Re: linearization - luminance, chroma or density?
Re: linearization - luminance, chroma or density?
- Subject: Re: linearization - luminance, chroma or density?
- From: email@hidden
- Date: Mon, 22 Jan 2007 19:31:18 +0100
I agree with Graeme.
1. In order to achieve lower L you have to add black in the max
chroma case.
2. Adding black strongly reduces chroma for most inkjet inks. This
can easily be confirmed by checking an inkjet profile. Note how all
the colors at the bottom seem to "implode" once black is added.
3. So, if a color reaches maximum chroma at some point, and beyond
that it merely reduces L but at more or less similar chroma, as most
of the inkjets do, than that is preferable over adding black which
significantly reduces chroma.
4. In addition you may achieve darker, more saturated colors with
less ink for even lower L values.
In other words: a cyan wedge ending in a dark, saturated blue, is
preferable over a cyan wedge ending in a light, saturated cyan, then
adding black/magenta to achieve a desired L.
Unfortunately this also means that conventional density linearisation
also fails. Conventional density requires at least two parameters:
1. The part of the spectrum that defines the most absorbing
frequencies for a particular color.
2. A maximum absorption which is used as the max density reference.
Since the color changes as the wedge progresses, defining the
absorption filter is a bit of a problem.Whether there is any
reasonable relation between maximum printable color on a particular
substrate and maximum density for a selected absorption filter, is
impossible to tell without actually printing and looking at the ink/
substrate behavior.
So, in my opinion neither of the options mentioned will help. Neither
just L, nor Chroma, nor Density will cut it. The best option I have
come up with so far is an algorithm based on accumulated error
distribution:
1. Accumulate all deltaE between consecutive patches of a wedge,
2. redistribute the patch values so all deltaE between steps are equal
This will at least give you a perceptual "linear" wedge, much like
conventional densitometry and conventional printing inks. Next step
is how to determine the maximum useful "density", which usually is
right before the point where the deltaE becomes noisy. I think a
directional/vector-based deltaE would be useful here.
This process is also useful in a profiling session after
linearisation. Since the profiling process uses interpolation of a
Lab table for which it is desirable to have the Lab values
distributed as "evenly" as possible.
Regards,
Oscar Rysdyk
On Jan 22, 2007, at 24:10 AM, Graeme Gill wrote:
Ray Maxwell wrote:
All that is necessary, is to do an a* vs b* plot of step wedges
for each color. From this you can clearly see when you have
achived maximum chroma. Moving beyond this point is folly. You
cannot get a larger gamut than this point. You are only adding
muddy density beyound this point.
You can't know that for sure if you're only looking at a* and b* -
the gamut is 3D, so you need to be looking at L* as well, if
you really want to be sure that the gamut is at a maximum.
A colorant could double back in an a* b* projection, yet still be
expanding
the gamut by decreasing the L*. Now for various good reasons you
may choose
not to use some of that gamut, but it's still gamut you are forgoing.
Graeme Gill.
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