[metapost] MetaFont: Unexpected behavior of intersections times

luigi scarso luigi.scarso at gmail.com
Wed Apr 6 11:23:21 CEST 2011

On Wed, Apr 6, 2011 at 10:54 AM, Boguslaw Jackowski
<B_Jackowski at gust.org.pl> wrote:
> Hekko, Everybody,
> Larry:
>> 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?
> Dan:
>> I once had two semicircles that crossed
>> (theoretically) at the midpoint of each, and MP said
>> they did not cross.
> Larry:
>> 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
I will send you off-list tonight the example from which I've take the paths
(I hope to show more for bachotex).
In the context of MetaFont, I consider this case a "border case"
because it seems to happen
in situations where, in the end,  the pixels are anyway correctly put .
-- otherwise the font designer should change
something in his/her code if he/she uses directly these kind of paths.
At least this is my brief  experience .

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