how to match LUT in VFB in Photoshop to raw EXRs

[emartin8907]

Hi,

I am picking up another artists work and can't figure something out.

He was applying LUTs in the VFB and just saving out the RGB, he had 'convert to LOG before applying LUT' ticked on.

If I use the render elements to rebuild the RGB in Photoshop for more control (GI, Refract, Reflect, Spec) it loses the LUT because that only gets saved on the RGB. Also I want to render on the cloud which doesn't even apply the LUT to the RGB, just gives me the non LUT version.

So I am trying to match the LUT from the VFB in Photoshop, but I cant seem to get it to match.

I need to first convert it to LOG and then apply that LUT, but I can't figure out how to convert to LOG the same way VFB is doing it. I've tried many 'conversion LUTs' but they are all from rec 709 to LOG or some camera to LOG. Does anyone know if there is a linear to log or something that will do the same thing VFB is doing?

Cheers
Eric

[piotrus3333]

do it in VFB. check convert to log and apply “empty” LUT. just save a LUT from VFB with no adjustments.
but Photoshop will still most likely give you slightly different results.

[emartin8907]

thanks piotrus, I just gave it a try it sounded promising but it doesn't seem to work out. with the layer on normal mode there isn't even a difference in the image in photoshop, only when you go to multiply or some others but they are all extremely off.

I'm getting by with lin2log node in nuke right now. just have to run my raw renders through there and then into photoshop.

[piotrus3333]

used to work just fine in max and Next since the update with ability to save LUT result from VFB. probably Next 4.2.
I was putting raw log renders into photoshop to avoid its 32 bit color mode hurdles but keeping dynamic range as wide as possible.
rebuild your beauty in ps - save exr - load this into VFB - save log color with empty LUT trick.
keep in mind I haven’t touch VRay during last three months so something might work differently now.
empty LUT attached.

[emartin8907]

ohhh very cool!!

Thank you so much for sharing that zeroLut, I did as you said and in Photoshop it is a perfect match from what I get in nuke when I did lin2log and applied the LUT after, it's just a touch more contrasty than the RGB saved out of the VFB originally which is fine.

I am still confused though how did you make that ZeroLutVFB that you shared?

And so this has to be done on a per image basis? I suppose so does the nuke step, at least this keeps it all in PS. hate having to step into another program for one dumb thing.

[piotrus3333]

this “empty” ZeroLutVFB was saved from VFB the way you normally save LUT but with no adjustments. it does nothing but still tricks VRay to make linear to log conversion during saving.
or google for “lin2log in photoshop” if that would be more convinient.