[argyllcms] Re: Of ink limiting and maximizing gamut
- From: "Alastair M. Robinson" <profiling@xxxxxxxxxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 29 Jan 2009 01:37:06 +0000
Hi :)
Graeme Gill wrote:
There two "blue-turns-purple problem"s that I'm aware of:
Perceptual hue non-linearity of L*a*b* space.
Yup, that's the one I was referring to - the one which means an L*ab
plot of a constant hue with increasing saturation wouldn't necessarily
be a straight line.
The best idea I have off the top of my head, is that when calibration
curves are being applied, the in-profile ink limiting is unaware of
> exactly what ink is being put down
so you have to err on the cautious
> side when setting the total ink limit, thereby unreasonably limiting the
> gamut.
Ah, yes, good point - so the test chart samples, for instance, [0, 50,
50, 0] when searching for a saturated red, and the resulting ink
coverage could well be as low as [0, 20, 20, 0] once it's gone through
the linearization curve and base gamma adjustment. Yup, that explains
my problem, I think.
This is also an issue when the profile channels are actually composites of
combinations
of light and dark.
Yup, I think this issue is why I've seen unexpected paper-soaking in a
couple of tests. I also have no idea quite how linear the relationship
is between input-value and ink coverage in pl/mm². In Gutenprint's full
raw mode I would imagine pretty good, but I imagine any miscalibration
of dropsizes could have a big impact there.
Since the very beginning I've had some hooks in place inside Argyll to
allow this to be taken into account, but without having settled on
a calibration table format, the hooks aren't currently able to be used.
Interesting! Thanks for the input :)
All the best,
--
Alastair M. Robinson
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