This is done using the existing Emission node and closure (we may add a volume
emission node, not clear yet if it will be needed).
Volume emission only supports indirect light sampling which means it's not very
efficient to make small or far away bright light sources. Using direct light
sampling and MIS would be tricky and probably won't be added anytime soon. Other
renderers don't support this either as far as I know, lamps and ray visibility
tricks may be used instead.
This works pretty much as you would expect, overlapping volume objects gives
a more dense volume. What did change is that world volume shaders are now
active everywhere, they are no longer excluded inside objects.
This may not be desirable and we need to think of better control over this.
In some cases you clearly want it to happen, for example if you are rendering
a fire in a foggy environment. In other cases like the inside of a house you
may not want any fog, but it doesn't seem possible in general for the renderer
to automatically determine what is inside or outside of the house.
This is implemented using a simple fixed size array of shader/object ID pairs,
limited to max 15 overlapping objects. The closures from all shaders are put
into a single closure array, exactly the same as if an add shader was used to
combine them.
This is the simplest possible volume rendering case, constant density inside
the volume and no scattering or emission. My plan is to tweak, verify and commit
more volume rendering effects one by one, doing it all at once makes it
difficult to verify correctness and track down bugs.
Documentation is here:
http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/Materials/Volume
Currently this hooks into path tracing in 3 ways, which should get us pretty
far until we add more advanced light sampling. These 3 hooks are repeated in
the path tracing, branched path tracing and transparent shadow code:
* Determine active volume shader at start of the path
* Change active volume shader on transmission through a surface
* Light attenuation over line segments between camera, surfaces and background
This is work by "storm", Stuart Broadfoot, Thomas Dinges and myself.
