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

[Øyvind Kolås <pippin gimp org>]

On Wed, Mar 12, 2014 at 10:27 PM, Elle Stone
<ellestone ninedegreesbelow com> wrote:
> > 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.

In the end; you'd have the buffers for individual ink plates in GIMP
(more likely than having it as an n-component buffer as you would if
there was a CMYK mode along with RGB); and need to preview/softproof
it. In some cases lcms might provide what is needed; in others like
silk screen printing a couple of metallic inks on colored fabric -
maybe not.

Today I wrote a proof of concept spectral soft proofing op for GEGL.
  https://git.gnome.org/browse/gegl/tree/operations/workshop/ink-simulator.c

The settings are edited in a little multi-line editor, with a default
configuration like this:
-----
illuminant = 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
substrate  = white
ink1 = cyan
ink2 = yellow
ink3 = magenta
ink4 = black opaque
-----

It currently uses an "RGBA float" as input; this could be changed to
multiple buffers or an n-channel babl format. The 4 inks in order,
though the order only matters if inks are suffixed with opaque. One
can either pass in a 20 space separated numbers between 0.0 and 1.0
(or higher), or one of few hardcoded colors.

The op is in the operations/workshop/ directory of GEGL; so you have
to type make && sudo make install; in the workshop dir to install it.
For quickly preparing a file to use with the default settings;
decompose a color imake to CMYK and recompose it to RGBA; then invoke
the ink-simulator from the GEGL tool.

The model is far from complete. It lacks spectral curves for commonly
used illuminants, sane spectral responses for the inks, it doesn't
take any form of dot gain into account - and has a naive idea about
types of ink.
With the addition of a couple of animated frames and more tweaks; one
could add the ability to preview glossy or metallic inks as well.

/pippin