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<H1><A NAME="SECTION0014110000000000000000"></A>
<A NAME="10985"></A>
<BR>
6.11 Making a 3-D RGB color cube
</H1>
<P>
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 <I>x-y-z</I> 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.
<P>
The method for generating the 6 color faces utilizes
<I>$AWK</I><A NAME="11010"></A> in two steps. First, a <I>z</I>-grid is composed
which is 256 by 256 with <I>z</I>-values increasing in a planar
fashion from 0 to 65535. This <I>z</I>-grid is common to all
six faces. The color variations are generated by creating a
different color palette for each face using the supplied
<I>$AWK</I><A NAME="11015"></A> script <U>rgb_cube.awk</U>. 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 (<IMG
WIDTH="37" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img111.gif"
ALT="$r,g,b$">) is to be held fixed at 0 or 255,
is to vary in <I>x</I>, or is to vary in <I>y</I>. If the
color is to increase in <I>x</I> or <I>y</I>, a lower case
<I>x</I> or <I>y</I> is specified; if the color is to decrease
in <I>x</I> or <I>y</I>, an upper case <I>X</I> or <I>Y</I> is
used. Here is the shell script and accompanying <I>$AWK</I><A NAME="11021"></A>
script to generate the RGB cube:
<P>
<P>
<BR>
<P>
<BR CLEAR="ALL">
<HR>
<BR>
<PRE>
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
$AWK -f rgb_cube.awk r=x g=Y b=0 < /dev/null >! rgb_cube.cpt
grdimage rgb_cube.grd -Crgb_cube.cpt -JX -K -O -X2.5i -Y-2.5i -B256wesn >> example_11.ps
psxy -W0.25pto -JX -R -K -O -X2.5i << END >> example_11.ps
0 0
20 20
20 235
0 255
END
psxy -W0.25pto -JX -R -O -X-5i << END >> example_11.ps
255 0
235 20
235 235
255 255
END
\rm -f rgb_cube.cpt rgb_cube.grd .gmtcommands
</PRE>
<P>
<BR>
<BR>
<P>
<BR CLEAR="ALL">
<HR>
<P>
The <I>$AWK</I><A NAME="11096"></A> script <U>rgb_cube.awk</U> is as follows:
<P>
<P>
<BR>
<P>
<BR CLEAR="ALL">
<HR>
<BR>
<PRE>
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;
}
</PRE>
<P>
<BR>
<BR>
<P>
<BR CLEAR="ALL">
<HR>
<P>
The cube can be viewed in Figure <A HREF="node84.html#fig:GMT_example_11">6.11</A>
<P>
<P></P>
<DIV ALIGN="CENTER"><A NAME="fig:GMT_example_11"></A><A NAME="11136"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 6.11:</STRONG>
The color cube</CAPTION>
<TR><TD><IMG
WIDTH="432" HEIGHT="587" BORDER="0"
SRC="img112.gif"
ALT="\begin{figure}\centering\epsfig{figure=eps/GMT_example_11.eps}\end{figure}"></TD></TR>
</TABLE>
</DIV><P></P>
<P>
<A NAME="11126"></A>
<P>
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<ADDRESS>
Paul Wessel
2001-04-18
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