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From: | Elle Stone |
Subject: | Re: [Openexr-devel] OpenEXR files with nonlinearly encoded RGB |
Date: | Sat, 21 Feb 2015 05:28:49 -0500 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0 |
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
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