Old algorithm:
Raytrace from one transparent surface to the next step by step. To minimize
overhead in cases where we don't need transparent shadows, we first trace a
regular shadow ray. We check if the hit primitive was potentially transparent,
and only in that case start marching. this gives extra ray cast for the cases
were we do want transparency.
New algorithm:
We trace a single ray. If it hits any opaque surface, or more than a given
number of transparent surfaces is hit, then we consider the geometry to be
entirely blocked. If not, all transparent surfaces will be recorded and we
will shade them one by one to determine how much light is blocked. This all
happens in one scene intersection function.
Recording all hits works well in some cases but may be slower in others. If
we have many semi-transparent hairs, one intersection may be faster because
you'd be reinteresecting the same hairs a lot with each step otherwise. If
however there is mostly binary transparency then we may be recording many
unnecessary intersections when one of the first surfaces blocks all light.
We found that this helps quite nicely in some scenes, on koro.blend this can
give a 50% reduction in render time, on the pabellon barcelona scene and a
forest scene with transparent leaves it was 30%. Some other files rendered
maybe 1% or 2% slower, but this seems a reasonable tradeoff.
Differential Revision: https://developer.blender.org/D473
This was the original code to get things working on old GPUs, but now it is no
longer in use and various features in fact depend on this to work correctly to
the point that enabling this code is too buggy to be useful.
This can for example be useful if you want to manually terminate the path at
some point and use a color other than black.
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D454
Volumes can now have textured colors and density. There is a Volume Sampling
panel in the Render properties with these settings:
* Step size: distance between volume shader samples when rendering the volume.
Lower values give more accurate and detailed results but also increased render
time.
* Max steps: maximum number of steps through the volume before giving up, to
protect from extremely long render times with big objects or small step sizes.
This is much more compute intensive than homogeneous volume, so when you are not
using a texture you should enable the Homogeneous Volume option in the material
or world for faster rendering.
One important missing feature is that Generated texture coordinates are not yet
working in volumes, and they are the default coordinates for nearly all texture
nodes. So until that works you need to plug in object texture coordinates or a
world space position.
This is work by "storm", Stuart Broadfoot, Thomas Dinges and myself.
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.
