Render colour primaries

[LL_Will_J]

www.anderslanglands.com/blog//2016/06/23/render-color-space.html#4

This is a very interesting blog post about rendering in different colour spaces using Arnold. It covers Spectral, Rec 709, Rec 2020 and ACEScg.

But it posed an obvious question for me:

What colour primaries is Vray using? Is it all happening behind the scenes or is it something I can change as an option somewhere?

I have read that Vray has a combined RGB and spectral approach - but I have no idea what gamut is being used.

I see that Maya lets you change this now in its colour management tools - but I don't know if that is having any effect for vray.

I would love to have some more information on this - as Colour pipeline seems like a never ending discussion at our studio!

Many thanks

Will

[vlado]

V-Ray uses sRGB primaries by default, but this is really only relevant if you use any V-Ray features that deal with spectra - like light temperature, camera white balance as temperature, physical sun and sky. If you don't use any of those, then it doesn't matter in the least what color space V-Ray works with and you can declare the RGB values to mean whatever you want - provided that you use the correct OCIO or LUT transform to convert those primaries to whatever displays you are using (normally sRGB), and make sure that your lights and textures also use that color primaries. F.e. it is quite possible to set up an ACEScg pipeline by making sure that all scene materials, lights and textures are defined in ACEScg and you have proper ACEScg profile for your monitors.

Best regards,
Vlado

[LL_Will_J]

Thanks mate. That is good to know. sRGB - nice and simple.

But I am still a bit confused about the practical way you would render in a different colour space. Would you be able to expand a bit on this?

Reading this post from the Maya manual...

https://knowledge.autodesk.com/suppo...501F1-htm.html

The ideal workflow should be changing my rendering colour space for my output device. So if I want to output to a P3 projector I would set my rendering space to a DCI-P3 colour space and if I want to output to a UHDTV I would set it to a rec2020.

What are the practicalities of changing these settings in Vray? I take it that those Maya colour management preferences have no effect on Vray? So using your example - Where do I have the options to define my materials, lights and textures in ACEScg? Also what would happen if I have a ACEScg rendering space and I want to use the temperature based tools you have mentioned (unpredictable resulting colours I take it?)

For me making TVCs out of the box is perfect. But I would be interested to learn how the Big Boys with carefully managed colour pipelines would use Vray in different ways.

Cheers

Will

[vlado]

One of our clients recently approached rendering in ACEScg like this (maybe you can consider this client to be a "big boy" ) They didn't use Maya's color management yet.

1. They converted their 8-bit color textures to ACEScg but with sRGB profile baked in. When bringing these textures into Maya, they would add V-Ray texture gamma attributes to the File nodes to convert from ACEScg sRGB to ACEScg.

2. Any other bit depth would be plain ACEScg.

3. Any color nodes such as Ramp and VRayUserColor may have to be passed through the VRayOCIOTex texture applied to put the color in ACEScg.

4. In the V-Ray VFB, the OCIO color correction needs to be enabled for conversion from ACEScg to ACES for display.

Ideally, we should recognize the ACEScg color space specified in File node textures, similar to how we now recognize the sRGB one. For the temperatures, we would need to switch the internal V-Ray spectral color space to ACEScg (right now it can only be switched between sRGB and CIE).

Best regards,
Vlado

[LL_Will_J]

Thanks Vlado, This is really interesting to read.

So as I understand it Vray is a lot like Nuke then. The inputs and outputs can be defined as whatever you want - but Vray itself is pretty much Colour space agnostic (with the exception of colour temperatures).

Is adding quick setups for workflows involving other colour gamuts in the planning? It seems like when we are finally creating UHDTV content - we will have to switch between these settings (possibly on a per job basis).

Your post has also lead to me reading a bit more about spectral rendering. Which I am a bit unclear about how this is handled in Vray. Here you talk about Spectral colour space - and using sRGB. Does that mean you are using a combined approach of calculating wavelengths of rays and RGB values for rays? or is spectral meaning something different from wavelength calculations in this context? It seems that the term spectral relates to quiet a lot of elements of the Vray renderer (like refraction dispersion) - so would definitely be keen to find out more.

Hope this isnt too tedious. Cheers for taking the time as ever.

Regards

Will

[vlado]

So as I understand it Vray is a lot like Nuke then. The inputs and outputs can be defined as whatever you want - but Vray itself is pretty much Colour space agnostic (with the exception of colour temperatures).

Yes, exactly.

Is adding quick setups for workflows involving other colour gamuts in the planning? It seems like when we are finally creating UHDTV content - we will have to switch between these settings (possibly on a per job basis).

Yes, as these workflows are becoming more popular, we are working to make switching between them easier.

Here you talk about Spectral colour space - and using sRGB. Does that mean you are using a combined approach of calculating wavelengths of rays and RGB values for rays? or is spectral meaning something different from wavelength calculations in this context? It seems that the term spectral relates to quiet a lot of elements of the Vray renderer (like refraction dispersion) - so would definitely be keen to find out more.

Normally, V-Ray treats all colors as triplets of floating point values; for the most part V-Ray doesn't really care what the three numbers actually mean. However some calculations in V-Ray are based on light spectra; the conversion of these spectra to floating-point color triplets assumes that the three numbers mean something specific - a color in some predefined internal renderer color space. This means that when converting from Kelvin temperature to RGB colors, V-Ray must know what that internal renderer color space is. Right now we support CIE and sRGB and we plan to support a few additional ones like ACEScg. Additionally, V-Ray may assign a specific wavelength for some types of rays - f.e. when calculating dispersion. For such rays, calculations are assumed to be performed for the specific wavelength only. Which makes V-Ray a hybrid RGB and spectral renderer.

Best regards,
Vlado