Interesting that you should mention this.  I recently got the O.K. from one of
my professors to do an undergraduate research project next semester to add
backwards raytracing to POV-Ray.  I've been doing a lot of thinking about this
over the past month and I've got some ideas on how to do it which turned out to
be very similar to the "photon map" by Henrik Wann Jensen.  Jensen's work with
photon maps seems to have been a success, both in terms of image quality and in
rendering speed.  You can read about it at:
http://www.gk.dtu.dk/home/hwj/
http://www.gk.dtu.dk/home/hwj/papers/ewr7/index.html

An example of rendering time (from Jensen's paper):
  Machine: 100Mhz Pentium with 32 megs of RAM
  -A diffuse Cornell box (standard radiosity test scene)
    Preprocessing (about 300k 'photons'): 67 sec
    Rendering (at 1280x960): 8 min
  -A glossy Cornell box
    Preprocessing (about 380k 'photons'): 56 sec
    Rendering (at 2560x1920): 50 min

A render of the diffuse Cornell box at the same resolution by the Radiance
renderer took 60 minutes.

My original work will not be as extensive as Jensen's, but I will try to do it
in such a way that future enhancements will be easy for myself or others to
add.

Because I'll be doing this for college credit, I will be able to justify
spending some extra time on POV. :-)  However, input from other POV programmers
will be very helpful.  If I can get enough done, I really want to combine this
with your dispersion patch, which could produce some really nice images.

-Nathan Kopp

Daren Scot Wilson wrote:
> 
> Alright, everyone keeps asking about caustics, of one sort or another,
> so maybe next week I'll tinker around with adding it.
> 
> Before I start, though, has anyone made or found a list of previous
> attempts that have failed?  I'd hate to reinvent a wheel and discover
> the hard way that it won't roll...
> 
> --
> Daren Scot Wilson
> Member, ACM
> dar### [at] pipelinecom
> www.newcolor.com
> --
> "A ship in a harbor is safe, but that is not what ships are built for"
>                      -- William Shedd

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