RALCOLOUR_MATCH(9) RALCOLOUR_MATCH(9)
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NAME
ralcolour_match - this is not a program, this is a set of instructions
for the VDU to printer colour matching procedure.
ORIGIN
Rutherford Appleton Laboratory
AVAILABILITY
The source code of this set of programs together with information on
how to install them is available from the University of Kent.
DESCRIPTION
ralcolour_match is a set of programs and files used in the VDU to
printer colour matching procedure. This procedure is meant for users
who have Colour PostScript printers (running PostScript level 1 with
colour extensions) and want to enhance the quality of their hardcopy
output. This procedure gives a cheap and easy to use solution to the
problem, but does not attempt to give an absolute and general
solution.
This manual page describes the whole procedure and the format of files
containing data collected by taking measurements. The description on
how to use the relevant programs of this procedure is given separately
and can be obtained by referring to the appropriate manual page.
The colour matching procedure consists of three stages:
- gray balance and contrast adjustment,
- incorporation of the suplementary black printer, and
- saturation improvement and colour correction.
The first two stages involve printing relevant images, taking some
measurements, storing measurement tables into a file, running programs
that produce colour matching tables and justifying achieved results.
This version of software implements only the first stage of the whole
procedure. The other two stages will be supported in the next release.
ralcolour_match consists of programs:
ralmatch
- command line version of the filtering program
ralmatchx
- Motif version of the filtering program
16shades
- the monitor calibration program
midtone
- the program used to measure values of Red, Green and Yellow
colours needed to produce monitor's midtone (midtone is the gray
colour consisting of 50% of black and 50% of white pigment)
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charts.ps
- PostScript program for printing colour charts
black.ps
- PostScript program for printing a black square
ralcol_match
- the program to produce monitor and printer tables for ralmatch
and ralmatchx programs.
MEASUREMENT MATERIAL
In order to conduct measurements, the Kodak Reflection Density Guide
(catalogue No 146 5947, Kodak Publications Q16) is needed.
STAGE I
Stage I of the procedure ensures that colours that are gray on the
monitor are also gray on the printer and that the lightness of the
screen grays linearly maps to printer lightness values. To complete
Stage I of the procedure, execute the following steps:
[1] Do the monitor calibration - Run the 16shades program. It will
display 16 gray squares with colours going from black to white in
equidistant steps. Adjust monitor's brightness and contrast so
that as many squares as possible appear distinct.
[2] Determine the monitor midtone colour - Run the midtone program.
It will display a gray background and a rectangle with a
chequered pattern of black and white squares in the middle of the
screen. Adjust the colour of the background so that the
difference in the colour between the rectangle and the background
disappears. Type man midtone for more information on how to run
the program. It is desirable to have more than one measurement
result and to have up to 6 people involved, each performing this
step separately. Results are then stored in the monitor
measurement file (the name of the file is user defined; the
format of the file is described later in the text).
[3] Determine density of the black print - Send the black.ps
PostScript file to the printer involved in the colour matching
exercise. A black square on the white paper will be produced.
Measure the density of the black patch using the Kodak Reflection
Density Guide in the following way: look through each of the
holes from the guide and compare the colour seen through the hole
with the colour from the Guide surrounding the hole. When the
closest match of the two colours is achieved, read the number
below the colour from the Guide. This is the density of the black
square. Note: if the colour seen through the hole does not match
any in the Guide, find two squares from the Guide, one lighter
and the other darker than the colour from the hole and manually
interpolate their density values. Write the results to the
printer measurement file (the name of the file is user defined;
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the format of the file is described later in the text).
[4] Print colour charts - Send the charts.ps PostScript file to the
printer involved in the colour matching exercise. A set of
printouts will be produced, each consisting of 4, 5 X 5 squares
filled with a solid colour. Values of the three colour
coordinates (Cyan, Magenta and Yellow) will be printed below each
square. Each quarter of a page should contain at least one gray-
scale colour square. These colour charts are used in the
subsequent printer measurement.
