Volume rendering: multiple scattering

This is mostly a contribution from Raul 'farsthary' Hernandez - an approximation for 
multiple scattering within volumes. Thanks, Raul! Where single scattering considers 
the path from the light to a point in the volume, and to the eye, multiple scattering 
approximates the interactions of light as it bounces around randomly within the 
volume, before eventually reaching the eye.

It works as a diffusion process that effectively blurs the lighting information 
that's already stored within the light cache.

A cloudy sky setup, with single scattering, and multiple scattering:
http://mke3.net/blender/devel/rendering/volumetrics/vol_sky_ss_ms.jpg
http://mke3.net/blender/devel/rendering/volumetrics/sky_ms.blend

To enable it, there is a menu in the volume panel (which needs a bit of cleanup, for 
later), that lets you choose between self-shading methods:

* None: No attenuation of the light source by the volume - light passes straight 
through at full strength
* Single Scattering: (same as previously, with 'self-shading' enabled)
* Multiple Scattering: Uses multiple scattering only for shading information
* Single + Multiple: Adds the multiple scattering lighting on top of the existing 
single scattered light - this can be useful to tweak the strength of the effect, 
while still retaining details in the lighting.

An example of how the different scattering methods affect the visual result:
http://mke3.net/blender/devel/rendering/volumetrics/ss_ms_comparison.jpg
http://mke3.net/blender/devel/rendering/volumetrics/ss_ms_comparison.blend


The multiple scattering methods introduce 3 new controls when enabled:
* Blur: A factor blending between fully diffuse/blurred lighting, and sharper
* Spread: The range that the diffuse blurred lighting spreads over - similar to a 
blur width. The higher the spread, the slower the processing time.
* Intensity: A multiplier for the multiple scattering light brightness

Here's the effect of multiple scattering on a tight beam (similar to a laser). The 
effect of the 'spread' value is pretty clear here:
http://mke3.net/blender/devel/rendering/volumetrics/ms_spread_laser.jpg

Unlike the rest of the system so far, this part of the volume rendering engine isn't 
physically based, and currently it's not unusual to get non-physical results (i.e. 
much more light being scattered out then goes in via lamps or emit). To counter this, 
you can use the intensity slider to tweak the brightness - on the todo, perhaps there is a more automatic method we can work on for this later on. I'd also like to check 
on speeding this up further with threading too.

1 files changed, 3 insertions, 1 deletions

diff --git a/source/blender/render/intern/include/volume_precache.h b/source/blender/render/intern/include/volume_precache.h
index 78409e4c646..9d87a219c82 100644
--- a/source/blender/render/intern/include/volume_precache.h
+++ b/source/blender/render/intern/include/volume_precache.h
@@ -28,4 +28,6 @@
  
 void volume_precache(Render *re);
 void free_volume_precache(Render *re);
-int point_inside_volume_objectinstance(ObjectInstanceRen *obi, float *co);
\ No newline at end of file
+int point_inside_volume_objectinstance(ObjectInstanceRen *obi, float *co);
+
+#define VOL_MS_TIMESTEP	0.1f
\ No newline at end of file