[argyllcms] Re: Printer grey-scaling profiling inaccuracy (using scanner)

Milton Taylor wrote: 


Graeme,

I've been further evaluating the printer profile I got out from argyll cms - yes I've nearly done this to death I think - but I have a problem which I would appreciate some advice on.

Basically, even though the color matching seems quite good (comparing reflective target to printout under same light), when I print a grey scale test chart through the new printer profile, it's got a decidely green cast in the mid section.
To double check a couple of things, I visually compared a freshly printed grey scale chart (made with no cms conversions), with my IT8 reflective target. This confirms what my eye is telling me, that if you feed in grey values 

Milton,

what do you mean with "gray values"? Which "values" do you call "gray"? Do you mean printer specific DeviceRGB values, where R=G=B? Notice, that particularly for device specific color spaces it is not necessarily granted, that R=G=B generally applies for neutral colors, though in your case, this likely happens to apply (at least with original consumables), since the manufactorer has likely implemented/calibrated the driver in a way such that it accepts/expects sRGB input.

as inputs to the printer it does indeed generate visually neutral greys on the paper. This is not really surprising as the printer should only be using black ink at that point,

I would not necessarily expect that. Very likely, the driver does not print neutral colors with black ink only, particularly not lighter gray tones.

assuming the black ink really is black of course.

Thinking my profiles must be up the creek, I carefully recreated both the scanner profile and printer profile, but got the same results. Both profiles showed low avg and peak dE values.

Looking at the before and after numbers in soft proof, I can see the printer profile is boosting the green channel.

So then I had a look at what was happening in the scanner. If I scan the IT8 target, assign the profile, all of the Lab colours are spot on compared with the reference data, including the greyscale patches - no chroma to speak of. So the profile itself is good.

[Except of course that the IT8 was made on different paper with different inks - maybe this is a factor. Just as printer profiles have to made with the same paper and inks, does the same apply to making a scanner profile? i.e is that profile really ink/paper specific?]

If I then scan grey scales made from the printer - which visually look neutral next to the greys in the IT8 - and assign the scanner profile, the Lab numbers are definitely not neutral. Converting to sRGB at that point verifies that Green is down below the other two channels. Hence this is obviously why the printer profile is boosting green.

Looks like you are experiencing metamerism, i.e. you're experiencing the limitations of using a scanner as poor man's colorimeter.

I know there is a comment in the docs about often getting a color cast using a flatbed to measure the targets...and I suppose this is what I'm seeing here? But why is it happening...how can I prove it?

Is it possible to get metamerism effects with greys though?

Of course. Particularly the neutral colors on the IT8 target do not have a flat spectral reflectance at all, since they are all mixed from CMY colorants (and for your printer it's hard to say how the spectrum looks, since it depends on the inks, black generation done by the driver, ...).

Can the scanner look at one print, say printed with Epson K3 black ink on Kodachrome Professional paper, and see the greys as neutral, then look at another print made with dye-based black ink on Ilford pearl paper, and see non-neutral greys? Even though they both look neutral to the human eye?

Yes, even if the scanner "sees" the same RGB numbers, when it "looks" at the printed gray ramp and at the IT8 gray ramp, the eye can still see these colors differently (and vice versa). The scanner "sees" colors in a different way, than we do.

Regards,
Gerhard

Secondly, what sampling algorithm does scanin use to read the patches off the targets? Does it average or median, and typically how many points would it take from each patch?

Thanks,
Milt



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