Uniform Perceptual Lab
Uniform Perceptual Lab
- Subject: Uniform Perceptual Lab
- From: "Bruce J. Lindbloom" <email@hidden>
- Date: Wed, 16 Jul 2003 12:03:16 -0500
A couple weeks ago, Robin Myers wrote:
>
CIE 1976 L*a*b*, commonly referred to as "Lab", was designed as a
>
representation for qualitative comparisons of color differences. It was
>
pressed into service as a PCS by the ICC, but it was not designed for the
>
job. Once in use it's warts started to show, which are the non-uniformities
>
of perceptual mappings in the L*a*b* space. The main reasons for this usage
>
are that there are equations for converting between XYZ and L*a*b*. Color
>
spaces derived from direct human perceptual evaluations, such as Munsell, do
>
not have any equations that easily convert between their perceptual
>
coordinates and XYZ. XYZ is important because it is a direct correlate of
>
the cone responses of the human visual system and are easily derived from
>
spectral measurements. There are no programs available that allow one to
>
convert between XYZ and Munsell with user defined lighting (there is one
>
program available from GretagMacbeth but it is basically a table lookup
>
system and I believe it only works in CIE Illuminant C).
Robin is, of course, correct on all of this. I have repeatedly tried in
years past to derive equations that do this mapping, but had only limited
success. A LUT seems to be the only practical candidate for a useful
implementation.
Pursuant to this, I have built such a LUT, implemented as an ICC color space
profile. It was derived from the public domain Munsell data set, adapted to
D50. This profile may be useful to manufacturers of printer profiling
software, or authors of "smart" CMMs. It is probably not directly useful to
end users (which is the reason for the cross posting of this message).
This profile will map CIE Lab into Uniform Perceptual Lab ("UP Lab") in a
way that straightens out the curved perceptual hue lines and makes the
distorted chroma rings circular again. The necessary inverse transformation
is also built into the profile.
You may learn more about it (and download it) here:
<
http://www.brucelindbloom.com/UPLab.html>
I would be interested in hearing about anyone's experiences with it.
--
Bruce J. Lindbloom
email@hidden
http://www.brucelindbloom.com
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