(11) A 3-D RGB color cube
In this example we generate a series of 6 color images, arranged so that they can be cut out and assembled into a 3-D color cube. The six faces of the cube represent the outside of the R-G-B color space. On each face one of the color components is fixed at either 0 or 255 and the other two components vary smoothly across the face from 0 to 255. The cube is configured as a right-handed coordinate system with x-y-z mapping R-G-B. Hence, the 8 corners of the cube represent the primaries red, green, and blue, plus the secondaries cyan, magenta and yellow, plus black and white.
The 6 color faces are generated by feeding grdimage three grids, one for each color component (R, G, and B). In some cases the X or Y axes of a face are reversed by specifying a negative width or height in order to change the variation of the color value in that direction from ascending to descending, or vice versa.
A number of rays emanating from the white and black corners indicate the Hue value (ranging from 0 to 360). The dashed and dotted lines near the white corner reflect saturation levels, running from 0 to 1 (in black font). On these 3 faces the brightness is a constant value of 1. On the other 3 faces of the cube, around the black corner, the white decimal numbers indicate brightnesses between 0 and 1, with saturation fixed at 1.
#!/usr/bin/env bash
# GMT EXAMPLE 11
#
# Purpose: Create a 3-D RGB Cube
# GMT modules: set, grdimage, grdmath, text, plot
# Unix progs: rm
gmt begin ex11
# Use gmt plot to plot "cut-along-the-dotted" lines.
gmt set MAP_TICK_LENGTH_PRIMARY 0
# First, create grids of ascending X and Y and constant 0.
# These are to be used to represent R, G and B values of the darker 3 faces of the cube.
gmt grdmath -I1 -R0/255/0/255 X = x1.nc
gmt grdmath Y = y1.nc
gmt grdmath 0 = c1.nc
# Second, create grids of descending X and Y and constant 255.
# These are to be used to represent R, G and B values of the lighter 3 faces of the cube.
gmt grdmath 255 X SUB = x2.nc
gmt grdmath 255 Y SUB = y2.nc
gmt grdmath 255 = c2.nc
gmt plot @cut-here_11.txt -Wthinnest,. -R-51/306/0/1071 -JX8.4c/25.2c -X6c
gmt set FONT_ANNOT_PRIMARY 12p,Helvetica-Bold
gmt grdimage x1.nc y1.nc c1.nc -JX6c/-6c -R0/255/0/255 -X1.25c
gmt plot -Wthinner,white,- @rays_11.txt
gmt text --FONT=white -F+a+f <<- END
128 128 -45 12p 60@.
102 26 -90 12p 0.4
204 26 -90 12p 0.8
10 140 180 16p G
END
echo 0 0 0 128 | gmt plot -N -Sv0.4c+s+e -Gwhite -W2p,white
gmt grdimage x1.nc c1.nc y1.nc -JX6c/6c -Y6c
gmt plot -Wthinner,white,- @rays_11.txt
gmt text --FONT=white -F+a+f <<- END
128 128 45 12p 300@.
26 102 0 12p 0.4
26 204 0 12p 0.8
140 10 -90 16p R
100 100 -45 16p V
END
echo 0 0 128 0 | gmt plot -N -Sv0.4c+s+e -Gwhite -W2p,white
echo 0 0 90 90 | gmt plot -N -Sv0.4c+s+e -Gwhite -W2p,white
gmt grdimage c1.nc x1.nc y1.nc -JX-6c/6c -X-6c
gmt plot -Wthinner,white,- @rays_11.txt
gmt text --FONT=white -F+a+f <<- END
128 128 135 12p 180@.
102 26 90 12p 0.4
204 26 90 12p 0.8
10 140 0 16p B
END
echo 0 0 0 128 | gmt plot -N -Sv0.4c+s+e -Gwhite -W2p,white
echo 0 0 128 0 | gmt plot -N -Sv0.4c+s+e -Gwhite -W2p,white
gmt grdimage x2.nc y2.nc c2.nc -JX-6c/-6c -X6c -Y6c
gmt plot -Wthinner,black,- @rays_11.txt
gmt text -F+a+f <<- END
128 128 225 12p 240@.
102 26 270 12p 0.4
204 26 270 12p 0.8
END
gmt grdimage c2.nc y2.nc x2.nc -JX6c/-6c -X6c
gmt plot -Wthinner,black,- @rays_11.txt
gmt text -F+a+f <<- END
128 128 -45 12p 0@.
26 102 0 12p 0.4
26 204 0 12p 0.8
100 100 45 16p S
204 66 90 16p H
END
echo 0 0 90 90 | gmt plot -N -Sv0.4c+s+e -Gblack -W2p
echo 204 204 204 76 | gmt plot -N -Sv0.4c+s+e -Gblack -W2p
gmt grdimage x2.nc c2.nc y2.nc -JX-6c/6c -X-6c -Y6c
gmt plot -Wthinner,black,- @rays_11.txt
gmt text -F+a+f <<- END
128 128 135 12p 120@.
26 102 180 12p 0.4
26 204 180 12p 0.8
200 200 225 16p GMT
END
rm -f *.nc
gmt end show
The RGB color cube