surface_bounds = function(nodes)
	local minx,miny,maxx,maxy
	for _,node in pairs(nodes) do
		if minx == nil or node.x < minx then
			minx = node.x
		end
		if maxx == nil or node.x > maxx then
			maxx = node.x
		end
		if miny == nil or node.y < miny then
			miny = node.y
		end
		if maxy == nil or node.y > maxy then
			maxy = node.y
		end
	end
	return minx,miny,maxx+600,maxy+600
end