#ESCAPECHARS ON

DIM colored = ".\\Lena.ppm", grayscale = ".\\LenaGry.ppm"
DIM head, tail, r, g, b, l, ptr, blobsize

FILEGET(FILEOPEN(colored, BINARY), FILELEN(colored)): FILECLOSE(FILEOPEN) ' Load buffer
blobsize = INSTR(FILEGET, "\n255\n") + 4 ' Get sizeof PPM header
head = @FILEGET + blobsize: tail = @FILEGET + FILELEN ' Set loop bounds

FOR ptr = head TO tail STEP 3 ' Transform color triplets
  r = PEEK(ptr + 0, 1) ' Read colors stored in RGB order
  g = PEEK(ptr + 1, 1)
  b = PEEK(ptr + 2, 1)
  l = 0.2126 * r + 0.7152 * g + 0.0722 * b ' Derive luminance
  POKE(ptr + 0, CHR(l))(ptr + 1, CHR)(ptr + 2, CHR) ' Write grayscale
NEXT

FILEPUT(FILEOPEN(grayscale, BINARY_NEW), FILEGET): FILECLOSE(FILEOPEN) ' Save buffer