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

[Elle Stone]

On 02/20/2015 09:03 PM, Piotr Stanczyk wrote:
> Elle,
>
> The intent is very much that of a scene linear encoded data for colour
> data in an OpenEXR file.
> Of course, you can put anything in there you like, but of the
> applications I am aware of all will interpret the data as linear,  scene
> referred.
>
> Out of interest, what is your motivation for wanting to encode exr files
> with a non-trivial gamma?
>
> Piotr

Hi Piotr,

I'm not sure what you mean by "non-trivial gamma". In an ICC profile 
color managed workflow using matrix profiles, the profile's tone 
reproduction curve (TRC) is commonly either the sRGB TRC or the lstar 
TRC or a simple gamma curve such as gamma=1.0, 1.8, or 2.2.

The motivation for writing nonlinearly encoded RGB data is to use 
OpenEXR to transport high bit depth RGB data between various ICC profile 
color managed editing applications that can work at 32-bit floating 
point precision, for example Krita (painting), GIMP development branch 
(photographic editing), and darktable (raw processor).

The RGB data that's being transported from one image editor to the next 
might be scene-linear. But more likely it's already been processed to be 
very far from scene-linear. So in this context OpenEXR is being used as 
a data container to transport data between applications that aren't 
expecting scene-linear data (though of course the data might be 
scene-linear, depending on the user's editing goal).

So on the one hand, it seems arbitrary and pointless to insist that data 
be converted from whatever ICC profile working space the user has 
chosen, to a linear gamma ICC profile, before being saved to disk and 
then reopened in another editing application, whereupon a second ICC 
profile conversion would be required to transform the data back into the 
user's desired ICC profile RGB working space.

And on the other hand, there is a desire to read and write OpenEXR files 
in accordance with the online OpenEXR PDF documentation, which seems 
open to diverging interpretations.

Elle