[argyllcms] Re: CMYK Profile Problem
- From: Nikolay Pokhilchenko <nikolay_po@xxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Mon, 15 Mar 2010 22:43:18 +0300
Graeme, thank You for paying your attention!
Graeme Gill wrote:
> 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.
I've noticed, that for RGB profile with not-neutral RGB 0,0,0 point, the result
still is OK, because the tone change from white to black is stretched.
> > 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 ?
Yes, It may be rather non-neutral, but still dark.
> > 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.
...
> 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.
As I mentioned above, the minor tone shift may be acceptable. In some cases
even the heavy tone shift at blacks can be preferred in an effort to the
contrast.
> > 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.
Yes.
I think there should be a compromise between the dark neutrality and the
contrast. I think the two values: the "lightness" and "unwanted tone shift"
should be computed in perceptual space for every candidate point, then the best
point by lowest sum of this values should be selected.
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