Re: [Openexr-devel] Color Space Settings

[Florian Kainz]

Hi Jamie,

in OpenEXR files the "chromaticities" attribute defines the color
space of the pixels.  The attribute specifies the CIE (x,y) coordinates
of the red, green and blue primaries and the white point.  By default
that is, if the chromaticities attribute is not present, the primaries
and white point specified in Rec. 709 (Rec. ITU-R BT.709-3) are used.

OpenEXR images are meant to be "linear" and "scene referred." The
values stored in the pixels are proportional to the amount of light
in the depicted scene.  There is no gamma, and the images have not
been color-rendered.

The OpenEXR sample image collection includes a couple of files that
may help you test your code.  The Chromaticies subdirectory contains
four versions of the same image, encoded in different ways:

    Rec. 709 primaries and white point, RGB
    Rec. 709 primaries and white point, luminance/chroma
    CIE XYZ primaries, equal-energy white point, RGB
    CIE XYZ primaries, equal-energy white point, luminance-chroma

As Lars Borg mentioned, the chromaticities attribute is not enough
to uniquely specify what an image is meant to look like when it is
displayed.  One possible solution to this problem is described in
this document: http://www.openexr.com/UsingOpenEXRandCTL.pdf

Regarding some of the other comments in this thread:

- Storing gamma 2.2 encoded pixels in an OpenEXR is a bad idea; image
  processing software does not expect this encoding, and there is not
  even a generally agreed-upon way to specify that the pixels have
  been encoded this way.

- The white point in the chromaticities attribute defines the
  "relative strengths" of the RGB primaries.

Florian



Jamie L. Finch wrote:
> I am working on loading and saving color space information in several 
> file formats.
> JPEGs and TIFFs contain a tag to know of the image was created as sRGB 
> or Linear.
> What I want to do is identify the color space of the loaded image and be 
> able to set
> the color space when I save the image.   Common color spaces are Linear, 
> sRGB
> and rec709.
>  
> I would like some sample openexr images with color space setting in them and
> any documentation about the color spaces.
>  
> Jamie Finch
> Senior Programmer
> NewTek, Inc.
>
>
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