RT CPU vs RT GPU problems

[DariusK]

Hi,

I am experiencing a lot of problems with RT GPU mostly. First GPU materials look nothing like CPU when I render. It was actually same in 3DS Max as well so I don't know what is the problem with that or am I missing something. The biggest issue I encountered is v-ray Skin material renders black on RT GPU and if I change option to not re-size the textures the render crashes 98% of the time.
Here is an example ( v-ray skin material applied only to the face )

http://imgur.com/BjwwXqg

[vladimir.nedev]

First GPU materials look nothing like CPU when I render. It was actually same in 3DS Max as well so I don't know what is the problem with that or am I missing something.

The MODO standard material is not translated very faithfully to RT GPU as it contains a number of non-physical options.
For RT GPU, it is best to use V-Ray's own material.
Note that the latest nightly builds contain a number of improvements related to RT GPU (shader tree blending, gradient texture with "incidence" as input).
These improvements are not in the official 3.01.01 build.

You can get information about RT GPU support in V-Ray for MODO here : http://docs.chaosgroup.com/display/V...+Compatibility
This page is missing information about V-Ray geometry/materials/textures, we will add a separate page for them in the V-Ray for MODO docs (as they are always supported for Production/RT CPU rendering).
Until then, you can use the 3ds Max help:
http://docs.chaosgroup.com/display/V...orted+Features

The biggest issue I encountered is v-ray Skin material renders black on RT GPU

Note that the V-Ray Skin material is not yet supported in RT GPU. You can use the V-Ray Fast SSS2 material instead.

and if I change option to not re-size the textures the render crashes 98% of the time

This might be caused by your GPU not having enough memory to fit everything.

Greetings,
Vladimir Nedev

[DariusK]

Thanks for a quick reply. I was more talking about the rendering difference between GPU and CPU. Thanks for the information about v-ray skin didn't know that.
here is another screenshot with comparison of GPU and CPU renders, nothing was changed, I am using vray cam and vray sun, same differences appear by using modo cam and sunlight.

http://imgur.com/xJLKDBe
p.s. I am using GTX780 6GB so I thought it should be enough for full res materials ;]

[vladimir.nedev]

I was more talking about the rendering difference between GPU and CPU.

Do you have GI enabled in your scene ?
One small caveat is that RT GPU always has GI enabled (the option is ignored).
So if you want to compare it with CPU rendering, you need to have GI enabled.

I am using GTX780 6GB so I thought it should be enough for full res materials

Hm, in that case, it might be an actual bug. I will give it a test a bit later.
But it will be very helpful if you send me the scene with the textures ( e-mail is vladimir.nedev[at]chaosgroup[dot]com )

Greetings,
Vladimir Nedev

[DariusK]

Ahh yes Gi totally forgot about that, thank you for that.
Here are the results with GI turned on still bit of the difference in color and shadows, not sure why is that.
I will send you the scene later today.

thank you

[vladimir.nedev]

and if I change option to not re-size the textures the render crashes 98% of the time.

In the latest nightly builds, we have some fixes for crashes with out-of-memory conditions.
I tested the fixes with the scene you sent me, it was very helpful.
If you can, give it a try to see if it still crashes for you.

I am using GTX780 6GB so I thought it should be enough for full res materials

I got my hands on a GPU with more RAM (my previous was 1GB so full res textures on your scene were never going to work on it).
It turns out I made a mistake when setting the default value for "GPU texture format".
It should be "8-bit per channel", whereas I had it set to "16-bit per channel" by default.
With 8-bit textures, the full size textures in your scene take ~900MBs, with 16-bit textures they take ~1800MBs.
I am guessing you are using the same GPU for your monitor, that reduces the amount of RAM available to RT GPU as well.

Here are the results with GI turned on still bit of the difference in color and shadows, not sure why is that.

I am not sure whether you used the official 3.01.01 build or a nightly build, but the nightly builds had a bug with how the "invert" option of texture layers is handled in RT GPU in V-Ray for MODO.
In you scene, all the opacity textures on the hair had "invert" enabled, so that was one source of the problems.
This is fixed in the latest nightly builds.

Another problem is that RT GPU in V-Ray for MODO doesn't respect the "double sided" option of the MODO material.
It always renders it double-sided, so it changes the shadows in the hair.
I still need to check if that could be fixed. It is quite important though.

