Re: [Gimp-developer] Soft proofing and the GIMP Display Filters and Color Management settings

[Gez <listas ohweb com ar>]

El mié, 12-03-2014 a las 17:27 -0400, Elle Stone escribió:
> On 03/11/2014 02:39 PM, Omari Stephens wrote:

> Hopefully the printer people will correct me if I'm speaking nonsense 
> here. CMYK printer profiles have four channels because ink produces 
> color subtractively, but not perfectly, as inks are not as "narrow pass 
> reflective" as one might like. So using C+M+Y to make black produces a 
> muddy black and uses a lot of ink, which is sloppy to print. So the 
> fourth color is black.

That's spot on. Another reason is that black ink (carbon based) is
cheaper than color inks (and 1 ink pass is cheaper than three pases, and
it dries faster).
However, because blank ink isn't perfect either, a pure black pass
doesn't look "deep" enough in large areas and will look rather like dark
gray than like black, so it's common to add a little C, M and Y to get a
"rich" black.

> More than four colors of ink gives smoother color reproduction and also 
> may extend the available color gamut, depending on the inks. The 
> corresponding ICC profile is a Lookup Table profile, which basically 
> says "r% ink-1 + s% ink-2 + t% ink-3 + u% ink-4 + . . . + z% ink-n" 
> (where r, s, t, u, . . . z are arbitrary percentages) equals a 
> particular location in the CIELAB reference color space, for all 
> possible combinations of various percentages of the n available inks.

The color profile also contains additional information like black
generation, and TAC (Total Area Coverage percentage, a maximum ink
coverage recommended for the media used). If you go beyond that value
the media will take longer to dry, dot gain could saturate and mud
details, etc.

> > In the New GEGL World, converting between different channel layouts is
> > going to be a reality, and we should at least put _some_ thought into
> > what that means for color management.  Of course, this is way out of my
> > depth, and I have no idea.
>
> I'm also curious as to what gegl n-channel editing might be like. Soft 
> proofing to an n-channel printer is a one use case for n-channel 
> editing, when the goal is to convert to the n-channel ICC profile and 
> tweak the channels while soft proofing. Hopefully again the printer 
> people will correct me if I'm speaking nonsense.
>
> Dan Margulis gives examples of image editing in an artificial CMYK 
> matrix color space, requiring four channels.

Margulis is a respected name, but I'd take what he says with a grain of
salt. The last time I checked he still insisted that doing creative
editing in device CMYK is a great idea and that "color management
failed", something that contradicts the direction of the entire graphic
industry for the last 15 years.

> Would there be a use case for editing in n-space (as opposed to soft 
> proofing to an n-space output profile), where n is greater than 4?

If you have to treat one of the CMYK primaries as a spot color, or if
you need an extra spot color, then yes.
It's indeed useful and a quite common requirement in the print industry.
For instance, if you have a color that can't be achieved mixing CMYK
inks (saturated greens, oranges, blues, etc.), an extra print pass is
used, inking with a specially formulated ink that reproduces the exact
color you need.
That's what a "spot color" means. When you want to get red mixing 100%
yellow and 100% magenta (and you want that exact combination) you're
using both yellow and magenta as they were spot channels. It has nothing
to do with CMYK, because you're overriding color management and using an
arbitrary mix, not a colorimetric translation.
Perhaps this is not a popular point of view, but in my opinion, using
CMYK just because you want to tweak channels manually (as if it was
possible to predict the printed output of that procedure) is a bad idea.
If you want to work on a computer screen and send the output to print,
the most reliable way to get the color appearance properly translated is
a solid color managed workflow.

Spot plates only have to be color corrected for previewing purposes, but
they won't be separated in individual channels. They are extra channels,
completely separated from the CMYK process colors. The only interaction
with the CMYK channels is defining overprinting/knockout.

Gez