[NTG-context] METAPOST: How do I get this 'max' cutafter to work?

[Hans Hagen]

On 4/21/2020 1:29 PM, Gerben Wierda wrote:
> I have these two paths, but cutting doesn’t work because the path to be 
> cut takes the wrong intersection regardless of how I direct the path to 
> be cut after (reverse or not).
> 
> The path to cut after is a triangle with the base horizontal and the 
> path to cut is a vertical line. The top of the triangle is the endpoint 
> of the line and is selected as intersection by cutafter. This happens 
> also if I reverse the triangle path, strangely enough. I have an 
> alternative cutting algorithm, but that cuts as long as the length of 
> cuttings is not 0, which is the case at the top too, so it won’t cut 
> further.
> 
> I need a reliable algorithm to make a ‘maximum cutafter’. What might be 
> a good approach?
> 
> G
> 
> path arrowHead;
> arrowHead :=
>    (542,-77.000186920166016)..controls 
> (545.33333333333337,-77.000186920166016) and 
> (548.66666666666663,-77.000186920166016)
>    ..(552,-77.000186920166016)..controls 
> (550.33333333333337,-73.666853586832687)
>    and (548.66666666666663,-70.333520253499344)
>    ..(547,-67.000186920166016)..controls 
> (545.33333333333337,-70.333520253499344)
>    and (543.66666666666663,-73.666853586832687)
>    ..cycle;
> 
> path connection;
> connection :=
>    (420.00004959106445,-367)..controls (420.66670256708773,-367) and 
> (421.333355543111,-367)
>    ..(422.00000851913427,-367)..controls (425.33333917170194,-367) and 
> (428.6666698242696,-367)
>    ..(432.00000047683727,-367)..controls (442.00000015894574,-367) and 
> (451.99999984105426,-367)
>    ..(461.99999952316273,-367)..controls (464.76142345324098,-367) and 
> (467,-364.76142382108867)
>    ..(467,-361.99999997656772)..controls (467,-293.99999999218926) and 
> (467,-226.00000000781074)
>    ..(467,-158.00000002343228)..controls (467,-155.23857623850762) and 
> (469.23857634039018,-153)
>    ..(472.00000014901161,-153)..controls (495.33333338300389,-153) and 
> (518.66666661699617,-153)
>    ..(541.99999985098839,-153)..controls (544.76142368504975,-153) and 
> (547,-150.76142367339932)
>    ..(547,-147.99999983662815)..controls (547,-125.99999994554271) and 
> (547,-104.00000005445727)
>    ..(547,-82.000000163371837)..controls (547,-78.666668047283764) and 
> (547,-75.333335931195691)
>    ..(547,-72.000003815107618)..controls (547,-70.333398183460417) and 
> (547,-68.666792551813217)
>    ..(547,-67.000186920166016);
> 
> show connection cutafter arrowHead;
> show connection cutafter (reverse arrowHead);

\startMPpage[instance=doublefun]

path arrowHead;
arrowHead :=
     (542,-77.000186920166016)..controls 
(545.33333333333337,-77.000186920166016) and
     (548.66666666666663,-77.000186920166016) 
..(552,-77.000186920166016)..controls
     (550.33333333333337,-73.666853586832687) and
     (548.66666666666663,-70.333520253499344) 
..(547,-67.000186920166016)..controls
     (545.33333333333337,-70.333520253499344) and
     (543.66666666666663,-73.666853586832687) ..cycle;

path connection;
connection :=
   (420.00004959106445,-367)..controls (420.66670256708773,-367) and
   (421.333355543111,-367) ..(422.00000851913427,-367)..controls
   (425.33333917170194,-367) and (428.6666698242696,-367)
   ..(432.00000047683727,-367)..controls (442.00000015894574,-367) and
   (451.99999984105426,-367) ..(461.99999952316273,-367)..controls
   (464.76142345324098,-367) and (467,-364.76142382108867)
   ..(467,-361.99999997656772)..controls (467,-293.99999999218926) and
   (467,-226.00000000781074) ..(467,-158.00000002343228)..controls
   (467,-155.23857623850762) and (469.23857634039018,-153)
   ..(472.00000014901161,-153)..controls (495.33333338300389,-153) and
   (518.66666661699617,-153) ..(541.99999985098839,-153)..controls
   (544.76142368504975,-153) and (547,-150.76142367339932)
   ..(547,-147.99999983662815)..controls (547,-125.99999994554271) and
   (547,-104.00000005445727) ..(547,-82.000000163371837)..controls
   (547,-78.666668047283764) and (547,-75.333335931195691)
   ..(547,-72.000003815107618)..controls (547,-70.333398183460417) and
   (547,-68.666792551813217) ..(547,-67.000186920166016);

pair a ; a := arrowHead intersection_point connection ;

draw connection cutafter arrowHead           withcolor red   withpen 
pencircle scaled 2mm ;
draw connection cutafter (reverse arrowHead) withcolor green withpen 
pencircle scaled 1mm ;

draw arrowHead;

draw a withcolor blue withpen pencircle scaled 3mm ;

currentpicture := currentpicture shifted (-bbwidth(currentpicture), 0) ;

draw connection cutafter a ;

draw arrowHead;


\stopMPpage

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                                           Hans Hagen | PRAGMA ADE
               Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
        tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl
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