Linear workflow in Silhouette? And example LUT files?

[Derek Melmoth]

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

[Paul Miller]

On 2/13/2013 12:38 AM, Derek Melmoth wrote:
> 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 lets you use a straight-up OCIO config, or you can select a
single LUT file. Sounds like you may need a custom config to do this, or
find a LUT that combines both conversions.

> 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.

Silhouette reads all the LUT formats supported by OCIO. Can you convert
your .CSP file to another format that is supported?

> 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?

If you're working with EXRs you're already using a linear workflow, so
that statement is a bit confusing. Switching to DPXs would require an
additional conversion (handled by OCIO), so there is no need to do that.

Silhouette v5 was just released and we're working on a point release
that adds/fixes a few things. I could look at getting the CSP bug fix in
there.

[Paul Miller]

> I wonder if the 1.0 clamping issue happens with LUTs only, or if it will
> also happen with a custom OCIO config?
> Also not sure if the issue is in the OCIO code, or if it's just in
> Silhouette. The CSP and Houdini LUTs work fine in Nuke's
> OCIOFileTransform node (Nuke 6.3v8 from May 1012). I may just be using
> an old version of Silhouette (4.5.3) with an older OCIO - I'll see if I
> can try a newer Silhouette version, maybe the bug was fixed!

Silhouette isn't doing any clamping of its own, so this must be
happening in the FileTransform.

> Also, I bring up DPXs because if I use DPX in Silhouette, I can set its
> "Interpretation" to "linear", and then simply apply the 3D LUT (a .cube
> that accepts CineonLog input) and it will look perfect! However there
> are a few downsides to this:
> - We'd need to convert our EXRs to DPXs.
> - It's not exactly a linear workflow, e.g. the "Gain" slider applies
> gain in log space instead of scene-linear space.

We put the Gain transform before the FileTransform because that is what
Mari does/did. I can see the reasoning of that, if the file LUT is meant
to model a specific special display.

Sounds like a custom Config is the way to go.