/* GIMP - The GNU Image Manipulation Program
 * Copyright (C) 1995 Spencer Kimball and Peter Mattis
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 */
static void
desaturate_region_luminosity (PixelRegion    *srcPR,
                              PixelRegion    *destPR,
                              const gboolean  has_alpha)
{
  const guchar *src  = srcPR->data;
  guchar       *dest = destPR->data;
  gint          h    = srcPR->h;

  while (h--)
    {
      const guchar *s = src;
      guchar       *d = dest;
      gint          j;

      for (j = 0; j w; j++)
        {
          gint luminosity = GIMP_RGB_LUMINANCE (s[RED_PIX],
                                                s[GREEN_PIX],
                                                s[BLUE_PIX]) + 0.5;

          d[RED_PIX]   = luminosity;
          d[GREEN_PIX] = luminosity;
          d[BLUE_PIX]  = luminosity;

          if (has_alpha)
            d[ALPHA_PIX] = s[ALPHA_PIX];

          d += destPR->bytes;
          s += srcPR->bytes;
        }

      src += srcPR->rowstride;
      dest += destPR->rowstride;
    }
}

http://www.jeffgraphics.in/2010/12/14/204/