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Warp wrote:
> Btw, it's a common misconception that light rays coming from the Sun are
> parallel.
>
> Actually the Sun is an area light. Its size is significant and thus the
> rays coming from it are not parallel. Light rays coming from opposite sides
> of the Sun disk hit a point in the Earth at different angles.
>
> Thinking about it, there should not be big difference between a circular
> light source with the radius of the Sun at a distance of the Sun from the
> Earth, and a circular light source at 1km of distance and respectively
> smaller radius (so that its apparent size is the same), given that our
> scene is some meters wide.
If the sun was a point light, the rays coming at Earth would form such a tight
(practically degenerate) cone that you might as well call them parallel, rather
than put a POV light at -1000000000000*x or whatever and get float problems.
This is only good for non-area-lighting though.
Parallel rays with area lights makes no sense, because it is the
non-parallelity(?) of the rays itself that gives soft shadows. So you are
perfectly right about that. However, the size of the shadow relative to the
object casting it (in soft shadows we will define the outline as the area
that's 50% light and 50% shadow) for all practical purposes remains constant, a
phenomenon really only possible with parallel rays, because the sun is at such
a large distance from the Earth. So when using hard lighting, you might as well
make it parallel.
--
David Fontaine <dav### [at] faricy net> ICQ 55354965
My raytracing gallery: http://davidf.faricy.net/
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