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

[Bassam Kurdali]

I've found it results in faster renders when you turn it off for
materials that aren't putting a lot of light into the scene (i.e. non
emmissive ones).. probably varies by scene.
On Tue, 2012-11-13 at 17:07 -0500, Matthew Heimlich 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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