[argyllcms] Re: Accounting for dot gain in press simulation
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 24 Feb 2011 18:23:41 +1100
Craig Ringer wrote:
I've recently been doing some more work using Argyll to produce better
profiles of the HP colour laser printer the newspaper I work for uses.
I've been using my i1Pro. The goal is to be able to use it for press
simulation, so we can soft-proof difficult images instead of just
sending them to the press and crossing our fingers. Unfortunately, I'm
running into issues that appear to be related to dot gain, and I'm not
sure where to go from here.
Hi,
it's certainly possible to do good laser printer proof
profile using Argyll, as we proved many times with the Colorbus
CLC1150 newspaper proofing systems.
The very short version: Can ICC profiles account for dot gain? If so,
can Argyll help create a profile that does?
Yes, ICC profiles account for the characteristics of the device.
The new profiles for the laser printer seem to be pretty good, at least
in terms of how well prints correspond to on-display previews in
Photoshop in absolute colorimetric proofing mode. Printing and measuring
another target, then using that as `profcheck' input, reports DeltaE of
2 on average, peak 5, min 1, so the profile for the laser doesn't seem
to be too far out. I'm delighted, as it's *massively* better than the
manufacturer-supplied profiles.
Sounds pretty good for a laser printer. The side to side page variation
and environmental sensitivity can be an issue. The control over
fine tints can be a challenge too, as it affects paper color simulation.
The trouble is arising when I try to produce soft proofs of the web
offset press using the colour laser printer. Most colours in the proofs
Soft proof ? I thought Soft Proof meant a display based proof.
I would tend to call a laser printer print a "Hard copy proof".
are a reasonable match, both with and without paper colour simulation,
but certain areas are printing WAY less saturated in the proof than in
the final press output. This is most notable in blue skies, where the
proof shows a fairly pale blue that's a reasonable match for the
on-screen preview, and the press produces a much richer, more saturated
blue. I see the same results - and the same issues with those results -
when using `tifficc' (lcms) and Photoshop's proof mode. I can't use
Argyll's own tiff conversion tool because it doesn't appear to support
any device simulation mode.
I'm not sure what you mean by that. The way to do a proof is to take
the Press CMYK (contone separation) and feed it into a link
made between the press profile and the laser printer profile.
Typically this would be an absolute colorimetric link for
background simulation.
Of course if your image setter isn't capable of a contone output
file, and is doing something funky like adding press curves that are
not applied to the contone output file, then this won't work so well.
The only sure fire way of coping with this is to take the image setter
output and de-screen it for proofing, but this takes specialised software
(which we had on the Colorbus product).
The press folks say the dramatic colour differences in select areas is a
dot gain issue caused by the inks bleeding out into the paper. That's an
issue I'm familiar with, but haven't seen having this dramatic an effect
before.
That sounds like a press-man's sort of explanation :-)
It doesn't make any sense from a profiling point of view though.
If that's what the press does with a certain CMYK input, then
that's what the instrument will measure, and that's how the
profile will represent its behaviour. (This is assuming you have managed to
profile the press - a non-trivial task typically, given the
cost of running it up for a test chart, and getting the press
to behave in a similar manner to "normal".)
This leads me to my main question: Can an ICC profile reasonably
account for the 30% dot gain of an offset web litho press, and if so are
there any facilities in Argyll CMS to create a profile that tries to
handle dot gain? Am I likely to be correct in suspecting that their
current profile just isn't a particularly accurate match for their
actual press output?
If the current press separation profile wasn't made from actually measuring
the press (or alternatively, if the press hasn't or can't be adjusted
to match the separation profile), then you will certainly
have problems with proofing, since you don't have a technical
means of looking up how the press behaves.
I'm thinking of doing a minimum print run of swatches on the press,
creating them using targen and using the old press profile as a guide
for swatch selection. Measuring that and feeding it to `profcheck'
shoudl give me a reasonable estimation of the accuracy of the current
profile (right?)
Yes.
and allow me to produce a new profile that might be
better if I've printed enough swatches. The press probably has a rather
non-uniform colour space, so I'd love some guidance about how many
swatches would be reasonable for getting a decent profile if I did want
to try to create a new one.
The more the merrier ! We tended to get by with a standard IEC chart,
although occasionally we would get to run a larger Argyll test chart.
I would tend to call most printing presses "well behaved" compared
to raw inkjets or even laser printers, so 1000 patches usually
gave a reasonable profile. If you can do 3000 and it makes any sense
(ie. the print run variation doesn't swamp other inaccuracies),
then by all means do so.
Just to complicate things a little more, the paper stock is off-white
(creamy), though at least it's without any FWA. The existing profile was
produced using that stock, so at least we're not lying to the CMS and
pretending to be printing on white stock.
Nothing unusual about that. If you were using Argyll to create a proofing
device link, then I would recommend you investigate the collink -w J,a,b
option to fine tune the paper color emulation by eye.
Graeme Gill.
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