And lastly, there are some differences in how the MODO material is supported between RT CPU and GPU in V-Ray for MODO.
If you want to match the results between CPU and GPU rendering, you should try to use the V-Ray material, with the MODO material it is harder to support everything on the GPU.
I will see if I can make it a little closer to RT CPU though.

Greetings,
Vladimir Nedev

[savage309]

Keep in mind that RT GPU and RT CPU a two completely different code bases and render engines. We do whatever possible to get the results as close as they could be, yet some differences are always possible since very different algorithms and approaches are being used. We that being said, take a look at the log, we try to notify about such stuff their (like, GPU always have 'scatter gi' on, uses specific scatter type, etc).

[vladimir.nedev]

The "double sided" option of the MODO material is (for the most part) fixed in RT GPU in V-Ray for MODO for the build from 2016.02.24.
Turns out the most important behavior when "double sided" is off - that back faces are invisible to camera rays, is already supported by the RT GPU core.
So, it was as simple as modifying the V-Ray for MODO exporter.
This makes it possible, for example, to put the render camera just outside a wall, invert the wall normals to point towards the inside of the house, and uncheck "double sided" for the wall material.
You will be able to see inside the house, but the walls will still cast shadows.
This setup can be found in a number of the MODO samples.
Here is an example:


There are still some specifics to MODO's "double sided" option that are not available in the RT GPU core it seems. We might add those in the future.

It seems though that the "double sided" option is not the main reason for the differences in the original scene. Still looking into that.

Greetings,
Vladimir Nedev

[Guilherme]

Hello,

UV maps aren't working on RT GPU, right? They work on RT CPU. Or i'm doing something wrong? Thanks

[vladimir.nedev]

Hello,

UV maps aren't working on RT GPU, right? They work on RT CPU. Or i'm doing something wrong? Thanks

Currently you must have only 1 UV map per mesh, or it might not work in RT GPU in V-Ray for MODO.
This is because MODO uses UV map names to link between textures and UV maps, whereas the GPU core is using 3ds Max style UV map indices.
This issue is on the short TODO list.

Greetings,
Vladimir Nedev

[Guilherme]

Thanks Vladimir, its working now.

[DariusK]

Thank you so much for all the updates. I will give it a go but from what I am seeing now it is much better already ;]
p.s. I am using a different video card for my monitor so it is all for rendering ;]

Thanks again.

[vladimir.nedev]

Currently you must have only 1 UV map per mesh, or it might not work in RT GPU in V-Ray for MODO.
This is because MODO uses UV map names to link between textures and UV maps, whereas the GPU core is using 3ds Max style UV map indices.

For today's nightly build ( 2016.03.01 ) I have modified my export code, so RT GPU in V-Ray for MODO will work with more than 1 UV map per mesh item.
The restriction now is that the scene cannot have more than 16 different UV map names across all mesh items or the incorrect UV map might be used for some meshes in RT GPU.
Also V-Ray Proxies might not be able to find any UV map after this change (in RT GPU).
To fix both of these we will need to extend the RT GPU code to support named UV maps, which is planned for later.

Some example scenes from the MODO content that previously didn't have correct textures in RT GPU:
Animation/Car_Rig_AP.lxo - this one has a total of 11 differently named UV maps
Render/GameController_01.lxo
Render/Telescope.lxo

Greetings,
Vladimir Nedev

[MildMustard]

I am having an issue where the colour from the texture map (material diffuse) isn't being used on the Fur Material with RT-GPU, it is working with RT-CPU though.

[vladimir.nedev]

The restriction now is that the scene cannot have more than 16 different UV map names across all mesh items or the incorrect UV map might be used for some meshes in RT GPU.
Also V-Ray Proxies might not be able to find any UV map after this change (in RT GPU).
To fix both of these we will need to extend the RT GPU code to support named UV maps, which is planned for later.

So, this improvement in the GPU core happened quite a while ago and I've just recently updated the V-Ray for Modo exporter to use the named UV sets.
Tomorrow's nightly build ( 2020.01.29 ) will have all the fixes, which means any mesh item or V-Ray Proxy can use up to 16 UV maps,
i.e. the restriction is no longer per scene, but per mesh, which should hopefully be much more convenient.

Greetings,
Vladimir Nedev