Wasn't it Gert Van den Eynde who wrote:
>Hi all,
>
>I have a scene with about 30.000 hexagonal-Z prisms which in their turn
>generate one large hexagonal-Z prism (i.e. a hexagonal-Z lattice from a
>numerical calculation). Each small prism has its own color based on the
>results from the calculation. If I just render this scene with the large
>prism a union{ prism ... prism... prism ..} of the small prisms, it takes
>about 10 seconds in total (parsing+rendering). However, when I use a
>clipped_by {plane {-z,0}} to cut the large prism in half to see the inside,
>rendering time becomes 10 hours. Is there a faster way (clipping each small
>prism seperately, specifying the bounding box by hand,...) to do this?

I guess that you're getting the same problem with clipped_by as is
generally encountered with difference. In a union, POV can apply
sensible bounds around each component, but when dealing with differences
and clipped_by it can only apply a bound around the whole object, which
means that each ray ends up being tested against every component.

The usual solution is to clip each component separately.

See <http://www.econym.demon.co.uk/holetut/index.htm> for full details
of the difference problem and examples of how to observe what's
happening by observing the stats.

-- 
Mike Williams
Gentleman of Leisure

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