Re: MP: problem connecting paths.

[Mikael Sundqvist via ntg-context <ntg-context@ntg.nl>]

Hi,

would it be an option for you to do

newcircleA := circleA cutbefore P1 cutafter P2 -- cycle ;

In that case you avoid extra points.

/Mikael

On Mon, Dec 20, 2021 at 5:30 AM Fabrice L via ntg-context
<ntg-context@ntg.nl> wrote:
>
> Dear list,
>
> I have a problem connecting path with the operator « & » (page 16 Metafun manual). The problem is illustrated by the code (and the joined PDF) which follows.
>
> I draw random cycled  forms (left part of the figure) and when they interact, they are modified to « fit together » (right part of the figure) . When building these new forms, I connect paths (usually two or three depending on position 0 of the path); the last point a path is the same as the first point of the next path ; for exemple, to modify the blue circle, named « circleA », I take the circle A from P1 to P2, then a segment from P2 to P1 and cycle to have a closed path. So I should be able to do :
>
> newcircleA := circleA cutbefore P1 cutafter P2 & (P2 -- P1) & cycle  ;
>
> The « & » operator is supposed to work (if I understand correctly) in this case, but I have this error message :
>
> "! Paths don't touch; `&' will be changed to `..’."
>
> Which I do not understand since the paths « touch ». There was a similar question last year, without a clear answer («
> METAPOST subpath rounding issue »https://www.mail-archive.com/ntg-context@ntg.nl/msg94294.html).
>
> A solution is to ignore the repeating points :
> newcircleA := circleA cutbefore P1 cutafter P2 -- (P2 -- P1) -- cycle  ;
> but this new shape has 9 points instead of 7. The problem is that I do these operations a number of times, and the length of a path (in the number of points) is increasing and cause trouble when I look for intersections with others shapes.
>
> Here is the MWE:
>
> \startMPpage
> path circleA , circleB ;
> pair P[];
>
> circleA := fullcircle scaled 1in randomized 3 ;
> circleB := fullcircle scaled 1in randomized 3 shifted(0.3in,0.3in) ;
>
> draw circleA withcolor blue ;
> draw circleB withcolor red ;
>
> P[1] := circleA intersectionpoint circleB ;
> dotlabel.lft("$P_1$",P[1]) ;
> dotlabel.rt("$\blue\tfx P_0^A$",point 0 of circleA) ;
> P[2] := reverse(circleA) intersectionpoint circleB ;
> dotlabel.rt("$P_2$",P[2]) ;
> dotlabel.rt("$\red\tfx P_0^B$",point 0 of circleB) ;
>
> path newcircleA , newcircleB ;
> % newcircleA := circleA cutbefore P1 cutafter P2 & (P2 -- P1) & cycle  ;
> % previous line gives an error :
> % "! Paths don't touch; `&' will be changed to `..'."
> newcircleA := circleA cutbefore P1 cutafter P2 -- (P2 -- P1) -- cycle  ;
> newcircleA := newcircleA xshifted 2in ;
> draw newcircleA  withcolor darkgreen ;
>
> newcircleB := (circleB cutafter P1)  -- (P1 -- P2) -- (circleB cutbefore P2 ) & cycle ;
> newcircleB := newcircleB xshifted 2in ;
> draw newcircleB withcolor black ;
> drawpoints newcircleA ; drawpointlabels newcircleA ;
>
> \stopMPpage
>
>
>
>
> I would appreciate any help ! Thanks !
> Fabrice.
>
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