LUTs interpreted differently in V-Ray 7?

[Alex_M]

Hi,

For some reason the rendered images look really bad with my LUTs applied after installing V-Ray 7. Please check the comparison below. I've loaded the same VRIMG file without making any changes to the layers.

[dimitar_panayotov]

Can you share a vrimg containing the color corrections and the LUT file as well? Please share them here and tag this thread to your ticket. Thank you.

[Alex_M]

Thanks for the quick reply, dimitar_panayotov ! I've just submitted a ticket (No. 310912) with the VRIMG and LUTs.

[dimitar_panayotov]

Hi, thanks for sharing the files.
Why do you use None for Display correction?
Does it look as before if you switch to sRGB in Display correction?

[Alex_M]

In V-Ray 6 the images looked fine when "Display Correction" is set to "None" so I always leave it at that when I use these 2 LUTs.
Switching "Display Correction" to "sRGB" doesn't fix the problem in V-Ray 7. Did you manage to match the look of the V-Ray 6 image in V-Ray 7?

[dimitar_panayotov]

Hi with V-Ray 7 we fixed a bug that was applying double sRGB when LUT files are used. You will need to rework your workflow in order to achieve the previous result.
But there is something else interesting. I saw that you were using AgX LUTs. They would also require a custom OCIO. None in this case wouldn't produce a correct result.
Why not using ACEScg or just sRGB in Display corrections including a Gamma workflow if you are using 3ds Max24 or Max 25?

[Alex_M]

Ah, I see. I like how AgX handles color so I use it instead of ACEScg and sRGB. I don't use it as OCIO, because then the Curves adjustment layers don't work as I want them to - they get applied before the OCIO correction, but I want them applied after OCIO. Otherwise I get bad colors. Is there way to make the Curves layers apply after OCIO?

[dimitar_panayotov]

Unfortunately, no. The curves layer is a regular adjustment layer, and it folds in the same category as the others.

[piotrus3333]

I've heard there is an issue with log conversion option in Lookup layer in VRay 7. Your image looks like this is the case.

[piotrus3333]

Hi with V-Ray 7 we fixed a bug that was applying double sRGB when LUT files are used. You will need to rework your workflow in order to achieve the previous result.
But there is something else interesting. I saw that you were using AgX LUTs. They would also require a custom OCIO. None in this case wouldn't produce a correct result.
Why not using ACEScg or just sRGB in Display corrections including a Gamma workflow if you are using 3ds Max24 or Max 25?

I've never heard about this bug - can you elaborate a bit?

[piotrus3333]

Ah, I see. I like how AgX handles color so I use it instead of ACEScg and sRGB. I don't use it as OCIO, because then the Curves adjustment layers don't work as I want them to - they get applied before the OCIO correction, but I want them applied after OCIO. Otherwise I get bad colors. Is there way to make the Curves layers apply after OCIO?

https://forums.chaos.com/forum/chaos...op#post1191941

this topic contains "decomposed" version of AgX ocio - it will allow you to use curves or any colour grading on AgX Kraken log encoded AgX primaries.

it is setup for ACEScg input so you would need to replace numbers in Composite section of VFB with colour matrix from original AgX ocio config file.

[Alex_M]

https://forums.chaos.com/forum/chaos...op#post1191941

this topic contains "decomposed" version of AgX ocio - it will allow you to use curves or any colour grading on AgX Kraken log encoded AgX primaries.

it is setup for ACEScg input so you would need to replace numbers in Composite section of VFB with colour matrix from original AgX ocio config file.

I will check it out. Thanks!

[piotrus3333]

I will check it out. Thanks!

I found a version for sRGB:

[Alex_M]

Cool! Thanks so much!

[yavor.rubenov]

I've never heard about this bug - can you elaborate a bit?

There were two separate issues actually.

Issue 1.
Let's assume the render itself uses sRGB rendering primaries.
If a Lookup Table layer is set to 'Work in sRGB' we would take the input linear color and do
1. Apply sRGB gamma on the color (~ 1.055*c^(1/2.4)....)
2. Apply the Lookup table on the color
3. Reverse the gamma back to linear ( ~ ((c+0.055)/1.055)^2.4 ...)

The bug caused the last step (3) to be skipped - so the color remained with the sRGB gamma. If you have an sRGB display correction after that - you essentially get double gamma..

Issue 2 - We didn't convert RGB primaries - so even if the render itself uses ACEScg primaries we still did the simple sRGB gamma. Now if the render is ACEScg and the LUT is set to work in sRGB we also convert primaries.