global procedure write_ppm(string filename, sequence image)
    integer fn = open(filename,"wb"),
            dimx = length(image),
            dimy = length(image[1])
    printf(fn, "P6\n%d %d\n255\n", {dimx,dimy})
    for y=1 to dimy do
        for x=1 to dimx do
            integer pixel = image[x][y]          -- red,green,blue
            sequence r_g_b = sq_and_bits(pixel,{#FF0000,#FF00,#FF})
                    r_g_b = sq_floor_div(r_g_b,{#010000,#0100,#01})
            puts(fn,r_g_b)
        end for
    end for
    close(fn)
end procedure