(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.

#!/bin/bash
#		GMT EXAMPLE 11
#
# Purpose:	Create a 3-D RGB Cube
# GMT modules:	gmtset, grdimage, grdmath, pstext, psxy
# Unix progs:	rm
ps=example_11.ps

# Use gmt psxy to plot "cut-along-the-dotted" lines.

gmt set MAP_TICK_LENGTH_PRIMARY 0

gmt psxy cut-here.txt -Wthinnest,. -R-51/306/0/1071 -JX3.5i/10.5i -X2.5i -Y0.5i -P -K > $ps

# 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 = x.nc
gmt grdmath -I1 -R Y = y.nc
gmt grdmath -I1 -R 0 = c.nc

gmt set FONT_ANNOT_PRIMARY 12p,Helvetica-Bold

gmt grdimage x.nc y.nc c.nc -JX2.5i/-2.5i -R -K -O -X0.5i >> $ps
gmt psxy -Wthinner,white,- rays.txt -J -R -K -O >> $ps
gmt pstext --FONT=white -J -R -K -O -F+f+a >> $ps << END
128 128 12p -45 60\217
102  26 12p -90 0.4
204  26 12p -90 0.8
10  140 16p 180 G
END
echo 0 0 0 128 | gmt psxy -N -Sv0.15i+s+e -Gwhite -W2p,white -J -R -K -O >> $ps

gmt grdimage x.nc c.nc y.nc -JX2.5i/2.5i -R -K -O -Y2.5i >> $ps
gmt psxy -Wthinner,white,- rays.txt -J -R -K -O >> $ps
gmt pstext --FONT=white -J -R -K -O -F+f+a >> $ps << END
128 128 12p  45 300\217
26  102 12p   0 0.4
26  204 12p   0 0.8
140  10 16p -90 R
100 100 16p -45 V
END
echo 0 0 128 0 | gmt psxy -N -Sv0.15i+s+e -Gwhite -W2p,white -J -R -K -O >> $ps
echo 0 0 90 90 | gmt psxy -N -Sv0.15i+s+e -Gwhite -W2p,white -J -R -K -O >> $ps

gmt grdimage c.nc x.nc y.nc -JX-2.5i/2.5i -R -K -O -X-2.5i >> $ps
gmt psxy -Wthinner,white,- rays.txt -J -R -K -O >> $ps
gmt pstext --FONT=white -J -R -K -O -F+f+a >> $ps << END
128 128 12p 135 180\217
102  26 12p  90 0.4
204  26 12p  90 0.8
10  140 16p   0 B
END
echo 0 0 0 128 | gmt psxy -N -Sv0.15i+s+e -Gwhite -W2p,white -J -R -K -O >> $ps
echo 0 0 128 0 | gmt psxy -N -Sv0.15i+s+e -Gwhite -W2p,white -J -R -K -O >> $ps

# 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 -I1 -R 255 X SUB = x.nc
gmt grdmath -I1 -R 255 Y SUB = y.nc
gmt grdmath -I1 -R 255       = c.nc

gmt grdimage x.nc y.nc c.nc -JX-2.5i/-2.5i -R -K -O -X2.5i -Y2.5i >> $ps
gmt psxy -Wthinner,black,- rays.txt -J -R -K -O >> $ps
gmt pstext -J -R -K -O -F+f+a >> $ps << END
128 128 12p 225 240\217
102  26 12p 270 0.4
204  26 12p 270 0.8
END

gmt grdimage c.nc y.nc x.nc -JX2.5i/-2.5i -R -K -O -X2.5i >> $ps
gmt psxy -Wthinner,black,- rays.txt -J -R -K -O >> $ps
gmt pstext -J -R -K -O -F+f+a >> $ps << END
128 128 12p -45 0\217
26  102 12p   0 0.4
26  204 12p   0 0.8
100 100 16p  45 S
204  66 16p  90 H
END
echo 0 0 90 90 | gmt psxy -N -Sv0.15i+s+e -Gblack -W2p -J -R -K -O >> $ps
echo 204 204 204 76 | gmt psxy -N -Sv0.15i+s+e -Gblack -W2p -J -R -K -O >> $ps

gmt grdimage x.nc c.nc y.nc -JX-2.5i/2.5i -R -K -O -X-2.5i -Y2.5i >> $ps
gmt psxy -Wthinner,black,- rays.txt -J -R -K -O >> $ps
gmt pstext -J -R -O -F+f+a >> $ps << END
128 128 12p 135 120\217
26  102 12p 180 0.4
26  204 12p 180 0.8
200 200 16p 225 GMT 5
END

rm -f *.nc gmt.conf
../_images/example_11.png

The RGB color cube