> That looks very interesting, i have some troubles understanding how it
> works though.  The phong model needs the light direction, how do you
> determine it with radiosity lighting?

This sampling is independent of the radiosity inter-diffuse stuff. It occurs
at each intersection point if there is a non zero phong in the finish. An
adaptive number (calculated with phong_size and a global parameter rad_phong
(a kind of density)) of rays are shooted near the direction = incident ray
reflected (samples rays are choosed with an adequate probability density
function -see the images- that's why it's called importance sampling) and
light is gathered. Yes that sounds cpu consuming - and it is, but for high
phong_size the number of samples can be low.

> Concerning practical usage - it would be interesting to see a version with
> reflective finish in comparison (which will also show highlights with
> hdri).

you mean a comparison with finish with a small amount of blurred reflection
(this is what i've used in my irtc image..) ?
for high phong_size (small blur) the results should look quite similar, and
for lower phong_size (bigger blur) the uniform distribution of the sampling
resulting from the blurred reflection will probably introduce some
differences (and blurred reflections are more annoying to declare than
phong+phong_size ;)

M

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