The GrADS Default Colors
GrADS is built with 16 default colors that are used in a variety of applications. Every color in GrADS has a unique color number that is used as an index to identify it in GrADS commands. Complete specifications of the default colors numbered 0 to 15 are given below:
Col# Description Sample R G BDisclaimer: The color samples may not be displayed properly.
0 background0 0 0 (black by default)
1 foreground255 255 255 (white by default)
2 red240 0 0
3 green0 220 0
4 dark blue31 61 250
5 light blue0 199 199
6 magenta220 0 99
7 yellow230 220 51
8 orange240 130 41
9 purple161 0 199
10 yellow/green161 230 51
11 medium blue0 161 230
12 dark yellow230 176 46
13 aqua0 209 140
14 dark purple110 0 220
15 gray125 125 125
The GrADS Default Rainbow Sequence
GrADS creates a default rainbow palette using the following sequence of 13 built-in colors:
9 | 14 | 4 | 11 | 5 | 13 | 3 | 10 | 7 | 12 | 8 | 2 | 6 |
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When drawing contour plots, the default behaviour of GrADS is to color code the contours and select an appropriate contour interval so that each contour is a different color and the colors span the range of the default rainbow sequence. The same principle is behind the selection of default contour intervals for filled contours and shaded grid plots.
The scripts "cbar.gs"
and "cbarn.gs"
will draw a
color key alongside a plot of filled contours or shaded grid cells; the script
uses the query shades
command to get
information about the contour levels and their color shades.
Defining new colors
For some types of displays, the 16 GrADS default colors may not be suitable
or adequate. It is possible for the user to define new colors using the set
rgb
command:
set rgb color# R G B
For example, let's create a palette of colors for plotting anomalies. We need to define new colors that will be shades of blue and red that range in intensity from fully saturated to very light. White will be the color in the center of the new anomaly palette.
* These are the BLUE shades set rgb 16 0 0 255 set rgb 17 55 55 255 set rgb 18 110 110 255 set rgb 19 165 165 255 set rgb 20 220 220 255 * These are the RED shades set rgb 21 255 220 220 set rgb 22 255 165 165 set rgb 23 255 110 110 set rgb 24 255 55 55 set rgb 25 255 0 0
Overriding the Defaults
Now that we have a set of newly defined colors (numbered 16-25), we can override the defaults and specify our anomaly palette with exact contour levels and the colors that go with them. This is accomplished by using the following commands:
set clevs lev1 lev2 lev3 ... levN
set ccols col1 col2 col3 ... colN
Contour levels and the colors that go with them are reset with every execution
of clear
or display
.
Thus, it may be easier to use these commands in a script to avoid typing them
over and over again.
Filled Contours or Shaded Grids: If you are specifying the levels
and colors for filled contours (set gxout shaded
) or shaded grid cells (set
gxout grfill
), then the number of colors specified with set ccols must be one larger than the number of contour
levels specified with set clevs. Continuing with our example of creating
an anomaly palette, the commands would have the following syntax:
set gxout shaded
set clevs -5 -4 -3 -2 -1 1 2 3 4 5
set ccols 16 17 18 19 20 1 21 22 23 24 25
Note the "0" contour level has been omitted, but color number "1" is still in
the palette. Drawing a plot with these specified clevs
and ccols
and then running the "cbarn.gs" script will
result in the following color key:
Here is example using 6 colors and 5 contour levels that shows how the filled contours (or shaded grids) relate to the data values:
col1: values <= lev1 col2: lev1 < values <= lev2 col3: lev2 < values <= lev3 col4: lev3 < values <= lev4 col5: lev4 < values <= lev5 col6: lev5 < values
Line Contours: If you are specifying the levels and colors for
line contours (set gxout contour
),
then the number of arguments to set clevs and set ccols should be equal -- one color for each contour.
Plotting Contours of Constant Color
It is sometimes preferable to plot line contours without the rainbow coloring.
An example might be a plot with sea level pressure contours in one color (red)
and 500 mb height contours overlaid in another color (blue). For drawing all
the contours in the same color, use the set
ccolor
command:
set gxout contour
set ccolor 2
d slp
set ccolor 4
d z(lev=500)
Omitting Colors
The default behavior of GrADS when plotting filled contours or shaded grid cells is to colorize all areas. To omit a particular color (or contour level) from the plot, simply assign the background color. For example:
set gxout shaded
set clevs -5 -4 -3 -2 -1 1 2 3 4 5
set ccols 0 17 18 19 20 0 21 22 23 24 0
This example is similar to the one given above, but notice where some
of the ccols
have been set to "0" (the background
color). The first, last, and middle colors have been omitted. These
commands set up a plot that will only shade areas where the anomalies
are between 1 and 5 and -1 and -5. The remaining areas will be black.
Plotting Non-Continuous Index Grids
Plotting grids with index values or non-continuous data (e.g. surface type
classification) is simplified by using the graphics output type fgrid
and the set fgvals
command.
set gxout fgrid
set fgvals 1 15 2 5 3 1
d sfctype
In this example, the variable "sfctype"
has three values: 1
represents land, 2
represents oceans, and 3
represents
sea ice. These commands would draw a plot with land grid cells filled with color
number 15 (gray), ocean grid cells filled with color number 5 (light blue),
and sea ice grid cells filled with color number 1 (white). If the first two
arguments to set fgvals were omitted, then the land grid cells
would not be omitted and only ocean and sea ice grid cells would be colored.