[argyllcms] Re: 0.60 CMYK profile misshaped

  • From: "Roberto Michelena" <colorsync@xxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Mon, 11 Dec 2006 23:07:10 -0500

Another way is the dot product with the vector to the 100% colorant
value to white (or some other choice related to the underlying
colorant vector direction) in a particular colorspace (XYZ, L*a*b* etc.).

This could be viewed as creating an "optimal" mix of the
co-ordinates as a measure, rather than just picking one
of the co-ordinates.

Sounds neat... will take a look at the idea. I wonder how will it
interact with the 'hook' shape.

XYZ isn't a good choice as absolute measure, as it is
linear light, so the blacks steps will be too big.
A gamma or L* curve applied to XYZ is needed at least.

Of course not raw XYZ.... more like XYZ emulating a reference curve
shape. For example, take offset printing, and try to fit the same
shape (scaling the endpoints).

Density is a log scale, which is why it's not too bad,
although in the Colorbus calibration system I set
a target density curve corresponding to a typical
20% dot gain press curve.

Really the problem is not that much the linearization (incremental dE
or incremental dE00 could be good choices also); the real problem is
determining the ink limits automatically.
Right now, the way I find ink limits is by plotting (in 3D) the
linearization curves, and superimposing that over the target gamut
shape (for example, target being ISOCoated). Then I can clearly see,
by rotating the graph and looking at it, which limits do I have to set
in each ink channel to be able to encompass the target gamut. I can go
that much, or higher; but not lower.
I've seen some systems (EFI/Best, for example, starting with v5)
trying to determine ink limits automatically, for example by chroma.
This fails in too many cases.

-- Roberto Michelena
  Lima, Peru

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