[Bf-cycles] sample with lamp changes light paths- normal behavior?

Bassam Kurdali bassam at urchn.org
Wed Nov 14 01:51:51 CET 2012


I've noticed it's off by default in the world (or is this a .B.blend
thing?) It does seem better to me (at least when I'm using an HDR for
the world, to turn this option on.
On Tue, 2012-11-13 at 23:37 +0100, Brecht Van Lommel wrote:
> If the material does not contain an emissive shader, the option does
> nothing. Ideally we should gray this out in such cases but it's slow
> to check this when drawing the UI.
> 
> Brecht.
> 
> On Tue, Nov 13, 2012 at 11:07 PM, Matthew Heimlich
> <matt.heimlich at gmail.com> wrote:
> > Are there any inherent pros/cons of leaving sample as lamp on for every
> > material? If there are cons, why is it enabled by default and not just when
> > you make an emissive material?
> >
> > On Nov 13, 2012 4:54 PM, "Brecht Van Lommel" <brechtvanlommel at pandora.be>
> > wrote:
> >>
> >> I would not call this a bug, not because it's great behavior but
> >> because it's part of an important optimization.
> >>
> >> When sample as lamp is disabled, what happens is that rays just bounce
> >> around based on the BSDF, and when they happen to hit a an emitter,
> >> then that illuminates the surface. In this case we don't know
> >> beforehand that we are going to hit an emitting surface, and so we
> >> don't know if it's a "shadow ray".
> >>
> >> With samples as lamp enabled, that same type of bouncing still happen.
> >> But in addition we now pick a points on emitting surfaces and trace
> >> shadow rays directly to it, which helps finding small emitting
> >> surfaces. In this case we know it's a shadow ray, and so the ray is
> >> tagged as such.
> >>
> >> The solution would involve executing shaders twice to distinguish
> >> between these two cases, so not ideal.
> >>
> >> Brecht.
> >>
> >> On Tue, Nov 13, 2012 at 10:01 PM, Bassam Kurdali <bassam at urchn.org> wrote:
> >> > Hey folks, I've constructed an artificial example here to illustrate the
> >> > issue.
> >> > I've got a plane (named selective) that has transparent shader and a mix
> >> > node with Light Path 'Is Shadow Ray' as input to factor. The idea is
> >> > that the shadows would be colored one color, but e.g. diffuse and glossy
> >> > reflections with another.
> >> >
> >> > This is being lit with a mesh cube and an emission shader.
> >> >
> >> > If 'Sample as lamp' is on on the Cube emission shader, the result is a
> >> > yellow transparent plane, with a blue shadow.
> >> >
> >> > If 'Sample as lamp' is off on the Cube emission shader, the result is a
> >> > yellow transparent plane, with a yellow shadow.
> >> >
> >> > Is this quirk a normal result? I realize that transparent shader is
> >> > somehow 'fake' - or is it a bug?
> >> >
> >> > blend file http://urchn.org/misc/test.blend
> >> > image1 http://urchn.org/misc/sample_aslamp.png
> >> > image2 http://urchn.org/misc/nosampleaslamp.png
> >> >
> >> > cheers
> >> > Bassam
> >> >
> >> > _______________________________________________
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