Slime wrote:

>> "Warning: Cylinder lights should be parallel when used with photons."
> 
> I'm willing to bet this is because of the way cylinder lights work. When
> an object is tested for illumination by a cylinder light, it checks two
> things: a) it is inside the cylinder that contains the light
> b) there is a clear path from it to the light source
> 
> But the light rays themselves are actually not parallel. (This can
> actually create ever-so-slightly incorrect shading.)
> 
> So, this works decently when doing backwards raytracing, since you can
> first see if you're inside the cylinder, and then if you are, trace a path
> to the light source to see if you're illuminated. But with photons, which
> simulate forward raytracing, the path is traced first. So, the photons
> will be shot from the light source, and they don't know if they're leaving
> the cylinder or not! So they could end up illuminating things that a
> cylindrical light source would otherwise not illuminate.
> 
> On the other hand, I was under the impression that parallel light sources
> affected nothing but shadow calculation, and didn't affect the starting
> position and direction of photons, but maybe that's wrong.
> 

Can anybody show a scene where cylindrical lights and photons are working 
correctly? I could not get it work as expected... 


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