In this example we generate a series of 6 color images, arranged in the shape of a cross, that 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 method for generating the 6 color faces utilizes
$AWK in two steps. First, a z-grid is composed
which is 256 by 256 with z-values increasing in a planar
fashion from 0 to 65535. This z-grid is common to all
six faces. The color variations are generated by creating a
different color palette for each face using the supplied
$AWK script rgb_cube.awk. This script generates a
``cpt'' file appropriate for each face using arguments for
each of the three color components. The arguments specify
if that component (
) is to be held fixed at 0 or 255,
is to vary in x, or is to vary in y. If the
color is to increase in x or y, a lower case
x or y is specified; if the color is to decrease
in x or y, an upper case X or Y is
used. Here is the shell script and accompanying $AWK
script to generate the RGB cube:
grdmath -I1 -R0/255/0/255 Y 256 MUL X ADD = rgb_cube.grd gmtset TICK_LENGTH 0 COLOR_MODEL rgb pstext -R0/8/0/11 -Jx1i < /dev/null -P -U"Example 11 in Cookbook" -K >! example_11.ps $AWK -f rgb_cube.awk r=x g=y b=255 < /dev/null >! rgb_cube.cpt grdimage rgb_cube.grd -Crgb_cube.cpt -JX2.5i/2.5i -R0/255/0/255 -K -O -X2i -Y4.5i -B256wesn \ >> example_11.ps $AWK -f rgb_cube.awk r=255 g=y b=X < /dev/null >! rgb_cube.cpt grdimage rgb_cube.grd -Crgb_cube.cpt -JX -K -O -X2.5i -B256wesn >> example_11.ps $AWK -f rgb_cube.awk r=x g=255 b=Y < /dev/null >! rgb_cube.cpt grdimage rgb_cube.grd -Crgb_cube.cpt -JX -K -O -X-2.5i -Y2.5i -B256wesn >> example_11.ps psxy -W0.25pto -JX -R -K -O -X2.5i << END >> example_11.ps 0 0 20 20 20 235 0 255 END psxy -W0.25pto -JX -R -K -O -X-2.5i -Y2.5i << END >> example_11.ps 0 0 20 20 235 20 255 0 END psxy -W0.25pto -JX -R -K -O -X-2.5i -Y-2.5i << END >> example_11.ps 255 0 235 20 235 235 255 255 END $AWK -f rgb_cube.awk r=0 g=y b=x < /dev/null >! rgb_cube.cpt grdimage rgb_cube.grd -Crgb_cube.cpt -JX -K -O -Y-2.5i -B256wesn >> example_11.ps $AWK -f rgb_cube.awk r=x g=0 b=y < /dev/null >! rgb_cube.cpt grdimage rgb_cube.grd -Crgb_cube.cpt -JX -K -O -X2.5i -Y-2.5i -B256wesn >> example_11.ps pstext -JX -R -G255 -K -O << END >> example_11.ps 10 10 14 0 -Times-BoldItalic 1 GMT 3 END psxy -W0.25pto -JX -R -K -O -X2.5i << END >> example_11.ps 0 0 20 20 20 235 0 255 END psxy -W0.25pto -JX -R -K -O -X-5i << END >> example_11.ps 255 0 235 20 235 235 255 255 END $AWK -f rgb_cube.awk r=x g=Y b=0 < /dev/null >! rgb_cube.cpt grdimage rgb_cube.grd -Crgb_cube.cpt -JX -K -O -X2.5i -Y-2.5i -B256wesn >> example_11.ps psxy -W0.25pto -JX -R -K -O -X2.5i << END >> example_11.ps 0 0 20 20 20 235 0 255 END psxy -W0.25pto -JX -R -O -X-5i << END >> example_11.ps 255 0 235 20 235 235 255 255 END \rm -f rgb_cube.cpt rgb_cube.grd .gmtcommands
The $AWK script rgb_cube.awk is as follows:
END{
z=-.5;
if(r=="X" || g=="X" || b=="X"){
xl=255; xr=0; xd=-255;
}else{
xl=0; xr=255; xd=255;
}
if(r=="Y" || g=="Y" || b=="Y"){
yb=255; yt=-1; yd=-1;
}else{
yb=0; yt=256; yd=1;
}
for(y=yb; y!=yt; y+=yd){
x=xl;
if(r=="x" || r=="X"){
if(g=="y" || g=="Y"){
printf("%7.1f %3d %3d %3d " ,z,x,y,b);
x+=xd; z+=256;
printf("%7.1f %3d %3d %3d\n",z,x,y,b);
}else{
printf("%7.1f %3d %3d %3d " ,z,x,g,y);
x+=xd; z+=256;
printf("%7.1f %3d %3d %3d\n",z,x,g,y);
}
}else if(g=="x" || g=="X"){
if(r=="y" || r=="Y"){
printf("%7.1f %3d %3d %3d " ,z,y,x,b);
x+=xd; z+=256;
printf("%7.1f %3d %3d %3d\n",z,y,x,b);
}else{
printf("%7.1f %3d %3d %3d " ,z,r,x,y);
x+=xd; z+=256;
printf("%7.1f %3d %3d %3d\n",z,r,x,y);
}
}else{
if(r=="y" || r=="Y"){
printf("%7.1f %3d %3d %3d " ,z,y,g,x);
x+=xd; z+=256;
printf("%7.1f %3d %3d %3d\n",z,y,g,x);
}else{
printf("%7.1f %3d %3d %3d " ,z,r,y,x);
x+=xd; z+=256;
printf("%7.1f %3d %3d %3d\n",z,r,y,x);
}
}
}
exit;
}
The cube can be viewed in Figure 6.11
