I had been meaning to implement Preetham's analytic sky model ever since I first came across it years ago. Well I finally got around to it and was pleased to find it's one of those few papers that gives you pretty much everything you need to put together an implementation (although with over 50 unique constants you need to be careful with your typing).
I integrated it into my path tracer which made for some nice images:
Also a small video.
It looks like the technique has been surpassed now by Precomputed Atmospheric Scattering but it's still useful for generating environment maps / SH lights.
I also fixed a load of bugs in my path tracer, I was surprised to find that on my new i7 quad-core (8 logical threads) renders with 8 worker threads were only twice as fast as with a single worker, given the embarrassingly parallel nature of path-tracing you would expect at least a factor of 4 decrease in render time.
It turns out the problem was contention in the OS allocator, as I allocate BRDF objects per-intersection there was a lot of overhead there (more than I had expected). I added a per-thread memory arena where each worker thread has a pool of memory to allocate from linearly during a trace, allocations are never freed and the pool is just reset per-path.
This had the following effect on render times:
1 thread: 128709ms->35553ms (3.6x faster) 8 threads: 54071ms->8235ms (6.5x faster!)
You might also notice that the total speed up is not linear with the number of workers. It tails off as the 4 'real' execution units are used up, so hyper-threading doesn't seem to be too effective here, I suspect this is due to such simple scenes not providing enough opportunity for swapping the thread states.
The HT numbers seems to roughly agree with what people are reporting on the Ompf forums (~20% improvement).