On Wed, Mar 11, 2009 at 10:33 AM, Rob Antonishen
<rob.antonis...@gmail.com> wrote:

> Sven mentioned other uses, like spot colour and halftoning.  I can't
> find any references on using gimp channels for spot colour, in fact
> google only finds me claims that a weakness of gimp is that it does
> NOT support spot colours.

GIMP does offer spot channels, but there is no (easy) way to output
the spot channels directly to a printer in grayscale.   Nor does the
channel mixer operate on the spot channels.

I use spot channels to create separations for printing.  Each spot
channel corresponds to a positive that will be used to create a screen
(plus one channel to represent the substrate).  In PS, one can turn
off the RGB channel and use the visibility of the spot channels to
simulate the final output on the printing press.  Turning off
everything except one spot channel renders that spot channel in
grayscale (which makes printing the single channel very easy).
Turning on two or more spot channels renders them in RGB.  This is
incredibly useful when setting up multicolor prints (think of a white
underbase for printing a bright color on a dark substrate).

I can work around most of these things in GIMP by doing things like
reloading the channel as a selection, creating a new layer and filling
it with black and printing just that layer - but it can be quite
cumbersome.  Also, PS supplies a very badly-named function called
"Apply Image" that allows you to take the contents of any given
channel and apply it to the contents of any other channel using the
available blending modes (multiply, screen, etc.).  This is the real
power behind channels.

99% of people will likely not need spot channels, but to the reaming
1% they are quite useful.  Hopefully with the coming of GEGL it will
become easier to do some of this pre-press separation and channel

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