Is mental ray capable of accurately rendering vector displacement?

[Trist]

G'Day,

It's great that Pixologic have finally added the function to export VDM's from ZBrush but I can't get a decent result in Mental Ray for 3DS Max (have tested on 2012 and 2013).

World maps seem to work better but tangent maps are more useful as you can deform them and I am currently working on a character animation.

Apart from the main problem with the lack of accuracy, Mental Ray renders out artifacts of black pixels, often forming lines or cuts in the geometry (not related to UV seams).

On the positive side Zbrush VDM's seem to work better than standard height maps but I can't get effects like undercuts to render properly.

I don't have VRAY or anything else to compare it to but I don't think the VDM from ZBrush is the problem as it looks to contain accurate information from what I can see.

Perhaps ZBrush and Mental Ray are not yet properly compatible in the displacement area in general?

If anyone can share any insight from personal experience of the above it would be greatly appreciated.

I am also wondering if NVIDIA have plans to improve the quality of displacement in general using maps generated in apps such as ZBrush.

Thanks everyone.

[Remydrh]

I don't have VRAY or anything else to compare it to but I don't think the VDM from ZBrush is the problem as it looks to contain accurate information from what I can see.

Zbrush actually outputs imprecise displacement at seams.

mental ray 3.10 has an added registry value to compensate for that but it has not made it to an OEM build yet.

I am not sure if the vector displacement in Zbrush is the same as in Mudbox. I have not had issue with the Mudbox vector displacement. Perhaps Autodesk could clarify that, the vector displacement shader in Maya is an Autodesk creation designed to work with Mudbox.

[Remydrh]

Take a look here for more information:

http://forum.mentalimages.com/showth...t=displacement

[Trist]

Hi Remydrh,

That's interesting that ZBrush outputs imprecise displacement at seams, I didn't know that. However some of the errors I am experiencing in my render are not located very near any seam so I'm not sure technically what the cause could be for that.

It's a shame the registry value that fixes the particular error from ZB maps you mentioned hasn't made it to the build yet, but I guess that's another thing to look forward to (like Ptex in Max).

I havn't really used Mudbox except for the trial so I'm no expert there but ZB VDM's look pretty much the same (of course there may be more information that isn't human readable in the map)

I will contact Autodesk if I can. It is likely that the vector displacement shader in Max (or Maya) is designed to work with Mudbox specifically but I would have thought that Pixologic had anticipated this difficulty when implementing the new feature.

Thanks for the link, I will take a look.

Cheers.

[Trist]

The artifacts found in the thread from the link are definitely not the same thing as what I am getting, BTW.

Thanks for the trouble though.

[gunter]

Trist, you could easily try the shared displacement mode in 3.10. You just have to set this in ~/.rayrc
registry "{_MI_REG_SHARED_DISPLACEMENT"} value "on" end registry

If that does not work for some reason we could take a closer look if you could make available the .mi
file and texture.

Gunter

[Trist]

Thanks for the response gunter,

Does it matter where this line:

registry "{_MI_REG_SHARED_DISPLACEMENT}" value "on" end registry

is actually entered in the rayrc registry file?

How do I know if it's in shared displacement mode when I launch Max 2013?

The render looks no different to me (again using Vector Displacement).

Sorry if I am misunderstanding your instructions, I don't do this sort of thing much.

[gunter]

Trist, could you please send us the .mi file ? I think this would save some time.

Gunter