On Thu, 18 Mar 2004 03:54:47 +0000, Mike Williams
<nos### [at] econymdemoncouk> wrote:

>Not for *unions*. For unions and merges it's possible to bound
>intelligently, and POV does so. It's possible to see the effect by
>rendering a simple pair of crossed cylinders with the switches 
> +MB0 +UD

Ah, I see.  Thanks for the example.  So, unions are able to be 'bound
intelligently'.

>
>With intersections and differences, however, there can be situations
>where POVs automatic bounding can't create such tight fits. 
>

Yes, and this is where my problem lies.  I've got a lot of cylinders
being differenced with other cylinders.

>For example, see <http://www.econym.demon.co.uk/holetut/index.htm>.
>A box with 1600 cylindrical holes in it took 26 minutes 25 seconds to
>render, but after rewriting the scene so that POV could bound it better
>it took 8 seconds.

Aha!  So, let me see if I've got this straight.  I place a large
number of objects in a CSG Union, and I should see very little
difference in speed.  However, if I place a large number of objects in
a CSG difference, my bounding boxes are not calculated very well, and
the render time may be quite poor.

Now, on your wonderful web page, you showed how to overcome this
problem - you broke your larger CSG difference into a bunch of small
CSG differences.  As you point out, the best you can do is to have
1600 slabs, each with only a single CSG difference in them.

Thanks so much, Mike!  And the rest of the responses I've recieved,
too -- this has been quite enlightening.  Already, now that I've
corrected the whole 'union-of-triangles vs. mesh' fiasco, I've
decreased my render time dramatically.

I'll try applying this information as well!

- How

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