Good demo for showing benefits of parallelism

[Bill Baxter]

I don't remember where it was, but somebody in some thread said 
something like gee it would be great if we had a demo that showed the 
benefits of having parallel threads.  (It was probably in either the 
Futures lib thread or in the discussion about varargs_reduce).

Anyway, a raytracer is a perfect example of "embarrasingly 
parallelizeable" code.  In the simplest case you can state it simply as 
"for each pixel do trace_ray(ray_dir)".

Bradley Smith posted code for a little raytracer over on D.learn under 
the heading "Why is this code slower than C++".  If anyone is interested 
in turning this into a parallel raytracer that would be a nice little demo.

Along the same lines, almost any image processing algorithm is in the 
same category where the top loop looks like "foreach pixel do ___". 
This is a big part of why GPUs just keep getting faster and faster, 
because the basic problem is just so inherently parallelizable.

--bb