[argyllcms] Re: CMYK Profile Problem

Nikolay Pokhilchenko wrote:
Hmm... Which way the ArgyllCMS determinate the printer black point for an RGB 
printer?

There's no such thing as an RGB printer, so in fact there is already a 
transform that
someone has set up to convert from RGB to the actual printer colorant. So we
assume that whoever set this up knows what they were doing, and RGB 0,0,0
must be the black point.

In my practice I can deal with different quality CMYK inks, so the "K" inks hue 
may
be quite high.

You mean it's not very neutral ?

I think it will be more robust to determine the black point only by gamut
envelope (boundaries) and to pay respect to vision adaptation to the printer 
black.

I don't know of any way of determining black reliably from the gamut boundary. 
The lowest
L* could be very non-neutral.

There is no such thing as black adaptation.

May be the optimal black point will be
at the intersection of the straight line Jab 100,0,0 to 0,0,0 and gamut 
boundary at
the gamut bottom.

Certainly one of the alternative is to look for the darkest black that has the 
same
hue as the paper. This should make the white to black axis look uniform.

Another variant is to determine the point at the straight line between
Jab 0,0,0 and 100,0,0 with J=Jmin, where Jmin - the minimum achievable J value 
for given
gamut with given TIL (without
regard for hue). The optimal black point will be the closest for the point 
determined.

Hmm. That would make an interesting not-quite-neutral "bend" target.

Graeme Gill.



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