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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
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