A common problem facing POVers is how to difference (ouch, that 
shouldn't be a verb!) a bunch of CSG objects from another one, without 
taking hours and hours to render.

Mike Williams has a great tutorial ( 
http://www.econym.demon.co.uk/holetut/ ) that talks about two of the 
methods (subdividing the object and using isosurfaces).  Another handy 
method is to use blobs with negative strength component.

In the first and second pictures I created a blob with a big, flat 
sphere and a bunch of negative cylinders, and then intersected the 
result with a box.  Render times were under 2 minutes, and, as the 
second picture shows, an advantage is that you don't need regularly 
spaced holes (as you would with the subdivision method.)

The third picture shows why I was needing a quicker difference.  I've 
been modeling "milling processes," where a milling tool rotates and cuts 
pieces out of a block.  If the block is a cylinder or a box, the 
subdivision method works fine, with a little prep work, but when you're 
carving out of some more irregularly shaped thing, this negative blob 
method is much easier.  In the fourth picture, I put the milling union 
back in (slightly smaller) as a separate blob, to see how the carving 
was done.

If anyone's interested, I can post the sources to PBSF or PTSF.

Dave Matthews

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