Overview of Color correction.
Color Pixmap input to pnm2ppa (ppm format, produced
by the ghostscript output devices ppmraw or ppm)
specifies the color of a pixel as three coordinates (Red, Green, Blue) in the
range 0-255. The program pnm2ppa converts these to relative amounts
of Cyan, Magenta and Yellow ink drops printed on the page.
Color correction allows the user to attempt to match the printed colors to some reference colors, such as those displayed on the terminal, or those printed on a different printer, or those printed on the same printer with a different driver (such as the Win9x drivers supplied by HP with their PPA printers).
The color output from pnm2ppa can be controlled in two ways:
/etc/pnm2ppa.gamma,
but it may also be specified with the pnm2ppa option
pnm2ppa ... -F gammafile ... .GammaR,
GammaG,
GammaB
or
RedGammaIdx,
BlueGammaIdx,
GreenGammaIdx) in the
pnm2ppa configuration file.
pnm2ppa will carry out "color correction",
unless it is called with the --noGamma command line option.
However, unless you set up a color correction scheme, the default
color correction does not change any colors.
Color correction is specified by three color intensity
functions (curves), one for each of
Red, Green, and Blue. In their simplest form, these curves
have a standard form controlled
by a single "gamma" parameter such as
GammaR. The more sophisticated correction, in the
file pnm2ppa.gamma, specifies these curves more generally
as a table of numbers that translate each possible
color intensity value to a new value.
Note that, in principle, different color corrections are needed for different print quality settings, and for different brands and qualities of paper!
At present the methods for constructing a color correction for
pnm2ppa are not very user-friendly. If you can improve
them, please help!.
Color correction with "gamma" values in the configuration file.
The PixMap (ppa) format specifies colors by an intensity for
each primary color, Red, Green and Blue, in the range 0-255, so
(0,0,0) is black, and (255,255,255) is white.
The standard color intensity enhancement function replaces the ppm
intensity i of each primary color by
Enh(i) which is the closest integer
to ( i / 256) raised to the power
Gamma, times 256.
You can specify the Gamma values in the configuration file
as decimal numbers GammaR, GammaG, GammaB,
but how do you find out what values to choose?. Perhaps by experimenting
with changing the Gamma values.
A source of information on Gamma color correction is www.cgsd.com/papers/gamma.html
One method for constructing a Gamma correction
has been built into pnm2ppa, but it
wastes quite a lot of ink, and many users
find it less effective than producing a customized
color correction curve as described in the next section.
You must first use pnm2ppa to print a color test page, and then
generate a file gamma.ppm to compare to it.
To do this, type
pnm2ppa -g -i - -o /dev/lp0
/dev/lp0 by whatever printer device your PPA printer uses,
or use ... -o - | lpr -l , etc.). Note also that
as pnm2ppa is not receiving any ghostscript input to tell it
the paper size, this must be correctly specified by a configuration
file entry or, e.g., a -s a4 option, if the default US Letter
paper is not being used. This procedure will print a color calibration
page.
The printed output is an array of sets of three colored bands, red, blue and green, each of which increases in brightness from left to right. The topmost set of bands should be referrred to as bands "0", the next ones as "1", etc.
Next, run the extra program calibrate_ppa which will produce
a (large, 10MB) PixMap (ppm) file:
calibrate_ppa -g -o gamma.ppm
gamma.ppm in the current directory.
To color-correct the printer so that its colors match those on the
terminal screen,
you should now attempt to view the file gamma.ppm
using some application
(such as the GNU image manipulation program gimp, or xv)
that can view .ppm files. For each color, identify the
printed band (0, 1, 2, ... etc)
that most closely corresponds to the brightness profile
shown on the screen. The default values are now:
RedGammaIdx 0
GreenGammaIdx 0
BlueGammaIdx 0
pnm2ppa
configuration file (usually /etc/pnm2ppa.conf).
The actual Gamma value is then 1.0 - 0.033*(Color)GammaIdx ;
To color-correct with respect to another printer (or the Win9x drivers),
you must manage to print gamma.ppm on that other device, then
make the comparison. (Or print the output of pnm2ppa -g
on that device; the top line is the same uncorrected color data as
gamma.ppm.)
Color correction using a "gamma file".
A "gamma correction file" is a binary file containing 3 x 256 numbers, which are lookup tables of corrected red green and blue color intensities.
The program pnm2ppa will use a default correction file
/etc/pnm2ppa.gamma if it exists, unless the
--noGamma option is used. The default correction file
location can be changed by using the pnm2ppa
option -F gammafile , where "gammafile" is the name
(including the full path) of the gamma correction file to be used.
Currently, there are two available tools for constructing a gamma correction file:
ppagammacorrect is available in the
PPA Color Correction Utilities section of
the pnm2ppa web page
http://sourceforge.net/projects/pnm2ppa,
(but at the time of writing,
this appears to be frozen in early stages of development, with
little documentation, and may not be fully functional).