octave> g=readpgm('ganymede.pgm');
magic number = P2
rows = 128 cols = 128
grey levels = 255
octave> gdisplay(g);



octave> c=readpgm('callisto.pgm');
magic number = P2
rows = 128 cols = 128
grey levels = 255
octave> gdisplay(c);



octave> gh=histogram(g/2,256);
octave> gh=gh/sum(gh); # make it a pmf
octave> ch=histogram(c/2,256);
octave> ch=ch/sum(ch); # make it a pmf
octave> mplot(0:255,gh,0:255,ch);



octave> cg=(c+g)/2;
octave> gdisplay(cg);



octave> cgh=histogram(cg,256);
octave> cgx=conv(ch,gh);
octave> mplot(0:255,cgh,0:255,cgx(1:256));