grd2cpt(1) GMT grd2cpt(1)
NAME
grd2cpt - Make linear or histogram-equalized color palette table from
grid
SYNOPSIS
grd2cpt grid [ -A[+]transparency ] [ -Ccpt ] [ -D[i] ] [
-E[nlevels] ] [ -F[R|r|h|c ][+c]] [ -Gzlo/zhi ] [ -I[c][z] ] [
-Lminlimit/maxlimit ] [ -M ] [ -N ] [ -Q[i|o] ] [ -Rregion ] [
-Szstart/zstop/zinc ] [ -T-|+|_|= ] [ -V[level] ] [ -W[w] ] [ -Z ]
Note: No space is allowed between the option flag and the associated
arguments.
DESCRIPTION
grd2cpt reads one or more grid files and writes a static color palette
(CPT) file to standard output. The CPT is based on an existing dynamic
master CPT of your choice, and the mapping from data value to colors is
through the dataas cumulative distribution function (CDF), so that the
colors are histogram equalized. Thus if the grid(s) and the resulting
CPT are used in grdimage with a linear projection, the colors will be
uniformly distributed in area on the plot. Let z be the data values in
the grid. Define CDF(Z) = (# of z < Z) / (# of z in grid). (NaNs are
ignored). These z-values are then normalized to the master CPT and col-
ors are sampled at the desired intervals.
The color palette includes three additional colors beyond the range of
z-values. These are the background color (B) assigned to values lower
than the lowest z-value, the foreground color (F) assigned to values
higher than the highest z-value, and the NaN color (N) painted wherever
values are undefined. For color tables beyond the standard GMT offer-
ings, visit cpt-city: http://soliton.vm.bytemark.co.uk/pub/cpt-city/.
If the master CPT includes B, F, and N entries, these will be copied
into the new master file. If not, the parameters COLOR_BACKGROUND,
COLOR_FOREGROUND, and COLOR_NAN from the gmt.conf file or the command
line will be used. This default behavior can be overruled using the
options -D, -M or -N.
The color model (RGB, HSV or CMYK) of the palette created by makecpt
will be the same as specified in the header of the master CPT. When
there is no COLOR_MODEL entry in the master CPT, the COLOR_MODEL speci-
fied in the gmt.conf file or on the command line will be used.
REQUIRED ARGUMENTS
grid Names of one or more grid files used to derive the color palette
table. All grids need to have the same size and dimensions. (See
GRID FILE FORMATS below).
OPTIONAL ARGUMENTS
-A[+]transparency
Sets a constant level of transparency (0-100) for all color
slices. Prepend + to also affect the fore-, back-, and nan-col-
ors [Default is no transparency, i.e., 0 (opaque)].
-Ccpt Selects the master color table to use in the interpolation.
Choose among the built-in tables (type grd2cpt to see the list)
or give the name of an existing CPT [Default gives a rainbow
CPT]. Yet another option is to specify
-Ccolor1,color2[,color3,a|] to build a linear continuous CPT
from those colors automatically. In this case colorn can be a
r/g/b triplet, a color name, or an HTML hexadecimal color (e.g.
#aabbcc ).
-D[i] Select the back- and foreground colors to match the colors for
lowest and highest z-values in the output CPT [Default uses the
colors specified in the master file, or those defined by the
parameters COLOR_BACKGROUND, COLOR_FOREGROUND, and COLOR_NAN].
Append i to match the colors for the lowest and highest values
in the input (instead of the output) CPT.
-E[nlevels]
Create a linear color table by using the grid z-range as the new
limits in the CPT. Alternatively, append nlevels and we will
resample the color table into nlevels equidistant slices.
-F[R|r|h|c][+c]]
Force output CPT to written with r/g/b codes, gray-scale values
or color name (R, default) or r/g/b codes only (r), or h-s-v
codes (h), or c/m/y/k codes (c). Optionally or alternatively,
append +c to write discrete palettes in categorical format.
-Gzlo/zhi
Truncate the incoming CPT so that the lowest and highest z-lev-
els are to zlo and zhi. If one of these equal NaN then we leave
that end of the CPT alone. The truncation takes place before
any resampling. See also manipulating_CPTs
-I[c][z]
Append c [Default] to reverse the sense of color progression in
the master CPT. Also exchanges the foreground and background
colors, including those specified by the parameters COLOR_BACK-
GROUND and COLOR_FOREGROUND. Append z to reverse the sign of
z-values in the color table. Note that this change of z-direc-
tion happens before -G and -T values are used so the latter much
be compatible with the changed z-range. See also manipulat-
ing_CPTs
-Lminlimit/maxlimit
Limit range of CPT to minlimit/maxlimit, and donat count data
outside this range when estimating CDF(Z). [Default uses min and
max of data.]
