clarification, RE: "Lab is always just right"
clarification, RE: "Lab is always just right"
- Subject: clarification, RE: "Lab is always just right"
- From: "Vanderlinden, Thomas M." <email@hidden>
- Date: Tue, 18 Sep 2001 10:21:12 -0400
good morning - - -
This is an attempt to be a little clearer
and answer a few follow-up questions
that were posed on this thread. :
== Apple CMM & ColorThink:
When used together (Apple CMM chosen as the CMM
in the ColorSync control panel), Lab space profiles
appear incorrectly in the 3-D Grapher,
showing a shape very different than the flat box
that displays when the Heidelberg CMM is used.
When other color spaces, like Adobe RGB 1998 etc.
are overlaid, ColorThink shows them as not contained
by this odd-shaped Lab space.
We're not talking about just a little tip here,
whole areas of color extend
outside of the odd-shaped Lab space.
Bruce J. Lindbloom's comments not withstanding,
>
...Many of the RGB reference space definitions are based on non-D50
>
reference whites. Notable examples include Adobe (1998), sRGB, BRUCE RGB
>
and others. In order to cast these profiles into ICC conformance (which is
>
D50 based), the reference primaries must be "adapted" from their original
>
reference white to D50 when the profile is _made_. There's more than one
>
way to do this and different methods will place the primaries in different
>
locations in XYZ (or Lab) space, producing gamuts of differing sizes and
>
shapes. There is no provision in the ICC format for the profile creator to
>
indicate the adaptation method used during profile creation.
>
Later on, when the profile is _used_ (by a CMM), these primaries may need
to
>
be further adapted to a non-D50 reference. Since the CMM cannot determine
>
by examining the ICC profile contents which method of adaptation was used
>
during creation, its own adaptation algorithm cannot be matched to it with
>
certainty. This may be the reason why Tom has found different results with
>
different CMMs -- each may use its own adaptation method.
The RGB reference spaces do not appear
to change display shape with a change in CMM.
== What was meant in this thread when we said "Lab space":
The profiles
pslabint.pf (which displays as Adobe Photoshop CIELab)
pslabpcs.pf (which displays as Adobe Photoshop CIELab)
Generic Lab Profile
Lab Color (a profile extracted from a file
that was converted to Lab by Photoshop 5.5)
all display in ColorThink 3-D Grapher
as exactly the same short box shape,
(or the strangely crippled space if you use
the Apple CMM)
Steve Upton, if I remember correctly,
said that the PCS is ICC Lab.
Chris Cox, of Adobe, if I remember correctly,
said that Photoshop uses ICC Lab, not CIE Lab.
The root of my inquiry was to be assured
that some (reflective scanner) captured colors
were not being shifted to fit inside the boundary
of the ICC Lab space in conversion from scanner RGB to Lab.
If I assume that the ColorThink 3-D Grapher
is giving a correct representation of these spaces
when the Heidelberg CMM is used,
then it appears that this ICC Lab space
is still useful as an archive color space.
Thanks to all for their patience and help
with my question.
- - -Tom Vanderlinden
email@hidden
Bridgeport National Bindery