Re: monitor profile
Re: monitor profile
- Subject: Re: monitor profile
- From: Tom Beckenham <email@hidden>
- Date: Thu, 28 Feb 2002 19:12:22 +1100
Yes, the PCS white point is always D50, and that is what you use to do
transformations. I'm talking about the 'wtpt' tag. The tag that allows
ColorSync to create AbsCol tables from the RelCol tables ('A2B1' tag). I've
just hacked my monitor profile programmatically to change just the 'wtpt'
tag. It certainly does affect AbsCol conversion in PhotoShop.
I thought the 'wtpt' tag specifies what your white point is when doing
AbsCol conversions with your monitor profile. For example, calibrate and
profile your monitor to D50, assign your monitor profile to an image, then
give that image to someone who has calibrated/profiled to D65. Using AbsCol
conversion will show you exactly what the first person was looking at
(ignoring the fact that the eye adjusts to white). i.e. it will have a
yellow tinge.
Prove It certainly sets the PCS white point to D50 as specified in the spec.
on 28/2/02 14:53, tlianza at email@hidden wrote:
>
Hello Henrik,
>
>
>
> By definition the white point must be scaled to D50 in a monitor
>
> profile. If a monitor profiling application allowed creation of D65
>
> monitor profiles, how would the result would be ICC compliant? Or
>
> maybe I didn't follow?
>
>
>
>
You are 100% correct. Tom Beckenham was led astray by one of the most ill
>
concieved and poorly documented parts of the ICC specification. The White
>
Point in a monitor profile must always be D50 (The PCS specified white
>
point). The Red Green and Blue XYZ tags are most definately NOT the XYZ
>
coordinates of the Red Green and Blue Phosphor. One would expect that the
>
White point tag and Colorant Tags might actually relate to the White point
>
and Colorants, but that was not meant to be. The XYZ tags actually are the
>
matrix which transforms the Colors in the monitor space to the D50 white
>
point PCS. I have complained about this section of the spec from the day
>
it was written. The adaptation method was never specified (although the
>
"Wrong Von Kreis" transform was recommended at one point) so it was
>
impossible to recover the actual display chromaticities from the profile and
>
white point from the profile. The next generation specification has a tag to
>
specifiy the adaptation method that is used. These tags carry an implied
>
adaptation matrix and the original color temp and chromaticities can be
>
deduced from the profile.
>
>
.
>
Tom Lianza
>
Technical Director
>
Sequel Imaging Inc.
>
25 Nashua Rd.
>
Londonderry, NH 03053
>
email@hidden
>
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