Re: debian/linux Colour Management Module
Bruce Perens says:
>Yes, some of us do real computer graphics on Debian. The color module
>belongs at the X level. I'd be interested in hearing about any you find
>(or make), one should be packaged for Debian. Could this not be done as
>a TCL/TK program?
>Has anyone interfaced a colorimiter? We have some lying around here, but
>I don't have the slightest idea how to use them (and I'm color-blind).
I'm pleased to hear that I'm not the only interested, as I'm looking around
for a couple of weeks.
Of course, the first idea seems to be that this is a matter of X, but a CMM
is also active when just sending a (colour) PostScript file to a printer or
another PostScript file which is thought for an expensive typesetter we
don't have at home. But I'll start at the other end.
High quality output of colour graphics have some trouble for most frequently
two reasons: (a) Most programs are RGB oriented (try imagin a POV
which is CMYK based!), which is not subject to a trivial conversion to
CMYK because addititive and substractive colour spaces are as different
as video and audio. (b) Neither RGB nor CMYK are device independent,
and neither show a linear property. While RGB just depends on a (calibrated)
monitor, CMYK usually depends on typesetter settings (resolution, screen
frequency, and angle, besides spot function, etc.), on the type of paper we
will be printing, the dyes the printer will use, the humity (season) of the printer
service' area, etc.
Lately I found out, that there is a `Internation Colour Consortium' (ICC)
composed, as usual, by the giants (Adobe, Agfa, Apple, SGI, Linotype, etc).
The have a draft of an pseudo industry standard which is public and specifies:
(roughly) All graphics pass through a CMM (colour management module)
which tries to find out the originator and an appropriate ICC-PROFILE.
This profile explains by means of tables and algorythms how to transform
the originator's colour space (e.g., the RGB of a particular screen) into
either CIE-Lab or CIE-XYZ, both of which are device independent. As a
next step, a destination device is looked for which has associated another
ICC-PROFILE, and which allows to perform the second transformation.
This nice idea, of course, depends by itself on the quality of the ICC-Profiles,
which not only are propietary, but have also space reserved for private
implementations (whose depeloper must be registered in ICC to get a label,
a matter of $2000 a year). So implementation is a two side matter: Write an
CMM, and provide an infinite amount of good profiles. (An ICC-Profile has
a structure which is very similar to a TIFF). I sent the ICC chairman a
message asking for a sample profile, but never got an answer; may be they
don't deal with some nobody as me.
I never have seen a colorimeter, so I've no idea how to interface it with
linux. (Shouldn't be that hard). I found in the `PostScript Corner' at a
swiss university, a short description for how to calibrate a colour space.
This could be a nice starting point. Also PANTONE's could be very interesting
to get into a ICC Profile, but in this case, I'm afraid some investigation in
matters of copyright will be necessary.
Regarding TCL/TK, I see no reason why not use it on front end software
(interactive PostScript spooler, tools for creating an ICC profile), but
I wouldn't limit it to this, so if X is not available, it's still useful.
Currently I'm struggeling with a postgres application, which I hope to finish
this weekend. Then I'll try to write some kind of library which implements
reading / writing of ICC-Profiles and performs the specified actions on
PNG files. But I've no idea how to integrate this into X and/or kernel level,
so I'll not be able to do this.
Another thing, which is not directly related, is an exhaustive interface to
Adobe's PPD files, altough it'll also help wih colour (available resolutions,
screen frequencies, etc.) This is at least worth another library.
I'd be happy to be able to share work on this, with the aim to provide a
new debian package. Help with the creation of Profiles will be needed in
any case, as this is really `no limits'. Anyway, as this is thought to be
matter of some well doted giants, the project is perfectly aligned with
Linux, Ghostscript and the like.
Sorry for the long message.
Ciccio C. Simon
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