> > I also started using a faster way to create random rays and also
> > implemented importance sampling that made the image converge (smooth
> > out) faster.
> Seems interesting. What is importance sampling ? Do you have a reference paper ?

It is really simple. Wikipedia also has an article about it. I also found some
ray tracing related articles but I don't now remember the source.

Normally you shoot rays in totally random directions inside a hemisphere and
weight the result according to cosine between normal and ray - in diffuse
reflections. (Mirrors are a different thing)

In importance sampling you don't shoot rays with uniform distribution. You shoot
them with cosine weighting (more rays to hemisphere pole, less to equator). And
because of this weighting you don't have to take the cosine into account
anymore. The real gain is that the image converges faster: you get more info
from interesting direction (normal). The result is still 100% unbiased.

See my thread in OT. Especially my last post where I realised that this works.
There is some wrong info also in the thread. My last post should be correct.

http://news.povray.org/povray.off-topic/thread/%3C47b81741%241%40news.povray.org%3E/

> The use of portals has the same effects when the are put around a light source.
> But a portal can be put in the hall of a door, increasing the path finding
> through the door. It can also be used on a refractive object to help to solve
> caustic (but results are not always good in this case).

Ok. I'll check that too. Bidirectional rendering looks interesting too.

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