[metapost] MetaFont: Unexpected behavior of intersections times

Boguslaw Jackowski B_Jackowski at GUST.org.pl
Wed Apr 6 10:54:19 CEST 2011

Hekko, Everybody,

> One helpful sufficient condition for one or more genuine
> intersections of the two paths p and q is this
> criterion:
> ($) existence of a quadrilateral Q [...]

Is it possible to construct such a quadrilateral efficiently?

> I once had two semicircles that crossed
> (theoretically) at the midpoint of each, and MP said
> they did not cross.

> Loosely interpreted, this sounds to me like a bug, so I
> would like to see a concrete example and its explanation.
> It seems to contradict the claim I have just made.

To me also sounds strange; it was several times mentioned on this
list that: `p intersectiontimes q = (-1,-1)' implies that the curves 
don't touch/cross each other, although `p intersectiontimes q <> (-1, -1)'
may sometimes occur for non-touching curves, but very close to each other.

I'd be astonished if the former assertion was invalid. But, frankly 
speaking, I am being astonished repeatedly... ;-)

Cheers -- Jacko

  Bogus\l{}aw Jackowski: B_Jackowski at GUST.ORG.PL
  Hofstadter's Law: It always takes longer than you expect, even
                    when you take into account Hofstadter's Law.

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