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

[Bassam Kurdali]

Thanks Brecht, makes sense. 
If anything, it should just be documented (might be a nice note in the
lightpath node). I'd much rather have better performance here since I
can and usually do enable 'sample as lamp' when it's needed. I figured
it was something like this, which is why I didn't just drop it in the
tracker.
thanks again.
Bassam
On Tue, 2012-11-13 at 22:54 +0100, Brecht Van Lommel 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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