Marti schrieb:
Hi Marti,
a few comments/questions below.
The algorithm is a little bit more complex. Here is a coarse outline:
1) A K tone curve from K-> K' is computed. That is, which amount of K'
on the output space gives same L* as which amount of K in input. That
is done by measuring ramps of (0,0,0, K) using the proof direction,
on both input and output profile. Then joining the obtained curves.
2) The tone curve is used on (0,0,0,K) -> (0,0,0,K') so pure black is
always preserved as pure black.
The chromaticity of the src and dst K-only axis may differ a couple of
dE_ab units. If [0,0,0,K] is mapped to [0,0,0,K'], but e.g.
[0.001,0,0,K] is mapped to [C',M',Y',K'], such that [0.001,0,0,K] and
[C',M',Y',K'] match colorimetrically, doesn't this produce a
discontinuity immediately near the K-only axis? Shouldn't there be done
a smooth blending? (sure, the limited CLUT resolution will smooth the
transition, but is this sufficent?).
Another related issue, where IMO the desired behaviour is not so clear,
and different people may have a different opinion:
Example:
Assume that [0,0,0,K] and [C,M,Y,0] have the same color in the source
color space.
If [0,0,0,K] is mapped to [0,0,0,K'] in the destination color space,
shouldn't also [C,M,Y,0] be mapped to [C',M',Y',0], such that [0,0,0,K']
and [C',M',Y',0] (in the destination color space) have the same color as
well?
Or in other words:
If two (different) CMYK values have the same color in the source color
space, wouldn't one expect, that the colors of the transformed CMYK
values also match in the destination color space?
2) AToBXX tags on output profile are reversed and a special BKToA is
computed. This is a new LUT has a extended PCS using Lab + K.
3) Then a devicelink is built by using the original AtoB plus this new
BKToA. Original K is passed to the extended PCS across tone curve.
The method for inversion is a modified Newton-Raphson extended to 3
dimensions. The original CMY + K across tone curve is used as a seed for
the search. The algorith iterates across Jacobian as far as the error
decreases and the system is convergent.
Isn't there a risk, that the Newton method gets stuck in a local minimum?
This have some nice side effects:
- If the color cannot be matched at all, the original separation is
keept as much as possible.
I've seen, you have again removed the MaximumError threshold and the
different handling below and beyond the threshold. That's IMO good,
since it also did introduce a discontinuity at the threshold.
Regards,
Gerhard
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