the source mesh to the target one.

Strat 1 seems quite ambitious to pull off. And strat 2 would basically require to work with a set number of head topologies, so we can have one groom hair mesh per topology…

Looks like a difficult problem, indeed. But at least, now I think I understand the core issue thanks to you.

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