Humanlike sight

For a game im making it would be cool to have a more natural look so id need two shaders/postprocessing-filters:
something like saturation=(saturation+brightness)/2 for the color-perception
and
pixel’s brightness-=(sum of the surounding pixels’ brightnesses)/4 or some working local tone-mapping

I dont know anything about cg, yet - except that its going to be a fragment-shader for the first…

Can anyone help and post a good tutorial-link or somethig?

Cg is very easy, i started out with the glow demo and learned.