MP: problem connecting paths.

* MP: problem connecting paths.
@ 2021-12-20  4:30 Fabrice L via ntg-context
  2021-12-20  7:21 ` Mikael Sundqvist via ntg-context
  0 siblings, 1 reply; 5+ messages in thread
From: Fabrice L via ntg-context @ 2021-12-20  4:30 UTC (permalink / raw)
  To: mailing list for ConTeXt users; +Cc: Fabrice L

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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

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I would appreciate any help ! Thanks ! 
Fabrice. 

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