Mike Williams wrote in message ...
>What happens is that when you do the union, POV can put a tight
bounding
>box around each cylinder, so for each ray it only has to perform
>cylinder tests for the few cylinders that that particular ray comes
>close to.
>
>When you do a difference, POV can't use the bounding boxes in the
same
>way. For each ray it ends up performing cylinder tests for every
single
>cylinder. You can see the effects of this if you look at the stats in
>the message window. You'll find that there are a *huge* number of
>cylinder tests, of which only a very small percentage were
successful.
>Whereas the union has about the same number of successful tests but a
>much lower number of unsuccessful ones.
>
>I did a scene a while ago with hundreds of small cylindrical holes in
a
>flat slab (rather than a torus) and experimented with cutting the
slab
>into smaller pieces with just a few holes in each piece. It turned
out
>that the rendering was fastest when each piece was so small that it
>contained only one hole.
>


Very informative; thank you! Throws a lot of light on some 'weird'
super-slow renders I've had .

Regards,
Anoop

Post a reply to this message