-M Overrule background, foreground, and NaN colors specified in the
master CPT with the values of the parameters COLOR_BACKGROUND,
COLOR_FOREGROUND, and COLOR_NAN specified in the gmt.conf file
or on the command line. When combined with -D, only COLOR_NAN is
considered.
-N Do not write out the background, foreground, and NaN-color
fields [Default will write them].
-Q[i|o]
Selects a logarithmic interpolation scheme [Default is linear].
-Qi expects input z-values to be log10(z), assigns colors, and
writes out z [Default]. -Qo takes log10(z) first, assigns col-
ors, and writes out z.
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more a|)
Specify the region of interest.
-Szstart/zstop/zinc or -Sn
Set steps in CPT. Calculate entries in CPT from zstart to zstop
in steps of (zinc). Default chooses arbitrary values by a crazy
scheme based on equidistant values for a Gaussian CDF. Use -Sn
to select n points from such a cumulative normal distribution
[11].
-T-|+|_|=
Force the color table to be symmetric about zero (from -R to
+R). Append flag to set the range R: - for R =|zmin|, + for R =
|zmax|, _ for R = min(|zmin|, |zmax|), or = for R = max(|zmin|,
|zmax|).
-V Verbose operation. This will write CDF(Z) estimates to stderr.
[Default is silent.]
-W Do not interpolate the input color table but pick the output
colors starting at the beginning of the map. This is particu-
larly useful in combination with a categorical color table. Can-
not be used in combination with -Z. Alternatively, use -Ww to
produce a wrapped (cyclic) color table that endlessly repeats
its range.
-Z Will create a continuous color palette. [Default is discontinu-
ous, i.e., constant color intervals]
-^ or just -
Print a short message about the syntax of the command, then
exits (NOTE: on Windows just use -).
-+ or just +
Print an extensive usage (help) message, including the explana-
tion of any module-specific option (but not the GMT common
options), then exits.
-? or no arguments
Print a complete usage (help) message, including the explanation
of all options, then exits.
GRID FILE FORMATS
By default GMT writes out grid as single precision floats in a
COARDS-complaint netCDF file format. However, GMT is able to produce
grid files in many other commonly used grid file formats and also
facilitates so called apackinga of grids, writing out floating point
data as 1- or 2-byte integers. (more a|)
NOTES ON TRANSPARENCY
The PostScript language originally had no accommodation for trans-
parency. However, Adobe added an extension that allows developers to
encode some forms of transparency using the PostScript language model
but it is only realized when converting the PostScript to PDF (and via
PDF to any raster image format). GMT uses this model but there are
some limitations: Transparency can only be controlled on a per-object
or per-layer basis. This means that a color specifications (such as
those in CPTs of given via command-line options) only apply to vector
graphic items (i.e., text, lines, polygon fills) or to an entire layer
(which could include items such as PostScript images). This limitation
rules out any mechanism of controlling transparency in such images on a
pixel level.
COLOR ALIASING
For best result when -E is used we recommend you do no append a spe-
cific nlevels. This way the original CPT is used exactly as is but the
z boundaries are adjusted to match the grid limits. Otherwise you may,
depending on the nature of the input CPT, miss aspects of the color
changes by aliasing the signal.
EXAMPLES
Sometimes you donat want to make a CPT (yet) but would find it helpful
to know that 90% of your data lie between z1 and z2, something you can-
not learn from grdinfo. So you can do this to see some points on the
CDF(Z) curve (use -V option to see more):
gmt grd2cpt mydata.nc -V > /dev/null
To make a CPT with entries from 0 to 200 in steps of 20, and ignore
data below zero in computing CDF(Z), and use the built-in master cpt
file relief, run
gmt grd2cpt mydata.nc -Crelief -L0/10000 -S0/200/20 > mydata.cpt
SEE ALSO
gmt(1), gmt.conf(5), grdhisteq(1), grdinfo(1), makecpt(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 grd2cpt(1)
gmt5 5.4.2 - Generated Wed Jun 28 18:09:08 CDT 2017