Re: [Openexr-user] Color Management with EXR

[Florian Kainz]

Bo Schwarzstein wrote:
> Hello folks,
>
> Would you like to explain how the OpenEXR format should be used in
> color management pipeline ?
>

OpenEXR files usually store "scene-referred" RGB data, that is,
linear real-world light values.  "Chromaticities" and "white luminance"
attributes in the file header define the relationship between RGB data
in the file and CIE XYZ tristimulus values.

Color rendering, or converting RGB values in OpenEXR and other HDR files
into colors on a display, has not been standardized.  A working group at
the Academy of Motion Picture Arts and Sciences is progressing towards
a digital color management standard for the movie industry.

The exrdisplay and playexr OpenEXR viewers include a mechanism for
describing the mapping between in-file and on-screen RGB values in a
portable way.  For more information, please see the OpenExrViewers.pdf
and UsingOpenEXRandCTL.pdf documents on the OpenEXR web site.

> In my opinion, the EXR support FP16/32 format, we can record linear
> light in real world with nearly unlimited dynamic range, take an
> example, in Paul Debevec's HDRI tutorial, the sun is brighter than the
> perfect diffuser nearly 46000 times, so if we record the reference
> white with CIE XYZ, the part about the sun on image should be 46000
> greater than the white part, a ( thick white paper or silk ) ? We can
> apply this image (also maybe RGBE format) to HDR illumination in
> generating CG content (gather in RSL?).

That sounds about right.

>
> But because even the high-end digital camera such as Panavision
> Genesis can't record scene in 46000:1 such a huge dynamics range, we
> still work with the limited dynamics range in CG production.

The dynamic range of a real scene that includes the sun is
significantly greater than 46000:1 if the scene includes
objects that are darker than a white diffuser illuminated
directly by the sun.

You are right, there is no motion picture camera that can record
the full dynamic range of an outdoor scene that includes the sun.
However, the sun is rarely depicted directly in movies.

HDR environment maps (also known as light probes) for image-based
illumination are captured separately, typically with a digital SLR
camera (using exposure bracketing) or with a special HDR panoramic
camera.  The dynamic range of captured environment map images can
exceed one million to one.