Description of the .cal format
Device calibration information. This is ASCII text, CGATS, Argyll specific format, used
to hold a description of device setup information that brings it to a
desired calibration state. Calibration files are created by synthcal, dispcal
and printcal.
While fully compatible with the CGATS.5 Data Exchange Format, the
particular required keywords and fields are unique to Argyll, hence an
Argyll specific file identifier CAL
is used to avoid confusion with standard ANSI or CGATS files.
The .cal format changes from
time to time with new releases, to add new functionality, but generally
retains backwards compatibility. Note that in the description below,
the word "may" indicates an optional component, while the word "shall"
indicates a necessary component.
Generally a .cal file contains only one table, the table containing the
setup information.
The table contains the following:
The file identifier (First 7 characters) shall be CAL.
A # character introduces a
comment.
There may be DESCRIPTOR, ORIGINATOR, or CREATED keywords and values (as per
CGATS).
There shall be a DEVICE_CLASS
keyword that has a value of "OUTPUT",
"DISPLAY" or "INPUT".
This indicates what type of device the calibration information is
suitable for.
There shall be a keyword COLOR_REP
that has a value that indicates what colorspaces of the device values.
The colorspaces shall be encoded with one or two letters
per
component. The device spaces shall use the
following letter encoding:
Cyan
C
Magenta
M
Yellow
Y
Black
K
Orange
O
Red
R
Green
G
Blue
B
White
W
Light Cyan
c
Light
Magenta
m
Light
Yellow
y
Light
Black
k
Medium Cyan
2c
Medium Magenta
2m
Medium Yellow
2y
Medium Black
2k
Light Light Black 1k
There may be an a prefix i
preceding the device space letter encoding, indicating that although
the space appears to be an additive space, it is in fact a subtractive
device.
Typical values might be: "RGB" for an RGB display, "iRGB"
for an RGB printer, "CMYK" for
a
printer, "RGB" for an RGB
scanner.
The NUMBER_OF_FIELDS keyword
shall have a value that indicates the number of fields in each data
set, e.g. 4 (as per CGATS).
The start of the declaration of the fields shall be marked by the BEGIN_DATA_FORMAT keyword (as per
CGATS).
The fields shall use the standard CGATS pattern of the full colorspace
identified followed by the individual colorant identifier. There shall
be an initial input index value identified by the letter I.
For an RGB device, the
following fields would be used:
RGB_I
The device value input value between 0.0 and 1.0.
RGB_R
The Red device value input value between 0.0 and 1.0.
RGB_G
The Green device value input value between 0.0 and
1.0.
RGB_B
The Blue device value input value between 0.0 and
1.0.
The corresponding field names for a CMYK device would be CMYK_I, CMYK_C, CMYK_M, CMYK_Y and CMYK_K, etc.
The definition of the fields shall be terminated by the END_DATA_FORMAT keyword (as per
CGATS).
The NUMBER_OF_SETS keyword
shall have a value that indicates the number of sets of data, e.g. 256 (as per CGATS).
The start of the values of the data sets shall be marked by the BEGIN_DATA keyword (as per CGATS).
Each set of data shall be on one line, and shall be separated by white
space. All values shall be normalized to lie between 0.0 and 1.0.
The end of the values of the data sets shall be marked by the END_DATA keyword (as per CGATS).
There may then be device type specific extra tables that hold target or
device behavior information. These will be specific to the tool that
creates that particular type of calibration.
Generally any other keywords and values will be ignored.