Re: Chromix ColorThink not using chromatic adaptation tag for plotting monitor profiles (?)
Re: Chromix ColorThink not using chromatic adaptation tag for plotting monitor profiles (?)
- Subject: Re: Chromix ColorThink not using chromatic adaptation tag for plotting monitor profiles (?)
- From: Steve Upton <email@hidden>
- Date: Tue, 19 Jul 2005 16:40:35 -0700
At 7:50 PM +0100 7/19/05, Alan Rew wrote:
>Dear list members,
>
>I've emailed Chromix support about this issue but I thought it would be
>useful to get some opinions from other potential users of this software.
>
>I'm using (the public beta of) ColorThink to plot the gamuts of monitor
>profiles made using Monaco Optix Pro 2.0.3.
>These profiles make use of the chromatic adaptation (chad) tag, so the white
>point stored in the profile is 5000K.
as with many things color, the answer to this is not so simple.
First, modern profiles adhering to the ICC v4.x spec will now have their white points adapted to D50 (which means, for a monitor profile, that the white point IS D50). The profiling software then includes a 'chad' tag to describe the equation used to perform this (chromatic) adaptation so CMM's can 'undo' it in the future if desired. This is all good and working fine in the above example.
I wrote about Chromatic Adaptation in our last newsletter (#18) for those who may want to read more:
<http://www.chromix.com/colornews/?pid=1.cs>
Now, about graphing in ColorThink.
ColorThink has always graphed in D50 Lab - the same space used as the Profile Connection Space (PCS) when profiles join together during color conversions. The idea is to have ColorThink display information in the same way as it is flowing through your workflow.
When graphing printer profiles, it makes sense to use the "absolute colorimetric" view of the profile's data so that the slightly colored paper white and slightly gray ink black are plotted showing these features. The gamut volumes shown also jibe well with the measurement data used to build them. So if you graphed your measurement data the cloud of dots should fit very closely to the gamut volume of the profile. (this is one of a number of handy profile verification techniques)
All's fine, all makes sense.
But, add a display or working space profile with a non D50 white point and it gets murky. My thinking in the past was to render the colors the same way as those for input and print profiles using absolute colors. Those of you who use ColorThink know that this results in a gamut volume that has a non D50 white point. The white point for sRGB, for instance, appears blue-ish and off-axis. Is this correct? There are certainly arguments that support it. Is it the most accurate? That's very tough to say. Is it the best for gamut comparison? Perhaps, perhaps not.
In thinking this through I came up with one clear idea. That no matter the monitor white point, to the "adapted eye" it will be the white reference (white point extremes aside) and all colors will fall out relative to it. Therefore it could be valid to produce a "relative" gamut volume for such devices where the white point aligns with the D50 axis. As a result of this rethinking, future versions of ColorThink will plot monitor / working space volumes as relative colorimetric colors. Will this affect how the gamuts overlay print gamuts? yes. Is that a good thing? Yes, I think so. I think it is the best choice to make when plotting multiple gamuts for comparison.
It is interesting to note that Adobe has made a change in Photoshop CS2 that is along the same lines. That is, if you choose to convert from a non-D50 space like sRGB to a print space using absolute colorimetric, you no longer get a blue white point as you did in earlier versions but rather a D50-source white point that is more in line with what people expect. Can it be argued that the other way is more correct? probably. But the reality is that people didn't get what they expected AND what they got was not of much value to them.
I am certainly open to discussion on this topic. I don't want to make these decisions in a vacuum and welcome feedback on or off-line.
to get back to Alan's post:
>
>It appears that ColorThink is not using this tag, so the gamut plotted has a
>white point of 5000K and (hence) is not the actual monitor gamut. I would
>like to see at least the option of ColorThink using this tag, thereby
>showing me the 'true' gamut of the profile.
The concept of a "true" gamut becomes rather foggy the closer you get to it - rather like trying to pin down an electron. ColorThink is not currently resurrecting your non-D50 white point, this is true. But it is likely giving you a better gamut for comparison than had it done so.
(That's not a bug, it's a feature!)
Regards,
Steve
________________________________________________________________________
o Steve Upton CHROMiX www.chromix.com
o (hueman) 866.CHROMiX
o email@hidden 206.985.6837
o ColorGear ColorThink ColorValet ColorSmarts ProfileCentral
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