[argyllcms] Re: targen options

Gerhard Fuernkranz wrote:
> On the other hand, I'm having trouble with my inkjet (-> RGB
> driver, only 3 dgrees of freedom, which should require fewer
> patches than CMYK) to get a neutral looking gray axis, though
> I'm using much more patches. A soft proof looks fine, but in
> the print, the patches of a gray staircase have color casts,
> not a particular trend into one directon, but into different
> [a,b] directions at different lightness levels. I've attempted
> to manually add additional patches near the gray axis, but
> only with very moderate success. Since I have no other
> explanation, I suspect that this driver has highly curved
> "corcscrew" characteristics. Assuming that my suspicion is
> true, do you see any reasonable way to build a profile which
> "unwinds" the corcscrew for such a weird device bahaviour?

It's an interesting problem. Some questions and suggestions come
to mind:

Are you converting from a device space (e.g. some sort of
RGB or a CMYK space) ? e.g. How is neutral defined, in terms
ofd L*a*b*, R=G=B, or K values ?

If so, are you using the same source profile for both the
former (satisfactory) situation, as well as the unsatisfactory
one of the RGB inkjet ?

If not, how sure are you that the source profile is accurate
in the greys, and that it is not that profiles test chart that
needs extra grey test patches. (This has certainly been my
experience in some situations.)

Another question/suggestion is on how you supplemented the
output device test chart. In theory the best way is
an iterative approach. Profile and link as usual.
Feed a series of (input space defined) grey values
into this link, and record the output device values.
Print and measure them. Add the device and PCS values
to the output device test chart and re-create the output
profile. Repeat if necessary.

If the above is not the whole problem (ie. lack of adequate sampling
along the grey axis), then it could be the limitation of the resolution
of the profile. The basic multi-dimensional representation is of relatively
coarse resolution (typically 9-33 per axis), and if the device behaviour
along the neutral axis changes more rapidly than can be represented
by the multi-d tables, the neutral will not be controlled sufficiently.
One way of tackling this as to add more points down the neutral axis
in the test chart (as you have been doing), but also upping the
resolution of the A2B and B2A tables. This is normally controlled by
the -q flags, but can be overridden in some of the programs. The cost
is generally an exponentially greater computation time of course.

The ultimate answer is that such systems benefit from a calibration
system. A calibration system would quite finely sample the individual
colorant channels response, and will quite finely control them. In this
way the neutral axis can be made almost as perfect as is possible,
before the profiling system gets to deal with it. (Argyll doesn't currently
contain tools aimed at this particular task.)

Graeme Gill.

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