[argyllcms] Re: "RGB"-printer and inversion in blacks

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Mon, 30 Apr 2012 15:41:04 +1000

Nikolay Pokhilchenko wrote:
> Can You take a look on the device (TI3 attached in archieve)?
> Perceptual and Saturation intents in ArgyllCMS profile are OK, but relative 
> intent is quite
> strange: there is lightness inversion at the black side of a gray gradient.

        thanks for drawing this problem to my attention.
It appears to be due to the way that out of gamut clipping transitions from
exact using L*a*b* within the gamut, to CIECAM02 out of gamut. The crossover
region (4 DE) sometimes gives rise to a dip, mainly because CIECAM02
models flare which compresses the dark region behaviour.

If I reduce the size of the transition region then this effect is
much reduced, although it does not remove other effects such
as cLUT interpolation artefacts, or other artefacts caused by the
devices darkest point not being neutral.

[This change will be in the next release. If you're compiling from
 source you could change xicc/xlut.c line 1348 from
   bf = cdist/4.0;
   bf = cdist/1.0;

Note as well, that perceptual intent returns the closest point
in the gamut, so there are no guarantees about monotonicity
or neutrality. If a gamut surface is convex (and many printer
gamuts are convex in places), then the closest point may "jump"
from one point on the surface to another as you move the target
color smoothly away from the surface. Such jumps can't be
represented in a B2A cLUT, and the resultant device value
interpolation may behave non-intuitively.

Graeme Gill.

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