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The intersect function

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I kinda buried the lead in the star drawing program. After years of struggling
to write this function in many clumsy ways. Finally, a super short 
implementation that's truly mind boggling. I only made some syntax changes.

It takes 4 points `a b c d` (each a 2 element array) and yields
a 2 element array of the intersection point (a->b)x(c->d). The source
mentions some cases where the function fails. Presumably this
is when one of the 3 intermediate matrices is not invertible (like if 
they don't intersect, maybe?).

% intersect adapted from https://www.ntg.nl/maps/18/23.pdf
/intersect { aload pop
1 dict begin {d c b a}{exch def}forall
  1 1
    1 1 [a _x  b _x  a _y  b _y  0  0] itransform % A B
    1 1 [c _x  d _x  c _y  d _y  0  0] itransform % A B C D
    0  0  6 array astore
    itransform 2 array astore
end }
/_x { 0 get }
/_y { 1 get }

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The intersect function luser droog <luser.droog@gmail.com> - 2022-01-24 09:45 -0800

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