Wasn't it Ross Litscher who wrote:
>Okay, so i've got this image, and i was hoping to do a difference of the
>back wall (a union of these mesh bricks) with a cylinder-cube union to cut
>away the wall, leaving an archway. now the bricks in the wall are individual
>bricks, not a texture. the original brick was a rounded cube kind of thing i
>made in Ray Dream, then exported with 3dWin4 to pov format.
>
>i tried the difference, getting a warning message, but it proceeded to start
>rendering. at an insanely slow rate. So i was wondering, is there a way to
>keep the brick as a mesh, and use the cylinder-cube union object in some way
>to dictate where not to place a brick?
>
>on the other hand, if i use a regular primitive box object instead of a mesh
>for the bricks, i could do a difference that would be more successfull (in
>terms of rendering speed), right?
>
>btw, this is my first extensive use of handcoding. so be gentle :)

Firstly, it would be useful to know what the warning was, and what the
code was that provoked it.

Secondly, differencing with collections of many small objects can
sometimes defeat POVRay's automatic bounding algorithm. Effectively you
end up with POV having to check every brick for every ray, rather than
only those that bricks that are close to the ray.

I'd suggest that you try converting it from 

difference {
  union {
    lots of {
      individual_brick
    }
  }
  arch
}

to

union {
  lots of {
    difference {
      individual_brick
      arch
    }
  }
}

Which will probably allow POV to apply sensible automatic bounds around
each brick.

Keep an eye on the statistics in the Messages pane. You want to see lots
of bounds tests and few "rounded cube kind of thing" tests (because they
are probably expensive.)

-- 
Mike Williams
Gentleman of Leisure

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