Mudbox 2018 - Ptex export distorted on certain faces

[philippfarmer]

Hi,

I'm currently painting a model in Mudbox 2018 using ptex. While painting everything seems fine. I can increase the subdivision level and paint without any distortions. But when I export the paintjob to a ptx file, mudbox for some reason distorts the texture on certain faces.
To be sure its not a vray bug (I first noticed the problem in my renders), I made a simple test inside Mudbox and re-imported the exported ptx file as a layer to the very model I exported it from. And well, I had the same distortions. That means for some weird reason Mudbox distorts the texture on some face's during the exporting process.
Btw I tried Mudbox 2019, same problem. It also doesn't matter whether I first subdivide and then setup ptex or the other way around. Same result all the time.

I hope anyone knows how to fix this very anoying problem.

The following images show the problem:

[Christoph_Schaedl]

Looks like the distorted mesh is subdivided once.
There are no supporting loops to hold the faces stable if you subdivide the mesh.

If you smooth the mesh the face are getting bigger cause of the subD.

[philippfarmer]

Yes, the mesh is subdivided with Mudbox's subdivision levels function. And I get your point of faces getting bigger that way, but thats exactly how its supposed to be. Otherwise there would be no way to get the texture working on a mesh using Vray Subdivision Attribute, for smooth renders. As you can see for most of the faces that got "bigger" because of the SubD Mudbox exports the texture just fine. It's only a few faces that it seems to use the level 0 mesh for calculating the final ptx. And I have absolutly no idea why.

[Christoph_Schaedl]

I assume thats an issue caused by how mudbox is storing the data.

Cause its using uvs todo that and not ptex. If you take a look in the mudbox file there is still a folder with textures beside the .mud file. If you import the ptex file its using the uvs to get the data back into the viewport.

If you use support loops the uvs will be stable and you dont have that issue. If you like to use displacement you need more poly in your base anyways.

[philippfarmer]

I guess you're right. Probably that's it. I was hoping there's a solution to this other than changing the mesh.