Misc technical notes
--------------------


Display Calibration
-------------------

Normally you don't use the vcgt tag to calibrate the display, you use
some other operating system mechanism to set the RAMDAC values, and then
store the curves you arrive at in the vcgt tag, so that they will be
loaded automatically by the operating system when that profile is used.
(Only Apple Mac uses the vcgt tag as far as I know, although there might
be ways of using the vcgt on windows and X11. MSWindowsXP has a
utility available, and there is also  Stefan Doehla's "xcalib").

There are four aspects I can imagine setting using the vcgt:
	1) white point color
	2) maximum brightness
	3) grey balance
	4) response curve ("Gamma" curve).

Both 1) and 2) are set by the maximum values in the three vcgt curves.
The relative values set the white point, the absolute values set the
brightness.

3) Is determined by the relative values of the three curves for
each input value, and might be selected to give a consistent grey
color to that set for the white point.

4) Is determined by the absolute values of the three curves, and might
be selected to make the display behave as if it had an ideal exponential
(or truncated exponential) response.

-> dispcal allows for all of the above.

- Would be good to have "instrument mode" to allow other (non RAMDAC)
manual adjustments made by the user. Include "white target", "Gamma target",
"brightness target" assistance.

-> dispcal allows for all of the above.



Things I'd like to standardise
------------------------------

ICC:
	Tag for TAC. Should include all combination of colorant
	summs limits. Should be min and max ?
	Option for LUT for abitrary limit (similar to gamut LUT).

	Tag for 3D gamut Hull using triangles.

	Standard rendering intent encodings, to be used as basis
	of extra per color component attribute. Backward compatible
	with current ICC rendering intents ?
	Use "schemes" too, to allow for future changes.

	Spectral sub-tag for spectrum encoding.

	New named color using spectral.

	New colorant tag using spectral.

Other:
	Add rendering intent plane support to TIFF etc.


ICC profile locations in various operating systems.
---------------------------------------------------

Someone emailed me about a standard Linux location for ICC profiles,
and I decided to research this a little more than has been done in
the past, so though others might be interested in the results.

> I was wondering if you are aware of any standard location in the Linux
> filesystem to put ICC profiles?

There was some discussion about this on the OpenICC mailing list a while
back:

Stefan Klein wrote:
> Marti Maria, developer of littlecms (www.littlecms.com), suggested
> /usr/share/color/icc and ~/.color/icc as intuitive paths. We decided to
> go along with this.

Kai-Uwe Behrmann wrote:
> I followed the suggestions of using
>  /usr/share/color           for global configuration data and
>  /usr/share/color/icc       for global ICC profiles
>  ~/.color and ~/.color/icc  as user pendant.

Googling for "common Linux ICC profile directory" brings up other references.

For instance Scribus puts its profiles in

    /usr/local/share/Scribus/profiles


SUN has been implementing CMM's on its Solaris system
for a long while (using Kodaks KCMS), and have already
established other conventions in its part of the the Unix world:
> 1. The current directory
> 2. Directories listed by the KCMS_PROFILES environment variable, which is a
> colon-separated list of directories
> 3. /etc/openwin/devdata/profiles
> 4. /usr/openwin/etc/devdata/profiles
as well as having extensive support for ICC profiles in Java, on multiple
platforms (again using KCMS and the native platform CMM's it seems).


SGI has a CMM called Coloratura on their IRIX Unix system, and uses
yet another convention:
> The Coloratura CMS searches for a file sequentially in the directories specified
> by the environment variable CMS_DEFAULT_PATH, which is a colon-separated list of
> pathnames similar to that used in the environment variable MANPATH.
> The value of CMS_DEFAULT_PATH defined in cms.h is
> /var/cms/profiles/local:/var/cms/profiles:.,
> which allows you to place profiles you prefer in /var/cms/profiles/local,
> and generic profiles, which might have the same names, in
> /var/cms/profiles or your working directory.


Apple in OSX (another Unix based system) chooses

    /Library/ColorSync/Profile
    /System/Library/Colorsync/Profiles
    /Network/Library/ColorSync/Profiles
    ~/Library/ColorSync/Profiles

and individual drivers store relevant profiles in their application
resources: eg.

    /Library/Image Capture/Devices/EPSON SScanner.app/Contents/Resources/
and
    /Library/Printers/Canon/BJPrinter/PMs/BJC8200PM.plugin/Contents/Resources/
and
    /Library/Printers/hp/Profiles
and many other places.

Microsoft Windows chooses:

\WINDOWS\SYSTEM\COLOR
\WINNT\SYSTEM32\COLOR
\WINNT\SYSTEM32\SPOOL\DRIVERS\COLOR

depending on the operating system version.
Photoshop on MSWindows seems to use its own location:

\Program Files\Common Files\Adobe\Color\Profiles

Note that Apple ColorSync, Microsoft ICM2.0 and KCMS as implemented
by SUN are all existing CMM's with detailed technical documentation
available via the web (as examples of how it has been done, and the variety
of facilities a CMM may make available.)