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Problem with UV border shriking on open mesh borders

ZBrush Usage Questions ZBrush 2020 Usage Questions
#1

I’ve consistently had a problem whereby open mesh borders UV’s will shrink along their edges once subdivided.

In the example here I unwrapped some UVs on the default male head ztool, and then made a copy and subdivided it, with smooth UV (suv) on . You can see that where the neck 3d mesh border was open, its resulting UVs shrank in.

The isn’t comaptible with any renderer AFAIK ( eg Renderman, arnold) which all typically can smooth internal,whilst preserving borders. The resulting bakes of this from zbrush render with a seam error.

Has anyone else encountered this ? Its a masjor issue for me currently

thanks

mesh subd

#2

Hello!

A few things here.

  • The Tool> Geometry > SMT button next to the “Divide” button controls whether smoothing will be applied to a mesh when subdividing. If you dont want any smoothing, disable this.
  • If you have an open section of mesh, and do not wish the border geometry to shrink too much when subdividing a mesh with smoothing active, make sure that all the open edges are Creased.

This should prevent most, but not all of the potential shrinking. You must still remember that:

  • The act of subdividing a mesh alters that mesh at ALL levels of subdivision. This is why, if importing a mesh from an outside program to generate maps that must match that mesh in another program, it is neccessary to store a morph target prior to subdividing, and restore that original mesh immediately prior to generating any maps. If you fail to do this, you are no longer generating the maps for the exact same mesh, which will result in issues along the borders.

The process for doing this can be found in the Multimap exporter documentation in the “Switch MT” section.

If you are exporting the altered geometry from Zbrush as the new base level geometry and applying the maps directly to that, then there is no issue. This is only an issue with imported meshes that have to match the UV/geometry in another program exactly. So if your workflow doesn’t require you to this, just export directly from ZB to make things less complicated.

  • Open volumes or 2d meshes are not preferred by Zbrush for many operations. This type of geometry can be situationally useful, but will be tricky to sculpt on near the open borders and not perform well for many operations, limiting what you can do with it. If planning on working at length in Zbrush, try to work on meshes that are closed volumes with thickness whenever possible, and remove unwanted geometry as part of the process of preparing export quality topology.
#3

Thanks for the reply.

However, my problem is nothing to do with the mesh subidiving, but what happens in UV space. Regardless of creasing the open mesh, or subdividing with SMT off / on or SUV on / off - the mesh border in UV space is the problem.

As soon as any sort of subdivision is applied, the mesh border in UV space shrinks in