Linear workflow in Silhouette? And example LUT files?


Derek Melmoth <derek....@...>
 

Hi!

I'm attempting to get a linear workflow working in Silhouette (4.5.3), which uses OpenColorIO. I'd like to load in scene-linear EXRs, apply a 1D linear-to-CineonLog transform, and then apply a 3D LUT.

Silhouette makes it very easy to apply a LUT file... so all I should need is a LUT file that contains both a 1D lin-to-log shaper LUT and a 3D LUT. I'm able to create such LUT files for some applications (e.g. I can create .CSP luts that work in RV and Nuke), but I'm having trouble finding a file format that works correctly in Silhouette.

First question:
Are there example LUT files available anywhere? I want to try all the different LUT formats (or at least the ones with 1D shaper luts), but can't find example LUT files anywhere, so I've been digging up the specs for each type and creating my own test LUTs in Python. If I had an example LUT file of each type it would be the most helpful thing in the world for me, and maybe for other developers just getting into OpenColorIO :D

Second question:
A single LUT file may be the simplest way for me to get a linear workflow in Silhouette, but perhaps it's not the best way or the recommended way? What's the recommended approach to linear workflow in Silhouette? Or should we switch to DPXs instead of EXRs? Is there a way to apply 2 LUT files instead of 1? Should I look into creating a new OCIO configuration?

Thanks!
-Derek



Anyone interested in what I've tried so far may read ahead:
==============================

So far here are the issues I've been having with the different LUT formats:

3dl
(Autodesk Apps: Lustre, Flame, etc. Supports shaper LUT + 3D)
Problem:
I don't think it can accept input values above 1.0, can it?

csp
(Cinespace (Rising Sun Research) LUT. Spline-based shaper LUT, with either 1D or 3D LUT.)
CSP is perfect and allows a 1D shaper LUT with arbitrary inputs and outputs, and we've been using these in RV for a while.
Problem:
In Silhouette CSP cannot be used right now because of a known CSP-reader bug in OCIO:
https://github.com/imageworks/OpenColorIO/pull/304
(It expects to read the string "CSPLUTV100", but instead reads "CSPLUTV100\n" with the newline character at the end, and fails to read the file.)

hdl
(Houdini. 1D Lut, 3D lut, 1D shaper Lut)
The 1D portion doesn't allow arbitrary input coordinates like CSP does, but luckily the inaccuracy in the darkest colours is only slightly noticeable.
Problem:
Unfortunately there's a bug that clips input values that are above 1.0, and this is a deal-breaker. So, scene-linear values from 0 to 1 work as expected, but anything above 1 fails to go above 0.6696 in log space. Is there a place to file a bug report for this?
Also the lut file's extension had to be renamed to .lut in order for Silhouette to read it :)


From the LUT list in the OCIO FAQ, it looks like the only other format that supports shaper LUTs is .cub (Truelight format). I will try this format next!

Are there any other LUT formats in OCIO that support 1D shaper LUTs? Or should I instead just use the 3D LUT that accepts CineonLog input?


Thanks for reading! And I must say I'm pretty thankful for OpenColorIO and the VES Cinematic Color white paper! :D
Cheers,
-Derek

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