Re: RelCol Mapping of Middle Gray
Re: RelCol Mapping of Middle Gray
- Subject: Re: RelCol Mapping of Middle Gray
- From: Graeme Gill <email@hidden>
- Date: Tue, 08 Mar 2005 13:58:26 +1100
- Organization: Argyll CMS
Michael Fox News Account wrote:
I’m curious about the RelCol intent mapping of the gray axis and,
specifically, middle gray. To start with, we know several things that
are immutable facts:
RGB = 128,128,128 = middle gray
It rather depends on which RGB space you're talking about. In some
RGB spaces, the above is not middle gray.
Reflective Density = 0.78,0.78,0.78 = middle gray
This rather depends on the colorants involved. With some
colorants, this is not middle grey.
In LAB, a*b* = 0,0 is neutral. It seems that the L value for middle
gray is dependent on the color space.
This rather depends on the white point the L*a*b* was computed relative
to, and the viewing conditions. Under some viewing conditions,
this will not appear neutral.
Given those things, if I profile my printer and I send it a file that
has RGB=128,128,128, I should be able to measure the result with a
reflective densitometer and get 0.78,0.78,0.78. Alternatively, if I
measure the result with a spectrophotometer, I should get a*b* values of
0,0.
Very, very unlikely that this will be the case. Only if the RGB space
is such that RGB=128,128,128 maps to a PCS value that just happens to
be relative colorimetric gamut mapped to a PCS value that just happens
to map to equal colorant output densities (measured in whatever status
filters you happen to be using), and that the the equal colorant densities
just happen to be a*b* = 0,0 relative to whatever L*a*b* white point
you happen to be using.
Note that the "Relative" part of "Relative Colorimetric" means that
the gamut mapping applied will be such that the white is assumed
to be relative to the white point of the output media. This
is based on the assumption that the viewer will be fully white adapted
to the white of the media. In this situation one would expect a PCS
neutral value to map to an a*b* that is a proportion of the media
white (depending on exactly how the white point adaptation is
managed, and the L* level of the neutral).
Graeme Gill.
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