gimp + selected color enhancement

/* Color Enhance 0.10 --- image filter plug-in for GIMP
 *
 * Copyright (C) 1999 Martin Weber
 * Copyright (C) 1996 Federico Mena Quintero
 *
 * You can contact me at martweb@gmx.net
 * You can contact the original GIMP authors at gimp@xcf.berkeley.edu
 *
 * 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
enhance_it (const guchar *src, guchar *dest, gdouble vlo, gdouble vhi)
{
  gdouble h, z, v;
  gint    c, m, y;
  gint    k;
  guchar  map[3];

  c = 255 - src[0];
  m = 255 - src[1];
  y = 255 - src[2];

  k = c;
  if (m < k) k = m;
  if (y  255) c = 255;
  m += k;
  if (m > 255) m = 255;
  y += k;
  if (y > 255) y = 255;

  dest[0] = 255 - c;
  dest[1] = 255 - m;
  dest[2] = 255 - y;
}

http://www.jeffgraphics.in/2010/12/15/selected-color-enhancement/