This’s an interesting article from http://www.matrixchinaunix.com/blog/article.asp?id=456.
I use Maxima to re-plot that picture.
f(x,y):=exp(-x^2-y^2/2)*cos(4*x)+exp(-3*((x+0.5)^2+y^2/2));
plot3d(’(if(f(x,y)>0.001)then 0.001 else if(f(x,y)<0)then 0 else f(x,y)),[x,-3,3],[y,-5,5]);
my_preamble: "unset surface";
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