[argyllcms] Re: Average bug/Density curves/ xicclu -g predictability issue
- From: Elena [service address] <1007140@xxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Mon, 24 Jan 2011 18:38:25 +0100
Hello Graeme
On 24-Jan-2011, Graeme Gill wrote:
> Yes, that's what I would expect. Same with the Argyll code. If you were to
> convert each pixel using xicclu -fif, and print it with a printer that
> can print each pixel (ie. not a printing press that could have registration
> problems), then the problem would not be evident, since there is no
> interpolation in device space.
Exactly.
> Sure. That will solve this particular problem. The problem is that it
> (typically)
> compromises the gamut severely. This article
> <http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.28.47>,
> illustrates the problem. The classic result of a poor choice of separation
> (ie.
> using the one in the PostScript Red Book, 7.2.3 in the 3rd edition), is a
> gamut that
> looks like the CMY cube with the addition of a thin "spike" at the black end,
> where
> the K has an effect. What's missing is the dark saturated colors...
Thanks for the link. Yes I agree on the gramut being reduced, see also my
answers to Gerhard.
After I have carefully analyzed again discrete slices from an argyll generated
B2A table, with some quick and dirty code written for the occasion and the
help of ICCProfileInspector's LUT dump capability, I'm more and more convinced
that my idea of interpolating between different K curves could really help.
To summarize: for each main diagonal in the Lab space (or whathever the 3D PCS
is)
going to absolute black [even those going to edge centers if you compute it
with pos/neg coordinates, so to include red->black, cyan->black, white->black,
etc.]
you compute a K separation curve, the way xicclu -fif would do if it had already
the suggested -p option available, using the -k or -K parameters given.
All points laying on the main diagonals would then get their proper separation,
optimally chosen and checked by the user with xicclu, if the case.
Those laying between diagonals are constrained to a K curve resulting from
interpolating the K curves of the diagonals around [allowing any colorimetrical
mismatch]. Since the pre-computed diagonals are just those going to absolute
black,
you can even think in angular terms around the main diagonal for interpolating
(a kinda conical space to interpolate K curves into by angulus/radius).
Operating this way, the gamut crop of a crude gcr/ucr would be greatly
minimized,
since many diagonals will now approximate the gamut shape. Also, for better
accuracy (at user's risk) you can also use more intermediate diagonals going
to black (even those at 22.5 degrees, for example).
Btw you told you have other priorities at the moment, so there's no problem.
I hope you will give some weight to the proposed idea though.
Bye
/&
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