converter for gcp list from M2_LRPT_Decoder for use with GDAL

gcp2gdal.lua

---------+---------+---------+---------+---------+---------+---------+---------+
--[[ arg[1]        - input.gcp
--   arg[2] arg[3] - sparse coff [50 [200]]
--]]

local gpcs = "-gcp %d %d %f %f"
local xcoff = arg[2] and tonumber(arg[2]) or 50
local ycoff = arg[3] and tonumber(arg[3]) or 200

local gcp, grid = {}, {}
local xmax, ymax
for line in io.lines(arg[1]) do
    if "" == line then goto skip end
    for x, y, lat, lon in string.gmatch(line, "(.+) (.+) (.+) (.+)") do
        x = tonumber(x)
        y = tonumber(y)
        lat = lat:gsub(",", ".")
        lat = tonumber(lat)
        lon = lon:gsub(",", ".")
        lon = tonumber(lon)
        table.insert(gcp, {x, y, lon, lat})
        table.insert(grid, {lon, lat, x, -y})
        xmax = x
        ymax = y
    end
    ::skip::
end

local wg = io.open(arg[1] .. ".gcps", "w+b")    -- for gdal_translate
local wp = io.open(arg[1] .. ".points", "w+b")  -- for QGis georef plugin
for i = 1, #gcp do
    local g = gcp[i]
    if  (g[1] % xcoff == 0 or g[1] == xmax) and (g[2] % ycoff == 0 or g[2] == ymax) then
        wg:write(" -gcp ", table.concat(g, " "))
        g = grid[i]
        wp:write(table.concat(g, ","))
        wp:write(",1,0,0,0\n")
    end
end
wg:close()
wp:close()