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From: William B Thompson <thompson@cs.utah.edu>
Organization: University of Utah
To: tiff@lists.maptools.org
Subject: Re: [Tiff] Floating-point 32bit sample data
Date: Wed, 24 Oct 2007 10:47:43 -0600
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Status:   

On Tuesday 23 October 2007 01:09, Nikita V. Borodikhin wrote:
> Are floating point RGB data stored gamma corrected or linearized?
> How should I use ICC workflow while processing data? ICC workflow is
> based on integer data, not float...

On Tuesday 23 October 2007 15:38, Chris Cox wrote:
> HDR data _really_ needs to be gamma 1.0, and floating point image data is
> best represented as gamma 1.0.

Photoshop CS3 appears to exhibit the following behavior with 32
bit(float)/channel images:

- An 8 bit/channel image without an attached color profile, read
  into Photoshop and then converted to 32 bit(float)/channel, is
  converted to linear gamma using the default color space.  If this
  image is saved as a 32 bit(float)/channel TIF, it gets an attached
  profile with the name of the default color space and "(Linear RGB
  Profile)" added to the end.  Gamma values are set to 1.0 in this
  profile (or possible they are missing?).

- An 8 bit/channel image with an attached color profile, read into
  Photoshop and then converted to 32 bit(float)/channel, is
  converted to linear gamma using the attached color color profile.
  If this image is saved as a 32 bit(float)/channel TIF, it gets and
  attached profile with the name of the original color profile with
  "(Linear RGB Profile)" added to the end.  Gamma values are set to
  1.0 in this profile (or possible they are missing?).

- A LogLuv TIF image read by Photoshop then written back out as a
  RGB floating point TIF is assigned the "XYZ Profile", which
  specifies linear gamma and an identity transform.  It appears that
  the RGB channels in fact have XYZ values, which is what is also
  done in EXR. Photoshop CS3 cannot write LogLuv TIF images.

The "(Linear RGB Profile)" versions of the ICC profiles do not
appear in Program Files\Common Files\Adobe\Color\Profiles in Windows
XP, nor does the "XYZ Profile".

The document describing the Adobe RGB (1998) Color Image Encoding
specifies that (0.0, 0.0, 0.0) should correspond to the color space
back point and (1.0, 1.0, 1.0) should correspond to the color space
white point.  It further specifies that "Component values outside
the range [0, 1] are not allowed for floating-point encodings."

Photoshop CS3 allows assigning profiles to 32 bit(float)/channel
images, but does not support profile conversion (which would have
the effect of rescaling the image).  It reads and writes 32
bit(float)/channel TIFF images containing values outside this range
without clipping.

	- Bill

-- 
William B. Thompson
Professor, School of Computing, University of Utah
50 So. Central Campus Dr., Rm 3190, Salt Lake City, UT 84112

thompson@cs.utah.edu, 801-585-3302 (ph), 801-581-5843 (fax)
http://www.cs.utah.edu/~thompson
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