Seems like Vray maya is subdividing UVs. I thought there was an option to turn it off

[Intuition]

I seem to remember being able to use a checkbox in the vray subdivision options that specified linear UVs. My Zbrush dispmap render looks like it is subdividing the UVs (smoothing them). I don't want Vray to do this to the borders or the rest of the geometry. Subdivide the mesh but do not distort the UV mesh at all borders or non-borders. Subdivide UV at borders checkbox doesn't seem to make any difference.

In Feb 2010 I remember using a linear UV checkbox. Was this removed?

Thanks.

---EDIT--


Yes, just tested in modo. If I use linear UVs there are no render distortions or jagged lines. If I uncheck linear UVs I get the same result in modo that I get in maya vray where there are borders showing up due to the UVs being subD' at render time.

[vlado]

Not sure what "linear UVs" are, but when you add the V-Ray subdivision attributes to a shape, there is an option whether UVs should be subdivided at borders or not.

Best regards,
Vlado

[Intuition]

Linear UV map is just a UV map that is not subdivided at all, borders and all points do not move even though the mesh is being subdivided at render time, which I thought was Vray default use of UV maps. I can't remember when it started (or why) but apps started offering subdivision in the UV space that rounds the result of the uv mesh map.

The problem with subdividing borders is that I have to use a sculpt program that is also subdividing the borders. modo can do both linear uv or subD uv but not a mode where it subDs the mesh except for the borders. Is Vray either not subD the uv or is subD the uv OR is it always subD the uv mesh except for the borders if you use the checkbox.

Maybe I was just using "subdivide at border" turned off in maya vray and thought it was linear UVs. Is there a way to tell vray not to change the UV map mesh at all during render time even though I am subD the mesh for displacement?

Here is a comparison of modo linear - subD UV. So if Vray is keeping the borders straight but still subD the non-borders then there is a mixed result.



I suppose I could tell Zbrush to use smoothUV which is Zbrush's way of saying subD UV (subdividing the UV, borders and all, at rendertime). Still I prefer using linear UV since its easier to deal with between apps. Mental Ray default in maya is linear UV unless you use approximation editor where I think more options become available.

So essentially I have 3 options for UV interpolation between different apps.

1. Linear UV



2. SubD UV <-- looks incorrect here since I sculpted in zbrush using smooth UV off. Notice the nose area with the edges.



3 subD with linear borders <- borders are straight but rest of uv is smoothed.



Can vray just do linear UVs? It seems Vray maya is either using 2 or 3 given the checkbox "subdivide UVs at borders".

[vlado]

V-Ray always subdivides the UVs - otherwise there are many issues with non-smooth mapping and so on. (There is currently no way to turn this off, but I'll make a note to look into it). The difference between subdividing the UVs at borders is probably best seen in the attached two examples - what would a "linear" mapping look like for this case?

Either way, if this is not what you need, you can always use Maya's own mesh smoothing.

Best regards,
Vlado

[Intuition]

Well, it depends on how the displacement map was generated. If I used the mesh you've shown in zbrush and I divided the mesh a couple times and then sculpted it and then brought the divisions down to level 2 for export I then have two options. I can either use "smooth UV" on or off (inside Zbrush export disp map options) which produces either a linear UV map like the one I am using here OR a smoothed UV map. If I use linear then when I bring back the obj into maya for use in mental ray I will need to subD it (mesh not uv) before applying the generated disp map. Then I can render it. If I use smooth UV button in zbrush then I'll have to tell maya mental ray to "smooth uvs" in the mesh checkbox attributes pictured below.

In either case I don't think I can tell zbrush to output a displacement map that has smoothing on except at the borders. I know mudbox does do this as that was the image I used in the 3rd example.

I mean, its not a problem overall because I can re-open my zbrush data and output a smoothed UV map. My main issue with using smooth UV maps is that each app has its own way of approximating the uv smoothing. Modo, maya, xsi, max, c4d all have slightly different subD smoothing. So by using linear uvs I am assured that there shouldn't be border or pixel distortion as long as destination app/engine offers linear UVs.

Where as when I use smooth uv each app/render engine will slightly be different in its interpretation between points in the uv window right, or do all render engines use the exact same subD interpretation code? So I tend to want to use linear to make sure no pixels are slipping across the subD UV mesh no matter how much or little the subD pulls on the main mesh. When you subD the uv mesh, borders or not, at all you will get into some small level of pixel distortion.

If someone isn't using an exported map from a sculpt app but is simply projecting a flat map onto say a grid as you have shown the linear UV could help.

Maya gives these options.



Default is linear. If I check the smooth UV checkbox I can then pick the border options.

I guess the scenarios are based on which sculpt app used and what settings at image export time.

1. Mudbox only can offer subD with linear border uvs OR all subD uvs. No linear uvs.

2. Zbrush can do either linear or subD, but not subD with linear borders.

3. modo either does linear or subD. It, like zbrush, can't do linear borders with subD mesh.

The main reason people like smooth UVs is that you can edit a smooth UV map in photoshop much easier. I just like having linear because it doesn't depend on the apps ability to subD correctly and uses the un-subD mesh UV which would be the same in any render engine. Harder to edit in Photoshop though..

I thought Vray used to not subD the uv map by default way back in the day. Is there any way to add this as an option/checkbox? This would be helpful in case I get assets from a client that has linear UVs and I don't have the source ZB file? In the meantime I can re-export my disp map from ZB with smooth UV turned on.

[vlado]

Is there any way to add this as an option/checkbox?

Yep, I've made a note for it.

Best regards,
Vlado

[Morbid Angel]

just a note, you should always output textures based on subdivided uvs anyway. If in zbrush you sculpted on non subdivided uvs, there is an option to import another model with subdiv uvs to it.

[Intuition]

Well, thankfully I can just load up the model and do another export with "smoothUV" button turned on in zbrush and it fixes my problem. Now I have to figure out if Vray can do multiple displacement maps on a single mesh. I am using UV tiles so I can maximize my detail. I have 9 different tiles. In mental ray there is a method of combining the displacement maps by applying the disp maps to UV islands individually even though they are on the same mesh. I haven't tried this in Vray before. Results coming soon.