Exporting LUTs from Nuke with matchgrade

[Rangi Sutton]

Hi all,

More of a nuke question than OCIO per-se, but figure this is the audience most likely to have solved this one!

I'd like us to be creating looks based of arbitrary nodes in nuke... lookup, colour correct, and grade would be popular ones to chain together.

I've been trying to wire the CMS test pattern into match grade and baking out a .csp file which OCIO subsequently references via shot environment variables.

To get the CMS test pattern to not clip the superwhites, I gain it up and gamma it a bit (eg 4, 0.7 respectively) before feeding into matchgrade and the colour nodes. I set the matchgrade to highest res (64).

When I'm lucky this works.. but more often these days I get a fairly crappy match for overall levels in the darks, and some wild curves with valleys in the brights.

So.. I'm wondering what people are doing to generate looks with nuke? Can you get matchgrade to work?  Do you try and stick to CDL's only? Use something like baselight plugin?

Cheers,

r.

Rangi Sutton | VFX Supervisor
CUTTINGEDGE  |  Brisbane Australia
Studio +61 7 3013 6236 | Mobile +61 41 521 1280

/SYD /BNE /MEL /TYO

[Elliott Smith]

Hi Rangi,

I've recently been using this method to matchgrade:

1: start off with a matchgrade node, adjusting the resolution, the crop area and if possible, making multiple samples in the sequence (so long as the source and target sequence match).

2: once close enough, i then color correct the matchgrade node

3: export out the matchgrade to a csp file

3: CMS Test pattern > ocio file transform(csp file) > copy the color correct > generate lut

[Rangi Sutton]

Hi Elliot,

Thanks for tips. I will try this at the studio today.. for some reason I had it in my head that GenerateLUT only created 1D luts. That'll be the key to what I'm doing.. as I'm not trying to match grades but rather build creative looks.

Cheers,

r.

Rangi Sutton | VFX Supervisor
CUTTINGEDGE  |  Brisbane Australia
Studio +61 7 3013 6236 | Mobile +61 41 521 1280

/SYD /BNE /MEL /TYO

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[Rangi Sutton]

Hey,

I played with Generate LUT.. no controls for the range it considers, and it doesn't take a comparison image the way Match Grade does.. which leaves it only really doing the range 0-1.

This is annoying with my current setup which assumes linear input and therefore super-white. For next project I'll ensure our OCIO always applies the looks by first jumping to log, and then I'd wire this up so it's doing the colour corrects in log..

Very invested now in linear space looks for current project though... buggar! Living and learning.

Cheers,

r.

Rangi Sutton | VFX Supervisor
CUTTINGEDGE  |  Brisbane Australia
Studio +61 7 3013 6236 | Mobile +61 41 521 1280

/SYD /BNE /MEL /TYO

[Aaron Barclay]

Hey rangi,

Use a shaper lut to normalise the values going in. Basically a log curve like cineon or lg2 from ocio.

You will need to reverse the process in the lut you are generating.

This becomes easier to do in ocio once you are familiar with the syntax.

Aaron.

[Rangi Sutton]

Hi Aaron,

Yeah, I get that.. it's very similar to how I"ve done it for previous shows where the LUTs where coming from baselight.. I'd need to jump to log before applying the LUT.

Currently, I have over 100 shots with a dozen luts between them on the show floor, that work assuming a 3D LUT taking scene-linear input. I don't want to have to re-create those or run ocio-bake stuff over the top of them.

A SHOT_LUT env var is used to say what that .csp file is per shot.

I can't, as far as I know, use environment variables to also control the shaper... hmm.. actually.. yeah now I think about it a second environment variable (say $SHOT_SHAPER_LUT) pointing to a 1D LUT that would switch between no-change (scene-linear->scene-lin) or a scene-linear->log.

That'll work!

Cheers,

r.

Rangi Sutton | VFX Supervisor

CUTTINGEDGE  |  Brisbane Australia
Studio +61 7 3013 6236 | Mobile +61 41 521 1280

/SYD /BNE /MEL /TYO