function rand(x) return math.random(0, x-1) end

w, h = img.width, img.height
if w==1 and h==1 then return end
m = w-1
r = rand(m*(h-1)-1)
cw = r%m+1
ch = math.floor(r/m)+1
cx = rand(w-cw)
cy = rand(h-ch)
return "Random crop: "..cw..", "..ch..", "..(cx+cw)..", "..(cy+ch)