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

Brecht Van Lommel brechtvanlommel at pandora.be
Tue Nov 13 22:54:09 CET 2012

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.


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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