rectangular smooth window function

min(a,b) = a>b ? b:a
max(a,b) = a>b ? a:b
lim(minval,maxval, value) = max(minval,min(maxval,value))

set pm3d
set pm3d interpolate 2,2
set isosamples 100,100
set view map
set hidden3d
unset key
set palette grayscale

sx = 4.5
sy = 3.5
margin = 1.5

sx2 = sx/2.0
sy2 = sy/2.0
m = 1/margin

f(x,y) = lim(0,1,m*(sx2-abs(sx2-x)))*lim(0,1,m*(sy2-abs(sy2-y)))
splot [-1:5][-1:5] 0.5*(cos(pi*(1-f(x,y)))+1)