> Such a cross section area might be better for the vista buffers,
> but the bounds are often used when considering rays from other
> directions, for example when tracing shadows and reflections.

True.  I was over simplifying the problem.

> My experiments show that, in this situation, the check happens
> with both +UR and -UR. I'm beginning to wonder if they've
> sneakily removed the UR functionality and forgotten to remove
> it from the documentation.

My thinking as well.

> Tight bounds are generally more important than tight vista buffers.
> In this case, if you force POV to use those manual bounds (not
> easy) then it runs 40% slower than using the automatic bounds.

Ok... may I asked how you forced it, since that is what I thought
the UR flag was for?  You've convined me that POV-Ray's
bounding was better then mine.  But I am curious how you tested
that.

> POV was right to overrule you. This remains true even if you
> move the light source to the same location as the camera
> (causing the shadow rays to point in the same direction as the
> viewing rays). POV's automatic bounds are still faster if there
> are no light in the scene.

I believe you.

> It's not an issue for bounded_by, but clipped_by is a CSG
> operation and therefore it is an issue for that.

Thanks.  That's one less thing I have to worry about.

> Even with these slightly more complicated objects, you are
> spending hours of effort to make optimisations that are saving
> about 0.05 seconds of tracing time.

True.  I'm doing this so I can learn what POV-Ray is doing.
I'll then try to optimize some of my much slower CSGs.  I'm
playing with the fast ones so I don't spend all my time waiting
whenever I make a tweak.  I know somewhere in my more
complicated CSGs there must be some big improvements to
be made.  I've got scenes that take weeks if not months to
render and they look simple compared to some of the stuff
I see posed here that takes hours.  Look at my post over in
the image area called "My second TRON subject".  I also
hope to play with animations so a 0.05 second improvement
per object per frame might pay off in the long run.  Even if
it doesn't I'm still learning alot.

Thanks for all the help,
Carl

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