Re: [Openexr-devel] OpenEXR files with nonlinearly encoded RGB

[Elle Stone]

On 02/21/2015 02:26 PM, Joseph Goldstone wrote:
> The ACES committee (an unfortunate change of name from the Image
> Interchange Framework committee) agreed with Florian on much of what was
> published in this document, but let go of indirect output referred
> imagery or output referred imagery in OpenEXR, IDCES and ODCES as
> explicitly named things, and a lot of other stuff. For example, the ACES
> 1.0 release carries a lot of the metadata as clip-level XML that the
> paper you reference proposes be carried as OpenEXR attributes
>
> The ACES 1.0 source and doc bundle can be reached from this page:
> http://www.oscars.org/science-technology/sci-tech-projects/aces
> And if you are going to be looking at how to make OpenEXR files for an
> ACES workflow, look at SMPTE ST 2065-4:2013, “ACES Image Container File
> Layout”.

On 02/21/2015 07:10 PM, Gonzalo Garramuno wrote:
> > This is handled by the ACESclip metadata and CTL.  It allows you to
> > have different Look Mod Transforms and save them along your image.
> > Or a gradeRef to convert from and to a different space (like log)
> > and apply ASC CDL transforms (SOPNode and SatNode).
> > mrViewer supports the first variety and is in progress to support
> > the second variety, too.

Thanks! for the links and the clarification on how ACES is used with 
OpenEXR files. I've been peeping over my shoulder at the ACES workflow 
and how people in the world of cinema handle color management for a 
while now.

The context of my question about the OpenEXR document on ACES wasn't 
about ACES per se but rather about how to use OpenEXR in a strictly 
photographic workflow, as a way to transport high bit depth RGB data 
between various photographic image editors that use ICC profile color 
management (the ACES-OpenEXR propsal mentions encoding nonlinear RGB data).

The specific question was whether photographic image editors should 
allow writing nonlinearly encoded RGB data to OpenEXR files, or whether 
instead the RGB data should be converted to a linear gamma ICC profile 
before being exported as OpenEXR.

In the usual photographic workflow, editing operations are performed 
directly on the RGB data to intentionally modify the scene-linear nature 
of the raw data that was captured by the camera. From the photographer's 
point of view, whether data should be exported as linear or nonlinear 
RGB depends on why the photographer wants to export the data. For 
example, if the goal is to rescale the image in another image editor, 
the data should be converted to a linear gamma ICC profile. But if the 
goal is adding or removing noise, usually the photographer will prefer 
results of operating on perceptually uniform data.

The question of writing nonlinearly encoded RGB values as OpenEXR came 
up because the GIMP devs are currently working on GIMP's OpenEXR 
read/write code. We (I contribute to GIMP development, mostly in the 
area of ICC profile color management) want to accomodate the needs of 
photographers while making sure we follow OpenEXR specifications and 
best practices.

On 02/21/2015 02:26 PM, Joseph Goldstone wrote:>
> The Academy doc set is in the unfortunate position of trying to provide
> color-managed substrate around which digital motion picture workflows
> could be built, while trying to stay agnostic with regards to workflow
> and not provide a ‘best practice’ there. I’m not any sort of official
> spokesperson for the Academy (just an 11-years-on-the-committee worker
> bee) but I think they will stay agnostic, and see what kinds of
> workflows are developed by productions and by interested individuals. My
> guess is that over the next year the ACES discussion group at
> https://groups.google.com/forum/?hl=en#!forum/academyaces
> <https://groups.google.com/forum/?hl=en#%21forum/academyaces>
> will move a bit from developer conversations to user conversations. And
> I hope that, supplanting that discussion group which is pretty linear
> and textual, we’ll see ACES usage discussions on websites like your own
> (which I wish had been there in the early 2000s when I was trying to use
> littlecms for movie production).

Does anyone use ACES in conjunction with ICC profile color management? 
If so, do they use OpenEXR to store RGB data?

I wish OpenEXR supported ICC profile color management by means of 
embedded ICC profiles.

If the data is converted to a linear gamma ICC profile before export as 
OpenEXR, then the chromaticities and white point can be used to 
construct an ICC profile. But many applications that do read ICC 
profiles, don't read or write OpenEXR chromaticities plus white point 
and don't have code that constructs ICC profiles from chromaticities 
plus white point.

Elle