AnAardvark opened this issue on Dec 15, 2009 · 9 posts
bagginsbill posted Wed, 16 December 2009 at 3:45 PM
Quote - it's logical to assume that the undersurface would reflect bottom-shadows of objects, but it's too cold now to go swimming and see if it actually does so. I just can't recall if it does that in a swimming pool or shallow shore area. no, wait, what I'm thinking is that the bottom-shadows would be tenuous or distorted due to caustics.
How could it not reflect shadows? It can only reflect the light that hits the bottom. If light fails to hit the bottom (a shadow) then it will fail to reflect light from there.
As to the nature of the shadow, it depends on the lighting. If there is bright sun, the shadow will be just as sharp as it usually is. The penumbra of a shadow is based on the angle subtended by the light source, and the distance between the occluder and the surface receiving the light/shadow.
It's hard to find an example. Check this one - not the bottom, but it is reflections of underwater shadows.
Now if the sun is not involved, and we're talking about artifical lights, or maybe the sky, then the same holds for those - whatever the penumbra is, it remains essentially the same, but for one exception.
Suppose you have a pool indoors, with a huge circular lit ceiling. So the incoming light forms a cone. Because refraction bends descending light more towards vertical, it means that the huge lit ceiling cone becomes compressed into a narrower cone. So the net effect is that the ceiling light appears to subtend a smaller angle than it would in the absence of water. Which means the penumbra shrinks, causing the shadows underwater to be sharper than in air.
Surprising, eh?
Observe this man's shadow.
Very sharp shadow there.
Observe the absence of caustics in his shadow. Caustics don't fundamentally change the shadow. They are a localized effect, bringing additional light from random small areas and concentrating it. Localized meaning the light from a neighborhood of a square inch might get concentrated into 1/4 square inch, making it 4 times brighter. That has no impact on the larger shape of a shadow.
Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)
