[argyllcms] Re: RGB profile for HP Color LaserJet. Targets.

  • From: "Alastair M. Robinson" <profiling@xxxxxxxxxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 18 Aug 2009 23:54:23 +0100

Hi :)

Nikolay Pokhilchenko wrote:

The printer have severe ink limits and it's gamut shape is rather flat from 
black side. Moreover, there is high saturation (or compression, or clipping) of 
colors at the gamut boundaries.

I see some "interesting" effects in the saturated greens and blues in particular - my guess would be these are resulting from the printer driver's own RGB -> CMYK conversion and gamut mapping. I suppose there's no easy way to disable some of this driver enhancement?

The noticeable issue is the sparse patch placing in the center and bottom of 
printer gamut in spite of the patches was placed by OFPS evenly and densely in 
the device space.

I've noticed the lack of near-grey coverage myself in the past, and for profiling my own printers I currently use a 2-page RGB target with some of the the patch values skewed inward towards the grey axis. That's the nice thing about Argyll - if you're trying to profile a badly-behaved device and need to augment the target, you can add or edit patches by opening the .ti1 in a spreadsheet.

1. The first step target data is insufficient for creation of even profile 
because of non-optimal patches spreading. When I checking the profile from 
792cie.ti3, I get the high invprofcheck (B2A) error. Or when I trying to 
increase colprof -r parameter, I'm get good invprofcheck (B2A), but bad 
profcheck (A2B).
2. When I creating the target by targen with -I option, the some regions of 
gamut are empty (having a holes). The holes in patch spreading depends of 
colprof -r parameter.

Are there any suggestions? May anybody propose the appropriate target creation 
way for the next profiling step for this printer (792 patches are needed)? Or 
another thoughts?

If you run profcheck -v on your .ti3 file and profile, you'll get a list of patches and a measure of the fitting error. This gives you a hint as to where the trouble-spots are,

What I think I'd do in this situation is hand-craft a second target by taking the RGB values of the trouble-spots and scattering a few points around each. For instance, your worst patch is: 0.709800 0.717650 0.258820, so I might add:
0.71 0.72 0.25
0.69 0.73 0.24
0.70 0.75 0.23
or something like that - just a cluster of points around the trouble spot.

If doing that for the points with the highest error gave me, say, 50 points, then I'd generate a target in the normal way, but with 742 rather than 792 patches, then adjust it in a spreadsheet.

What I usually do is take each full-spread patch, average the RGB values to give grey, then replace each RGB value with a mixture of the original and grey value, ramping the proportions so that the first few patches are using the original RGB values, the last few are using the greyscale equivalents of the original RGB values, and in the middle I use a 50/50 mix of the two.

I'd then add my hand-crafted patches around the trouble spots.

Now I'd generate the chart, print, read, and finally add these readings to the original readings, so I had a single .ti3 file containing 1584 patches, and profile using that.

With a badly-behaved device response to track, it's probably worth using the -qh flag to colprof.

Hope these thoughts are some help,

All the best,
--
Alastair M. Robinson

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