On 2023-12-24 19:34 (-4), Bald Eagle wrote:
> 
> Warp commented (on parabolic mirrors in general) that, "POV-Ray can make
> perfectly parabolic mirrors and reflect light from them. It's far from
> impossible."

Warp is, of course, right on this matter, and he even implemented a
parabolic mirror in 2 dimensions.  However, I have a quibble about
UW-Madison's terminology.  Those mirrors are what I consider *concave*,
not convex.  This macro implements a concave paraboloid in 3 dimensions:

#macro Concave_Paraboloid (yFocus)
  poly { 2, <-1, 0, 0, 0, 0, 0, yFocus * 4, -1, 0, 0> }
#end

The vertex is at the origin, and the concave surface faces upward for a
positive yFocus.  You would need to intersect this with a limiting
object of appropriate size.  I used poly{2} rather than quadric{}
because the latter is buggy.

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