1GRD2CPT(1) GMT GRD2CPT(1)
2
6 grd2cpt - Make linear or histogram-equalized color palette table from
7 grid
8
10 grd2cpt grid [ -A[+]transparency ] [ -Ccpt ] [ -D[i] ] [
11 -E[nlevels] ] [ -F[R|r|h|c ][+c]] [ -Gzlo/zhi ] [ -I[c][z] ] [
12 -Lminlimit/maxlimit ] [ -M ] [ -N ] [ -Q[i|o] ] [ -Rregion ] [
13 -Szstart/zstop/zinc ] [ -T-|+|_|= ] [ -V[level] ] [ -W[w] ] [ -Z ]
14 Note: No space is allowed between the option flag and the associated
16 arguments.
17
19 grd2cpt reads one or more grid files and writes a static color palette
20 (CPT) file to standard output. The CPT is based on an existing dynamic
21 master CPT of your choice, and the mapping from data value to colors is
22 through the data's cumulative distribution function (CDF), so that the
23 colors are histogram equalized. Thus if the grid(s) and the resulting
24 CPT are used in grdimage with a linear projection, the colors will be
25 uniformly distributed in area on the plot. Let z be the data values in
26 the grid. Define CDF(Z) = (# of z < Z) / (# of z in grid). (NaNs are
27 ignored). These z-values are then normalized to the master CPT and col‐
28 ors are sampled at the desired intervals.
29 The color palette includes three additional colors beyond the range of
31 z-values. These are the background color (B) assigned to values lower
32 than the lowest z-value, the foreground color (F) assigned to values
33 higher than the highest z-value, and the NaN color (N) painted wherever
34 values are undefined. For color tables beyond the standard GMT offer‐
35 ings, visit cpt-city: http://soliton.vm.bytemark.co.uk/pub/cpt-city/.
36 If the master CPT includes B, F, and N entries, these will be copied
38 into the new master file. If not, the parameters COLOR_BACKGROUND,
39 COLOR_FOREGROUND, and COLOR_NAN from the gmt.conf file or the command
40 line will be used. This default behavior can be overruled using the
41 options -D, -M or -N.
42 The color model (RGB, HSV or CMYK) of the palette created by makecpt
44 will be the same as specified in the header of the master CPT. When
45 there is no COLOR_MODEL entry in the master CPT, the COLOR_MODEL speci‐
46 fied in the gmt.conf file or on the command line will be used.
47
49 grid Names of one or more grid files used to derive the color palette
50 table. All grids need to have the same size and dimensions. (See
51 GRID FILE FORMATS below).
52
54 -A[+]transparency
55 Sets a constant level of transparency (0-100) for all color
56 slices. Prepend + to also affect the fore-, back-, and nan-col‐
57 ors [Default is no transparency, i.e., 0 (opaque)].
58 -Ccpt Selects the master color table to use in the interpolation.
60 Choose among the built-in tables (type grd2cpt to see the list)
61 or give the name of an existing CPT [Default gives a rainbow
62 CPT]. Yet another option is to specify
63 -Ccolor1,color2[,color3,...] to build a linear continuous CPT
64 from those colors automatically. In this case colorn can be a
65 r/g/b triplet, a color name, or an HTML hexadecimal color (e.g.
66 #aabbcc ).
67 -D[i] Select the back- and foreground colors to match the colors for
69 lowest and highest z-values in the output CPT [Default uses the
70 colors specified in the master file, or those defined by the
71 parameters COLOR_BACKGROUND, COLOR_FOREGROUND, and COLOR_NAN].
72 Append i to match the colors for the lowest and highest values
73 in the input (instead of the output) CPT.
74 -E[nlevels]
76 Create a linear color table by using the grid z-range as the new
77 limits in the CPT. Alternatively, append nlevels and we will
78 resample the color table into nlevels equidistant slices.
79 -F[R|r|h|c][+c]]
81 Force output CPT to written with r/g/b codes, gray-scale values
82 or color name (R, default) or r/g/b codes only (r), or h-s-v
83 codes (h), or c/m/y/k codes (c). Optionally or alternatively,
84 append +c to write discrete palettes in categorical format.
85 -Gzlo/zhi
87 Truncate the incoming CPT so that the lowest and highest z-lev‐
88 els are to zlo and zhi. If one of these equal NaN then we leave
89 that end of the CPT alone. The truncation takes place before
90 any resampling. See also manipulating_CPTs
91 -I[c][z]
93 Append c [Default] to reverse the sense of color progression in
94 the master CPT. Also exchanges the foreground and background
95 colors, including those specified by the parameters COLOR_BACK‐
96 GROUND and COLOR_FOREGROUND. Append z to reverse the sign of
97 z-values in the color table. Note that this change of z-direc‐
98 tion happens before -G and -T values are used so the latter much
99 be compatible with the changed z-range. See also manipulat‐
100 ing_CPTs
101 -Lminlimit/maxlimit
103 Limit range of CPT to minlimit/maxlimit, and don't count data
104 outside this range when estimating CDF(Z). [Default uses min and
105 max of data.]
