f(x,y)=cos(x)*cos(y) \
       + cos((sqrt(3)*x-y)/2)*cos((x+sqrt(3)*y)/2) \
       + cos((sqrt(3)*x+y)/2)*cos((x-sqrt(3)*y)/2)
g(x,y)=f(x,y)>0 ? 1: 0
set pm3d map
unset colorbox
set palette gray
set size square
unset key
unset xtics
unset ytics
set samples 512
set isosamples 512,512
set term png tiny crop
R=20
set xrange[-R:R]
set yrange[-R:R]
#
set out "g008.png"
splot g(x,y)
#
set out "f008.png"
splot f(x,y)
#
set palette color
set out "f008c.png"
splot f(x,y)