-- demo\rosetta\Bitmap_Greyscale.exw  (runnable version)

function to_grey(sequence image)
    integer dimx = length(image),
            dimy = length(image[1])
    for x=1 to dimx do
        for y=1 to dimy do
            integer pixel = image[x][y]          -- red,green,blue
            sequence r_g_b  =  sq_and_bits(pixel,{#FF0000,#FF00,#FF})
            integer {r,g,b} = sq_floor_div(r_g_b,{#010000,#0100,#01})
            image[x][y] = floor(0.2126*r + 0.7152*g + 0.0722*b)*#010101
        end for
    end for
    return image
end function

--include ppm.e   -- read_ppm(), write_ppm(), to_grey()  (as distributed, instead of the above)

sequence img = read_ppm("Lena.ppm")
img = to_grey(img)
write_ppm("LenaGray.ppm",img)