[argyllcms] Printer grey-scaling profiling inaccuracy (using scanner)
- From: Milton Taylor <milton.taylor@xxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Fri, 24 Mar 2006 18:21:01 +1100
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 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, 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.
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? 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?
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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