VFB LUT Export doesn't work

[JonasNöll]

I just tried it with a more complex scene that uses filmic tonemap and its possible to get something pretty close with this workflow. Thanks again

So is this a Photoshop issue or an issue with the generated LUT? From how piotrus descibed it, it seems to be more an issue by Adobe rather than Chaos.

[piotrus3333]

Although this trick is working, I'd rather have it fixed by the developers.

not a trick. just math. there is nothing to fix - VFB works exactly as PS in that regard.

[JonasNöll]

I think the idea would be that you can just export the LUT from VFB and load it into PS and it matches out of the box without having to fiddle around with this Channel Mixer and so on...
Again, is this an issue by Adobe or by Chaos?

[olly]

not a trick. just math. there is nothing to fix - VFB works exactly as PS in that regard.

Are you able to break the maths down please?

I've been doing this a long time and can't think how this has passed me by

I thought I understood the maths of compositing...

[piotrus3333]

I think the idea would be that you can just export the LUT from VFB and load it into PS and it matches out of the box without having to fiddle around with this Channel Mixer and so on...
Again, is this an issue by Adobe or by Chaos?

neither. this is how luts are handled most of the time (0-1 range). that is why log color input for luts is so common.

[piotrus3333]

Are you able to break the maths down please?

I've been doing this a long time and can't think how this has passed me by

I thought I understood the maths of compositing...

this is something that was called Exponential color mapping inVRay’s old days. same as VRay’s Reinhard with Burn 0.
y=x/(x+1) where x is raw render pixel value and y is tone mapped result. no matter what the dynamic range this will bring everything into 0-1 range. most of the time it is called basic/simple Reinhard or something along those lines.

[yavor.rubenov]

Hello,

Generally this is an issue with the LUT file format itself.

It contains e few things:

1. Domain min and max values - the minimum and maximum values the LUT can correct. Problems here:
- Those values are omitted by default and it is assumed that LUT is for values [0,1]. It is not exactly certain if all softwares actually support other values.
- The problem here is what value to put actually. We can easily determine the current image boundaries but what if you want to apply the same LUT to multiple frames of an animation ?

2. Three dimensional table with a fixed size of N [2,256]. So the file will contain N x N x N sample points. The sample points are equidistantly spaced. This is the key limitation. It means that if the range is [0,1] and we have 32 points - we have values for 0, 0.0322, 0.0644 and so on. If the range is [0-100] we get 0, 3.2258, 6.4516 and if the range is [0,10000] we get values for 0, 322.58, 645.16 and so on..
So if we want good precision and big enough range - we'd have to export huge LUT files that will be quite slow to apply.

Currently we export a 3D grid only in the [0,1] range and use N=32.

Best regards,
Yavor

[francomanko]

So "3D grid only in the [0,1] range and use N=32." aside ( why do so many threads feel like a maths exam)

So does in fact the VFB export luts correctly?

[seandunderdale]

So does in fact the VFB export luts correctly?

Not just that....because it may technically be correct...Id ask, What needs to be done to ensure a LUT from the VFB works correctly in Photoshop?

[JonasNöll]

So from how I understood it, it only works as expected as long as you don't have intensities higher than 1.
In other cases you will get wrong results and there is not really a way to transfer the adjustments in the VFB 1:1 to Photoshop (or any other software).

That's a bit of a bitter pill to swallow, hope there can be a solution for this. The trick with the channel mixer is great, but I think 99,9% of users are not aware of this and will wonder why their picture in PS looks wrong...

[piotrus3333]

Not just that....because it may technically be correct...Id ask, What needs to be done to ensure a LUT from the VFB works correctly in Photoshop?

exclude tone mapping step from lut and do it separately. adjustment layers, ocio plugin - whatever works for you.
I settled on Reinhard+curves as it’s available in almost all possible post production apps and was a standard for years with VRay.

practicality of using luts for tone mapping was discussed on aces forums a lot.

[piotrus3333]

That's a bit of a bitter pill to swallow, hope there can be a solution for this. The trick with the channel mixer is great, but I think 99,9% of users are not aware of this and will wonder why their picture in PS looks wrong...

funny story about those 99% - I tried sharing two yt videos about correct hdr compositing in Photoshop on two biggest VRay groups on Facebook as I heard “socials” are the place now for industry chitchat. both were blocked by moderators.

[JonasNöll]

funny story about those 99% - I tried sharing two yt videos about correct hdr compositing in Photoshop on two biggest VRay groups on Facebook as I heard “socials” are the place now for industry chitchat. both were blocked by moderators.

Thats unfortunate, but if you accept my 2 cents: In order to be succesful on Social Media your videos would need to be prepared in a slightly more "watchable" way. I think the content is great and it helped me with my issue, but if you wouldn't have specifically pointed me to that video I don't think I would have ever clicked it. I think the problem is more in the presentation than the lack of interest by users.

[piotrus3333]

Thats unfortunate, but if you accept my 2 cents: In order to be succesful on Social Media your videos would need to be prepared in a slightly more "watchable" way. I think the content is great and it helped me with my issue, but if you wouldn't have specifically pointed me to that video I don't think I would have ever clicked it. I think the problem is more in the presentation than the lack of interest by users.

heh, good point. not interested in reaching bigger audience though. too old for that game and this is the production value I can afford really. but I do enjoy sharing as this is how I learned ages ago. and if even two people find it usefull - I’m good and happy. on the line are even more obscure topics like colored film grain “in camera” in VRay and rule of thirds composition being a cancer on modern photography. not a vast audience for those but I will be delighted to deliver both.

going back to Photoshop used for hdr postproduction - my first quick tip is always: use Affinity.

[JonasNöll]

Sounds awesome, I subscribed and I will surely check this stuff out