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?)
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.