[Bf-cycles] Volume shaders, mirrors that project light sources

Lucas wsacul at gmail.com
Thu Nov 17 20:05:08 CET 2011

Sorry the permissions were wrong on the pictures, they should work now.

On Thu, Nov 17, 2011 at 10:27 AM, Brecht Van Lommel <
brechtvanlommel at pandora.be> wrote:

> Hi,
> On Thu, Nov 17, 2011 at 5:39 PM, Lucas <wsacul at gmail.com> wrote:
> > I'm hoping http://cgi.stackexchange.com becomes public soon and has
> lots of
> > blender support for some of these possibly remedial questions:
> > How is volume shading done with cycles in the current svn trunk?
> > In the following picture the transparent shader behaves intuitively but a
> > glass surface shader still casts a shadow.  I think this means I need to
> do
> > volume shading in addition or instead of surface shading, and in the node
> > editor view I see a block called 'material output' with a surface and
> volume
> > input and the surface input already hooked up, but what do I hook up to
> the
> > volume input?
> >
> https://picasaweb.google.com/103190342755104432973/Blender3D#5675708974274649330
> > Turning off shadows works for the glass objects but is only physically
> > correct if they are 100% white.
> Volume rendering is not supported yet. The link doesn't seem to work,
> but I think this is a case where you've got caustics that path tracing
> can't render well. There's no simple solution to this, it requires a
> more advanced rendering algorithm. A workaround is to make a shader
> that switches to a transparent BSDF for shadow rays, using the light
> path node.

I don't quite understand why the caustics doesn't work easily if the light
is passing through parallel glass planes- it seems like as many paths ought
to make it back from a diffuse bounce to the light source as they do in
free space, unless some shortcut/'cheating' is helping out with the free
space path, or the probabilistic path termination is really aggressive
after a single free space to glass transition.  Can I get correct results
(like the light source passing through the glass ball in
projecting and interesting light pattern on the wall and floor) if I crank
up the render cycles (going to try it myself but it may take a while).

> > Is there any way to pass hints to send more lightpaths bouncing off one
> > object to a secondary object?
> > In this example there are light paths that hit the diffuse bsdf of the
> > shadowed area of the plane and only a few of them hit the mirror and
> trace
> > back to the light source I have pointed at the mirror, resulting in only
> a
> > smattering of light reflected by the mirror.  Many many more render
> cycles
> > would fill in the gaps but at huge cost.
> >
> https://picasaweb.google.com/103190342755104432973/Blender3D#5675634439005970706
> >
> > One approach that comes to mind is to have a special shader that takes an
> > object as a target, so that the peak response is always pointed towards
> it
> > (but also has a softness factor that maybe would have to be increased
> > manually if the target object is large and close, or there could be an
> > automatic way of doing that).  Is that possible already?
> > My workaround is to use a mirror modifier, and I think some additional
> > compositing work could make it so light from that mirrored light source
> that
> > doesn't strike the profile of the mirror is masked out.
> Again the link doesn't work, but this seems another case that path
> tracing can't handle well. Caustics need a more advanced algorithm
> like bidirectional path tracing or photon mapping.
> Brecht.
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Thanks, that is very helpful.

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