[metapost] MetaFont: Unexpected behavior of intersections times

[luigi scarso]

On Sat, Apr 2, 2011 at 1:38 PM, luigi scarso <luigi.scarso at gmail.com> wrote:
> The following metafont code
>
> %%%%%%%%%
> batchmode;
> message "p1:=(33.09007380767,-33.8895224139) .. controls
> (33.370612519531,-33.905534970703) and (33.65115125,-33.9215475) ..
> (33.93169,-33.93756);";
> path p[];
> p1:=(33.09007380767,-33.8895224139) .. controls
> (33.370612519531,-33.905534970703) and (33.65115125,-33.9215475) ..
> (33.93169,-33.93756);
>
> p2:=(38.1058,-34.00049) .. controls (36.39993,-34.00049) and
> (34.44913,-34.0004125) .. (32.79190125,-33.86743875);
> numeric t,u; (t,u) = p1 intersectiontimes p2;
> show t,u;
> message "" ;
>
>
> message "p1:=(33.09007380767,-33.8895224139);";
> p1:=(33.09007380767,-33.8895224139) ;
> p2:=(38.1058,-34.00049) .. controls (36.39993,-34.00049) and
> (34.44913,-34.0004125) .. (32.79190125,-33.86743875);
> numeric t,u; (t,u) = p1 intersectiontimes p2;
> show t,u;
> message "" ;
>
> message "p1:=(33.93169,-33.93756);";
> p1:=(33.93169,-33.93756);
> p2:=(38.1058,-34.00049) .. controls (36.39993,-34.00049) and
> (34.44913,-34.0004125) .. (32.79190125,-33.86743875);
> numeric t,u; (t,u) = p1 intersectiontimes p2;
> show t,u;
> message "" ;
>
> bye.
> %%%%%%%%%
> gives as output
>
> This is METAFONT, Version 2.718281 (TeX Live 2009/Debian) (base=mf
> 2011.1.30)  2 APR 2011 13:33
> **a
> (a.mf
> p1:=(33.09007380767,-33.8895224139) .. controls (33.370612519531,-33.9055349707
> 03) and (33.65115125,-33.9215475) .. (33.93169,-33.93756);
>>> -1
>>> -1
> p1:=(33.09007380767,-33.8895224139);
>>> 0
>>> 0.94043
> p1:=(33.93169,-33.93756);
>>> -1
>>> -1
>  )
>
> Is it corrected ?
> Given that p1 is inside p2, perhaps in this case intersectiontimes
> correctly doesn't find any  intersection --- there are infinite
> intersections. But I'm a bit surprised .
>
Hm p1 isn't  a sub path of p2
With a subpath

p3 := subpath(0.4,0.8) of p2;
message "p3 := subpath(0.4,0.8) of p2;";
numeric t,u; (t,u) = p3 intersectiontimes p2;
show t,u


gives
t = 0.2179 , u =  0.48718

(but why t is not 0 or 1 ?)

-- 
luigi