Am 18.07.2016 um 16:18 schrieb fsv2712:
> More information about the reflector:
> If I make the reflector as the difference of two cylinders plus a box, it works,
> but only if the light source is displaced 2 unit out of the axis of the
> reflector. I think perhaps this is a bug. See the code:
> 
> #version 3.7;
> 
> global_settings { ambient_light 0
>     photons {spacing 1 max_trace_level 30 radius 5 autostop 0 media 50,1}
>     assumed_gamma 1.0
> }

I'm not an expert on the practical application of photons, and haven't
done much work on the photons code either, so can't give a precise
explanation of the magic behind it and why it makes your scene fail, but
your problem seems to be in the `spacing` parameter.

Note how the render statistics report the total number of stored photons
as 10 on surfaces and 28 in media. It seems plausible that they may all
happen to end up in places where they don't make much of a difference,
whether by chance or by some systematic effect emerging from the scene's
geometry.

To increase the total number of photons, and hence reduce the risk of
getting no photons at all where they're needed, reduce the `spacing`
parameter. For instance, a value of 0.1 seems small enough to make the
scene look roughly the way you seem to expect it to look like,
regardless of the `position` variable.

Alternatively, you can drop the `spacing` parameter entirely and specify
a `count` instead. To my knowledge, that's also what the people with a
knack for photons prefer.

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