Re: [Openexr-devel] Color Space, white balance...

[Florian Kainz]

Hi Antoine,

as you already suspected, there is not yet a single correct
and generally agreed-upon way to handle color HDR color.
At least initially, the data in the files should be linear
with respect to the depicted scene.  However, most people
do white-balance their images instead of storing absolute
scene colorimetry.  Transforming between XYZ and RGB color
spaces works the same way for HDR and LDR images.
Transforming between XYZ and LAB should work, too, but the
correct choice for the white stimulus is not obvious.

Regarding the relationship between the data in the file
and what gets displayed on the screen: the document at
http://www.openexr.com/OpenEXRColorManagement.pdf describes
conceptually what ILM does for visual effects.  As far as
I can tell, some other VFX houses work in a similar way.

The Academy of Motion Picture Arts and Sciences has formed
an Image Interchange Framework committee, which is working
towards a color management scheme that can be used throughout
the production of a movie, not just for visual effects.
The OpenEXR_Viewers software package includes two image
display programs which perform color rendering in a way
that is consistent with the committee's current ideas.
For more information see the Color Rendering section in
http://www.openexr.com/OpenEXRViewers.pdf.

Florian



Antoine Clappier wrote:
> Hi all!
>
> The following is more a discussion than a direct question.
>
> I am wondering what is the correct way to deal with Color Spaces in 
> OpenEXR format and HDRI in general.
>
> My current assumptions are that the HDRI data should be stored with 
> minimal modifications. That is: linear values (gamma = 1), no white 
> balance correction, etc.
>
> Now, the end user should be able to recover image information such as 
> white balance and recalculate the image for display from the HDR image 
> data stored in the OpenEXR file.
>
> This leads to the question to what should be stored in the EXR file? In 
> what Color Space the HDR data should be stored?
>
> Related discussion: I don't see in the Color Space literature how HDR 
> data can be transformed from one space to another.
>
>
> I realize that there is probably not a unique answer to my questions. I 
> hope with this post to start a discussion on the subject and get some 
> input from your various experiences!
>
> Let me know what you think.
>
> Regards,
>
> Antoine Clappier
>
>
>
>
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