> "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.

- Slime
[ http://www.slimeland.com/ ]
[ http://www.slimeland.com/images/ ]

Post a reply to this message