. Well, this is a trick, with this I mean that it looks like, but really is not physically correct, so it's just for show, but heck, that what's all about.The volumetric is faked at mesh side creating a volume that "traps" the light and has an aproximate shape of what your light cone should look alike. That's quite simple, besides most of the time if you don't do it quite right it will look just fine (as in my attached example). From my tests I've seen that the light volume has to have smooth edges, so it your windows is square you still have to round those edges or the trick will be seen.
On the Octane side there is one thing to keep in mind, since we are trying to make light go thru a specular material, sun won't be our first pick, better use a plane or similar emmitter. And it will add quite some noise, but it goes away after some samples at least in my tests. It obviously looks much better in PT or PMC. The material is a specular with 1 IOR value and a medium node. All that is common to fog materials, the fun part is when we add a light emmitter node to it so we can fake how the light comes thru. There is plenty of values to play with, from light temp to power, but the most important of all is the medium scale, that'll really control the volume density and how much light it sheds, so I've left a pin out so you guys can easily play with it.
The first example is with a full cone, no opacity, it looks good but too stiff (box_test.ocs) the second test is adding a gradient map to control the opacity (box_test_1.ocs) and it looks much more natural, mainly cause in the first example the light projected is contained by the cone and it's shadow is too crisp, where in the second one the light cone opens as it more transparent and the result is much more natural.
I've pack ocs, obj and blend files for you guys to play with, do some test, and any improvement on the overall setup or the material is more than welcome. I expect to see some god rays on the next competition, I for one am working atm on a scene to make good use of them, so have fun and good renders!
edit: I forgot, you can add some extra mood, adding a generator node to the scattering channel on the medium node, usually a turbulence, but check the others also.
