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NCL Graphics: Color Maps

This suite of examples shows how to change, create, draw, reverse, and otherwise manipulate colors and color maps (also known as "color tables").

NCL has a built-in list of available color tables and a suite of 650 named colors you can use.

Note: in NCL V6.1.0, a new color model was introduced, which replaces the concept of associating color maps with a workstation. For backwards compatibility, the old color model is still supported. Many of the examples here are for the workstation color map, but can still be very useful for both color models.

Here are some functions and procedures that are useful in working with color maps and colors in general:

draw_color_palette - draws the given colors or color map as a series of filled boxes.
get_color_rgba - chooses an RGB triplet or RGBA quadruplet for a scalar value, given a color map and a range of values.
namedcolor2rgb / namedcolor2rgba - returns the RGB triplets or RGBA quadruplets of the given list of named colors.
read_colormap_file - reads an NCL system colormap file or a user-defined colormap.
span_color_rgba - given the number of desired color values, return an array of RGB triplets or RGBA quadruplets that nicely span the given color map.
span_named_colors - returns an RGB array that is a span between given list of named colors.

Here are some older functions that are useful when working with the color map associated with the workstation:

gsn_define_colormap - defines a color map for the given workstation.
gsn_draw_colormap - draws the current color map for the given workstation.
gsn_draw_named_colors - draws the given list of named colors.
gsn_merge_colormaps - merges two color maps and sets this as the color map for the given workstation.
gsn_retrieve_colormap - retrieves a color map for the given workstation.
gsn_reverse_colormap - reverses the color map associated the given workstation. *

CMYK color

If you need to use CMYK color, then this is only supported with the old style color model (pre NCL V6.1.0), and hence you must use "oldps" or "oldpdf" as the output format. You can set this via a workstation resource, before you call gsn_open_wks:

 type = "oldps"     ; cannot use "ps" or "pdf"
 type@wkColorModel = "cmyk"
 wks = gsn_open_wks(type,"example")

Note: a user reported a noticeable degradation in the color quality when using the old postscript and pdf drivers along with the CMYK option. He said he's been able to submit RGB figures to various journals for the last few years, and never had a problem. If you have to send in CMYK graphics, then submit the figures as RGB (which is the default in NCL), and then use an external package like Illustrator to convert them to CMYK.

colormap_1.ncl: Demonstrates drawing the current color map associated with the workstation using gsn_draw_colormap.

Note: if you are setting a color map in your .hluresfile, then your output from this script may be different.

colormap_2.ncl: Demonstrates changing the color map associated with the workstation using: 1) one of the built-in color tables, 2) an array of named colors, or 3) RGB values.

The gsn_define_colormap function is used to set all three color maps.

See the "colormap_2a.ncl" example that follows, for a newer method of drawing a given set of colors or a color map, that doesn't require calling gsn_define_colormap.

If you like a particular color map you've created, you can put it in a file, and put it in a directory where you can then load it as if it were an NCL built-in color table. See creating your own color table file for more information.

colormap_2a.ncl: This example draws the same color maps as the previous example, but it uses the draw_color_palette procedure added in NCL V6.3.0. This procedure doesn't require that you first call gsn_define_colormap to set the color map.

Note that the color tables are drawn slightly differently than they are with gsn_define_colormap. For one, they are drawn left-to-right, top-to-bottom, rather than top-to-bottom, left-to-right.

colormap_3.ncl: Demonstrates how to reverse the workstation color map using gsn_reverse_colormap.

See the "colormap_3a.ncl" example that follows, for a different method of reversing a color map.

colormap_3a.ncl: This example is similar to the previous example, except it shows how to reverse the workstation color map using read_colormap_file to first read the color map, and then the array syntax "::-1" to reverse it. It draws the color map using the draw_color_palette procedure added in NCL V6.3.0.

colormap_4.ncl: Demonstrates merging two colormaps on the workstation using gsn_merge_colormaps. You can only do this with two colormaps whose sum total is fewer than 256 colors. The background/foreground colors are retained from the first colormap only.

Note: merging color maps is not really necessary under the new color model. See example newcolor_13.ncl on the RGBA examples page for a way to use multiple color maps on a single page.

colormap_5.ncl: Demonstrates adding colors to an existing colormap, using NhlNewColor. This is only necessary for NCL versions 6.0.0 or earlier, if you need to use one or more named colors and they are not in your current color map. In version 6.1.0 and later, named colors do not need to be added.

In order to add named colors to a colormap, you need to get their corresponding RGB triplet from the $NCARG_ROOT/lib/ncarg/database/rgb.txt file. This will be in values from 0 to 255, so you first need to divide the values by 255 to get an RGB triplet.

colormap_6.ncl: Uses hsvrgb to draw some illustrative HSV color wheels. See the next example for how to create a colormap using this function.

colormap_7.ncl: Uses values from the above HSV color wheels to generate colormaps than span from one color to another.

The colormap is drawn using gsn_labelbar_ndc.

colormap_8.ncl: Uses span_named_colors to generate a color table that spans between given named colors.

You can optionally set the attributes "NumColorsInRange" or "NumColorsInTable" to indicate the maximum number of colors allowed in the table, or the number of colors in each range between named colors.

colormap_9.ncl: Uses the span_named_colors function to create a color map that reverses itself at the midpoint.

The colors passed to the function are (/"red","green","blue","purple","blue","green","red"/), with 20 colors between each range.

colormap_10.ncl: Draws every NCL colormap as a horizontal labelbar. Only the first four frames are shown here. NCL V6.4.0 produces 24 frames with 240 color maps. NCL V6.5.0 produces 25 frames with 265 color maps.