generate_on_surfaces = function(
        splant,
        sradius,
        ssurface,
        savoid,
        seed_diff,
        lightmin,
        lightmax,
        nneighbors,
        ocount,
        facedir,
        depthmax,
        altmin,
        altmax,
        sbiome,
        sbiomesize,
        sbiomecount,
        airsize,
        aircount,
        tempmin,
        tempmax)
    if seed_diff == nil then seed_diff = 0 end
    if lightmin == nil then lightmin = 0 end
    if lightmax == nil then lightmax = LIGHT_MAX end
    if nneighbors == nil then nneighbors = ssurface end
    if ocount == nil then ocount = -1 end
    if depthmax == nil then depthmax = 1 end
    if altmin == nil then altmin = -31000 end
    if altmax == nil then altmax = 31000 end
    if sbiome == nil then sbiome = "" end
    if sbiomesize == nil then sbiomesize = 0 end
    if sbiomecount == nil then sbiomecount = 1 end
    if airsize == nil then airsize = 0 end
    if aircount == nil then aircount = 1 end
    if tempmin == nil then tempmin = -2 end
    if tempmax == nil then tempmax = 2 end

    minetest.register_on_generated(

        local searchblocks = table.getn(minetest.env:find_nodes_in_area(minp, maxp, ssurface)
        local surfaceblocks = {}
        for s in ipairs(searchblocks) do
            local pos = { x = searchblocks[s].x, y = searchblocks[s].y, z = searchblocks[s].z }
            local p_top = { x = pos.x, y = pos.y + 1, z = pos.z }
            if minetest.env:get_node(p_top) = "air" then table.insert(surfaceblocks, pos)
        end

        while #surfaceblocks > 0 do
            local r = math.random(1,#surfaceblocks)
            local pos = surfaceblocks[r]
            table.remove(surfaceblocks, r)

            local perlin1 = minetest.env:get_perlin(seed_diff, perlin_octaves, perlin_persistence, perlin_scale)
            local perlin2 = minetest.env:get_perlin(temperature_seeddiff, temperature_octaves, temperature_persistence, temperature_scale)
            local noise1 = perlin1:get2d({x=p_top.x, y=p_top.z})
            local noise2 = perlin2:get2d({x=p_top.x, y=p_top.z})
            [... etc. etc. etc. more code here ...]
            [... rest of this routine is a duplicate of the initial spawn ABM ...]
        end
    )
end