Hi, Алексей. Forget about RGB linearization for printers. It is useless.
When you're linearizing true CMYK output device in CMYK mode, it have a sence.
Using the linearization you can keep printing process precision without of
re-profiling. But when there is RGB-driven CMYK device, the conversion between
RGB to CMYK is hided from you. And when there is a need to re-calibrate, this
hided conversion doesn't allow you to calibrate device channels behavior. You
had to perform full re-profiling anyway. That is why nobody uses RGB
linearization for an output device.
To deal with highly non-linear RGB devices, where you had, for example, a lack
of dark patches, use preconditioning profile. Make first, "bad" profile by
standard ArgyllCMS workflow. You may use targen -V option at this stage, a
demphasis to dark region, at the target generation stage. Do not use standard
targets like IT8 in your quite extreme case. This is useless, at least at
printer profiling stage.
Having preliminary profile, perform main profiling. Use targen with -c
"Pre-profile.name" parameter. Targen will generate target with most appropriate
patches densities across the volume. And your problem with a lack of patches at
the dark regions will gone. You need not to use -V targen option when you have
preliminary profile. For future profiling use last valid profile as
pre-conditioning for targen.
Mention must be noted: If your printer black point has significant color cast,
the resulting profile produced by default colprof and collink utilities will be
bad. You will got color cast in darks because of output adaptation to actually
bad black point in perceptual and saturation (and similar) gamut mapping modes.
You need to disable black point adaptation or use the intents which doesn't
perform it. I had to disable black point adaptation in sources and re-compile
the ArgullCMS to get best profiles for such a bad cases.
HTH. Good luck!
четверг, 21 февраля 2019г., 10:02 +03:00 от Алексей Коробов
dmarc-noreply@xxxxxxxxxxxxx :
Dear Argyll team,
I use last version of Argyll together with LittleArgyll tool and manually
inserted arguments under Windows 10 64-bit Home. And I find that Argyll makes
RGB targets for Windows printer inproperly, especially for calibration. Manual
pages describe that RGB is transformed to CMY here, i.e. without K. Right, I
get graded CMY target for calibration. But if I add K patches, this doesn't
influence on calibration curves as the algorythm doesn't use K channel here.
The result is uneven K ramp on final IT8 print: dark areas are totally black,
white is grey enough. Of course, this happens with third-party inks on cheap
enough photo paper, but I get out more of this paper after manual settings
tuning. It's an obvious thing that for RGB printer colors should be converted
to full CMYK using some law, maybe using some pre-conditioning profile like
FOGRA ones. And then make upside down step for drawing calibration curves. But
now I find that Argyll doesn't have right tool to make it properly, so I ask
you to do it. I also note that target generation for RGB printer is always
uneven, it lacks of many dark patches by default and it is difficult to
correct the spread with arguments.
With regards,
Aleksei Korobov,
Yekaterinburg, Russia
