On Tue, 22 Jun 1999 00:37:05 -0400, Mark Wagner wrote:
>I was rendering a scene using the SuperPatch, and when I looked at the
>rendering statistics, they indicated that there was a CSG Merge in the
>scene, when I know that I didn't put one in.  Is this really an error, or is
>it just the way POV does things?  Here is the simplest code I could create
>that has this possible error:

It's just the way POV does things.  Here's why:

difference {
  object {A}
  intersection {
    object {B}
    object {C}
  }
}

is represented internally as

intersection {
  object {A}
  intersection {
    object {B}
    object {C}
    inverse
  }
}

which POV distills to

intersection {
  object {A}
  merge {
    object {B inverse}
    object {C inverse}
  }
}

Earlier versions of POV would have used union instead of
merge, but that can lead to weird artifacts if you're 
dealing with transparent surfaces.

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