grdhisteq − Histogram equalization for grid files
grdhisteq in_grdfile [ −Gout_grdfile ] [ −Cn_cells ] [ −D ] [ −N[norm] ] [ −Q ] [ −V ]
grdhisteq allows the user to find the data values which divide a given grid file into patches of equal area. One common use of grdhisteq is in a kind of histogram equalization of an image. In this application, the user might have a grid of flat topography with a mountain in the middle. Ordinary gray shading of this file (using grdimage/grdview) with a linear mapping from topography to graytone will result in most of the image being very dark gray, with the mountain being almost white. One could use grdhisteq to write to stdout an ASCII list of those data values which divide the range of the data into n_cells segments, each of which has an equal area in the image. Using awk or makecpt one can take this output and build a cpt file; using the cptfile with grdimage will result in an image with all levels of gray occurring equally. Alternatively, see grd2cpt.
The second
common use of grdhisteq is in writing a grid with
statistics based on some kind of cumulative distribution
function. In this application, the output has relative highs
and lows in the same (x,y) locations as the input file, but
the values are changed to reflect their place in some
cumulative distribution. One example would be to find the
lowest 10% of the data: Take a grid, run grdhisteq
and make a grid using n_cells = 10, and then contour
the result to trace the 1 contour. This will enclose the
lowest 10% of the data, regardless of their original values.
Another example is in equalizing the output of
grdgradient. For shading purposes it is desired that
the data have a smooth distribution, such as a gaussian. If
you run grdhisteq on output from grdgradient
and make a grid file output with the Gaussian option, you
will have a grid whose values are distributed according to a
gaussian distribution with zero mean and unit variance. The
locations of these values will correspond to the locations
of the input; that is, the most negative output value will
be in the (x,y) location of the most negative input value,
and so on.
in_grdfile
2-D binary grid file to be equalized. (See GRID FILE FORMATS below).
No space between the option flag and the associated arguments.
−C |
Sets how many cells (or divisions) of data range to make. |
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−D |
Dump level information to standard output. |
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−G |
Name of output 2-D grid file. Used with −N only. (See GRID FILE FORMATS below). |
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−N |
Gaussian output. Use with −G to make an output grid with standard normal scores. Append norm to force the scores to fall in the <-1,+1> range [Default is standard normal scores]. |
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−Q |
Use quadratic intensity scaling. [Default is linear]. |
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−V |
Selects verbose mode, which will send progress reports to stderr [Default runs "silently"]. |
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 "packing" of grids, writing out floating point data as 2- or 4-byte integers. To specify the precision, scale and offset, the user should add the suffix =id[/scale/offset[/nan]], where id is a two-letter identifier of the grid type and precision, and scale and offset are optional scale factor and offset to be applied to all grid values, and nan is the value used to indicate missing data. When reading grids, the format is generally automatically recognized. If not, the same suffix can be added to input grid file names. See grdreformat(1) and Section 4.17 of the GMT Technical Reference and Cookbook for more information.
When reading a netCDF file that contains multiple grids, GMT will read, by default, the first 2-dimensional grid that can find in that file. To coax GMT into reading another multi-dimensional variable in the grid file, append ?varname to the file name, where varname is the name of the variable. Note that you may need to escape the special meaning of ? in your shell program by putting a backslash in front of it, or by placing the filename and suffix between quotes or double quotes. The ?varname suffix can also be used for output grids to specify a variable name different from the default: "z". See grdreformat(1) and Section 4.18 of the GMT Technical Reference and Cookbook for more information, particularly on how to read splices of 3-, 4-, or 5-dimensional grids.
To find the height intervals that divide the file heights.grd into 16 divisions of equal area:
grdhisteq heights.grd −C 16 −D > levels.d
To make the poorly distributed intensities in the file raw_intens.grd suitable for use with grdimage or grdview, run
grdhisteq raw_intens.grd −G smooth_intens.grd −N −V
If you use grdhisteq to make a gaussian output for gradient shading in grdimage or grdview, you should be aware of the following: the output will be in the range [-x, x], where x is based on the number of data in the input grid (nx * ny) and the cumulative gaussian distribution function F(x). That is, let N = nx * ny. Then x will be adjusted so that F(x) = (N - 1 + 0.5)/N. Since about 68% of the values from a standard normal distribution fall within +/- 1, this will be true of the output grid. But if N is very large, it is possible for x to be greater than 4. Therefore, with the grdimage program clipping gradients to the range [-1, 1], you will get correct shading of 68% of your data, while 16% of them will be clipped to -1 and 16% of them clipped to +1. If this makes too much of the image too light or too dark, you should take the output of grdhisteq and rescale it using grdmath and multiplying by something less than 1.0, to shrink the range of the values, thus bringing more than 68% of the image into the range [-1, 1]. Alternatively, supply a normalization factor with −N.
gmtdefaults(1), GMT(1), grd2cpt(1), grdgradient(1), grdimage(1), grdmath(1), grdview(1), makecpt(1)