[argyllcms] Re: Comparing Input / Output ICC profiles the correct way?

Georg,

You make many complex points. Let me try...

> Assume I did have two printers with to ink sets which print the same
profiling
> data on the same media (same white point) which is then profiled by the
> same spectrophotometer. Comparing both CMYK ink sets (in terms of
> printable gamut) to see with which printer you can print "more colors".
> In this case it seems clear: absolute photometric.

To compare two different ink sets that way, for the purpose of comparing
gamuts, absolute colorimetry is the only meaningful way t do this, in my
opinion.

> But comparing an artificial input profile as for example Iso Coated ECI v2
> which has a default RI of perceptive (what I clearly not understand, how a
> single profile could have a RI. I though only the relation / translation
between
> two profiles could have a RI) with a printers (output) profile in terms of
> printable gamut with ideally (theoretically) delta E equal to ZERO (not
> possible in practice, I know!!).

Please understand that the "default" of any profile does not mean that the
profile is incapable of supporting the other usual rendering intents. An
legal output profile *must* have the full suite of required "input/output"
conversion tables (or tags). This means a perceptual, saturation and
colorimetric tables that convert from Device to Lab, and a perceptual,
saturation and colorimetric tables that convert from Lab to Device. The
default is just a textual documentation. For many profiles, a default makes
sense. 

> Assuming exactly the same media as used for the isocoated with also the
> same spectrophotometer then it would too be absolute RI. But using a
> different media with another white point then is should be relative RI?

No. If your goal is to compare gamuts in absolute terms, I would only use
absolute RI.

> What seems clear is that the perceptive and saturation RIs are not usable
for
> a gamut comparision of two profiles.

Ah! Not true. "Gamuts" are not absolutes. It is perfectly valid to talk
about perceptual gamut, saturation gamut and colorimetric gamuts because
these are all different and possibly valid projections of one space onto
another. See BabelColor PatchTool for more documentation (sorry Graeme, I
hope you don't mind...).

> 1) Why does a profile itself have a RI (a RI in relation to what? To the
> "unlimited" Lab PCS?)

All output profiles have four RI, perc, saturation, relCol and absCol.

> 2) how to compare gamuts of profiles under what circumstances concerning
> delta E == ZERO printable colours (ideally, theoretically) with which
rendering
> intent (assuming a most neutral comparison for the most general case, not
> assuming any special print job, just to say with this printer I can print
X% of
> the gamut in relation)

I am not sure I understand the question. What are you trying to demonstrate?

> But there should (at least I hope) be a standard for comparing presentable
> colours of different devices!?

The only "real" standard I found for comparing any prints to are the Munsell
Book of Colors. Or take NCS colors, or RAL, or PANTONE, if you have to.
Measure the original, build swatches of those measurements in InDesign,
convert using AbsCol to your printer profile and  compare to the actual
Munsell chips. Then you'll know. But keep in mind that whatever lighting you
observe the specimen under will influence your appreciation of the color
matches. I have done this experiment and found it useful.

Best / Roger




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