[Fsfe-uk] CMYK (was: Content creation)

[Alex Hudson]

On Mon, 2008-01-07 at 16:54 +0000, Ian Lynch wrote:
> So you have to map RGB to CMYK and it has been done in quite a few
> proprietary products so there is some sort of mathematical relationship.
> Sounds like a university applied maths project. Ok, I'm probably missing
> a lot but if that relationship was cracked and put out into the public
> domain surely it would be then relatively easy to build software to
> implement it? The mathematics wouldn't be patentable would it? 

The actual maths is relatively easy, and it's not patented, no. It's
just hard to do well in practice.

So, basic colour theory says that Cyan + Magenta + Yellow (C+M+Y) in
equal proportion is going to give you Black eventually, in the same way
you get white from red, green and blue. Practice is, for various
reasons, it comes out mucky brown. So, then people added Black (K) ink.
Problem is, C+M+Y == K, so if you want a dark colour, what proportion of
that darkness comes from the black? You could turn up the CMY values, or
you could increase the K value - you have two degrees of freedom there,
and they're not equivalent, and the relationship isn't overly
straightforward.

Then you get into issues like, well, what actual paint is being used?
How well does it mix chemically? Is the paper ok holding a lot of ink,
or will that cause things to run/smear/whatever? You know in theory what
tone is supposed to come out when you mix things in a certain way, but
it doesn't actually work in practice.

Then you have the issue of, how do I make sure what I see on the screen
is actually the same colour as what will print? What you have to do is
buy a colourimeter and actually *measure* the output of your monitor.
Printers do similar things with their print output. Then your software
is supposed to tweak images on your screen slightly - maybe via the
video drivers, who knows - so that the imperfections/setup of your
hardware is kind of "cancelled out" by the tweaks to the image, so that
the right colour ends up coming out.

If your images are in CMYK, and you're having to do all this colour
space conversion a. to get them into RGB and b. take into account the
oddness of your video hardware, that starts to become a more and more
complex task.

All these issues and more make CMYK "hard", because you're basically
talking about dealing with output which keeps needing to be bodged in
various ways to make the end result "right". 

Cheers,

Alex.