One easy one that ought to be there: a black-and-white film. I've had on
various occasions to produce POV scenes and then convert to monochrome. The
easiest way turned out to be defining all the objects to be gray - it's
hard to tell exactly how a color scene will look in monochrome, and I do a
lot of "postage-stamp" previewing. Two problems - firstly, you might
already have complicated colored objects that you don't want to rewrite; 
and secondly, this isn't true monochrome.  A true monochrome film can show
two identically gray objects, lit by a light [actually green] that lights
one [green] and not the other [red]. Gray objects and white light do not
simulate this.

	(One could also have a "pregray" option that converted everything to gray
first and ten rendered on only one channel; this would save a little
calculation time but not, I suspect, a lot. It might add it back on, too,
elsewhere, in ubiquitous "if(pregray)" conditionals.)

	The patch is obvious - just convert all three channels to their average
just between ray tracing and pixel storage, and add the keyword & an extra
field in the camera description. If I had a few hours to spend...

	Another possible effect in the same line would be a "human eye film" that
was monochrome at low light levels and trichrome at higher light levels. 

	-Robert Dawson

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