106 -M Overrule background, foreground, and NaN colors specified in the
108 master CPT with the values of the parameters COLOR_BACKGROUND,
109 COLOR_FOREGROUND, and COLOR_NAN specified in the gmt.conf file
110 or on the command line. When combined with -D, only COLOR_NAN is
111 considered.
112 -N Do not write out the background, foreground, and NaN-color
114 fields [Default will write them].
115 -Q[i|o]
117 Selects a logarithmic interpolation scheme [Default is linear].
118 -Qi expects input z-values to be log10(z), assigns colors, and
119 writes out z [Default]. -Qo takes log10(z) first, assigns col‐
120 ors, and writes out z.
121 -Rxmin/xmax/ymin/ymax[+r][+uunit] (more ...)
123 Specify the region of interest.
124 -Szstart/zstop/zinc or -Sn
126 Set steps in CPT. Calculate entries in CPT from zstart to zstop
127 in steps of (zinc). Default chooses arbitrary values by a crazy
128 scheme based on equidistant values for a Gaussian CDF. Use -Sn
129 to select n points from such a cumulative normal distribution
130 [11].
131 -T-|+|_|=
133 Force the color table to be symmetric about zero (from -R to
134 +R). Append flag to set the range R: - for R =|zmin|, + for R =
135 |zmax|, _ for R = min(|zmin|, |zmax|), or = for R = max(|zmin|,
136 |zmax|).
137 -V Verbose operation. This will write CDF(Z) estimates to stderr.
139 [Default is silent.]
140 -W Do not interpolate the input color table but pick the output
142 colors starting at the beginning of the map. This is particu‐
143 larly useful in combination with a categorical color table. Can‐
144 not be used in combination with -Z. Alternatively, use -Ww to
145 produce a wrapped (cyclic) color table that endlessly repeats
146 its range.
147 -Z Will create a continuous color palette. [Default is discontinu‐
149 ous, i.e., constant color intervals]
150 -^ or just -
152 Print a short message about the syntax of the command, then
153 exits (NOTE: on Windows just use -).
154 -+ or just +
156 Print an extensive usage (help) message, including the explana‐
157 tion of any module-specific option (but not the GMT common
158 options), then exits.
159 -? or no arguments
161 Print a complete usage (help) message, including the explanation
162 of all options, then exits.
163
165 By default GMT writes out grid as single precision floats in a
166 COARDS-complaint netCDF file format. However, GMT is able to produce
167 grid files in many other commonly used grid file formats and also
168 facilitates so called "packing" of grids, writing out floating point
169 data as 1- or 2-byte integers. (more ...)
170
172 The PostScript language originally had no accommodation for trans‐
173 parency. However, Adobe added an extension that allows developers to
174 encode some forms of transparency using the PostScript language model
175 but it is only realized when converting the PostScript to PDF (and via
176 PDF to any raster image format). GMT uses this model but there are
177 some limitations: Transparency can only be controlled on a per-object
178 or per-layer basis. This means that a color specifications (such as
179 those in CPTs of given via command-line options) only apply to vector
180 graphic items (i.e., text, lines, polygon fills) or to an entire layer
181 (which could include items such as PostScript images). This limitation
182 rules out any mechanism of controlling transparency in such images on a
183 pixel level.
184
186 For best result when -E is used we recommend you do no append a spe‐
187 cific nlevels. This way the original CPT is used exactly as is but the
188 z boundaries are adjusted to match the grid limits. Otherwise you may,
189 depending on the nature of the input CPT, miss aspects of the color
190 changes by aliasing the signal.
191
193 Sometimes you don't want to make a CPT (yet) but would find it helpful
194 to know that 90% of your data lie between z1 and z2, something you can‐
195 not learn from grdinfo. So you can do this to see some points on the
196 CDF(Z) curve (use -V option to see more):
197 gmt grd2cpt mydata.nc -V > /dev/null
199 To make a CPT with entries from 0 to 200 in steps of 20, and ignore
201 data below zero in computing CDF(Z), and use the built-in master cpt
202 file relief, run
203 gmt grd2cpt mydata.nc -Crelief -L0/10000 -S0/200/20 > mydata.cpt
205
207 gmt, gmt.conf, grdhisteq, grdinfo, makecpt
208
210 2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
2115.4.5 Feb 24, 2019 GRD2CPT(1)