[5] Measure densities and determine the printer response
characteristic - In order to measure densities, the Kodak Density
Guide and colour charts are needed. The measurement procedure is
similar to one described under paragraph 3. ( Determine density
of the black print ). Comparing the colours from the Guide and
colours from the charts, identify squares whose colours are
achromatic (i.e. gray scale). Read three values (for Cyan,
Magenta and Yellow) below these squares. Note there should be at
least one square per quarter page whose colour is gray scale. If
there is not a single achromatic square on a quarter page, find a
region on the page where a gray colour should be and manually
interpolate Cyan, Magenta and Yellow values. The density of these
colours should also be determined. Find the closest match of the
colour from the charts and the colour from the Guide and then
read the number below the colour from the Guide. Note: if the
colour seen through the hole does not match any in the Guide,
find two squares from the Guide, one lighter and the other darker
than the colour from the hole and manually interpolate their
density values. Following this procedure a list of colour value
triplets coupled with the density values will be produced. It is
desirable to have more than one measurement result and to have up
to 6 people involved, each performing this step of the whole
procedure separately. Each person produces a separate set of
measurement results and all results are then stored in the
printer measurement file one after the other (the name of the
file is user defined; the format of the file is described later
in the text).
[6] Produce monitor and printer tables - run the ralcol_match program
specifying the names of the monitor measurement and printer
measurement result files where the results of the above
measurements are stored. When invoking the program, names of the
files where monitor and printer tables will be stored should be
specified as well.
[7] Verification - use one of example images installed with this
package (or an image having plenty of gray scale colours and some
chromatic colours) and run ralmatch or ralmatchx filter program
specifying the just created monitor and printer table files.
Compare VDU and printer image (together with an old printer image
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produced without the filter program). The colour matched print
image should be lighter than both the VDU and the old image, but
grays from the VDU image should be reproduced as grays on the
colour matched print but could be chromatic on the old print. In
the case where grays are not reproduced as grays after colour
matching or if one is not satisfied with results in general, the
whole procedure should be repeated. Note the colour matched
picture can have unsaturated colours and poorly produced black
(i.e. the black is not black but dark gray), especially if the
printer uses black suplement (i.e. uses black ink in addition to
cyan, magenta and yellow ink in order to print all colour
variations). This is because stage I of the colour matching
procedure does not take into account black ink suplement and when
producing printouts switches off the black colour completely.
This problem will be cured after stage II of the colour matching
procedure.
STAGE II
The stage II is part of the next release.
STAGE III
The stage III is part of the next release.
FILE FORMATS
There are two types of input files for ralcol_match : monitor
measurements results and printer measurement results files. Their
formats differ slightly and will be described separately.
The two output files from the ralcol_match program are generated
automatically and their formats will not be discussed.
MONITOR MEASUREMENTS RESULTS FILE
This is a simple file containing the value measured by running the
midtone program. Each value measured by different person must be
written as a separate line.
example
0.728882
0.732697
This is a file containing values measured by running the midtone
program twice, each time having a different person involved in
measurements.
PRINTER MEASUREMENTS RESULTS FILE
The printer measurements results file consists of several separate
parts:
[1] a single line with two data values: one for the minimum density
(measured as a density of the white paper) and second value for
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the maximum density (measured as a density of a black patch
produced by setting cyan, magenta, yellow and black colours to
maximum).
[2] set of data values measured by a single person. This set consists
of several lines, each containing four values: cyan, magenta and
yellow colour values that produced the patch whose density was
measured and the measured density value.
Line containing minimum/maximum density and each set of measurements
done by a different person are separated from each other by one or
more line whose the first character must be # (hash). Any other
combination of characters (or user comment) can be written after the
first # character. It is also valid to put a heading at the beginning
of the file, assuming that each line of the heading begins with #
(hash) character.
An example of the printer measurements file format is given in the
CMYtoDensity file in the lib directory.
LIMITATIONS
The colour matching prodedure can be performed under following
conditions:
[a] operating system - Unix;
[b] language - C;
[c] windowing system - X11 version 5 (OSF/Motif Toolkit Version 1.1
or above
is required in order to run the ralmatchx program, but if not
available, the ralmatch program can be run instead);
[d] display - a minimum 8-bit colour;
[e] printer - PostScript level 1 with colour extensions;
[f] PostScript images - image has to be written using the RGB format
at 8-bits per component. The image has to have lines that are at
least 50 characters long. Note: all the limitations imposed on
the format of the PostScript image will be cured in future
releases;
[g] Current version of the ralmatch program favours cursor movements
using the BackSpace character. If this option is not available
with your system, the feature can be disabled from the Makefile.
SEE ALSO
ralcol_match, ralmatch, ralmatchx, midtone, 16shades, charts.
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AUTHORS
R Popovic and R J May ( RAL ID ) are the original authors.
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