[NTG-context] METAPOST subpath rounding issue

[ntg at scorecrow.com]

> On 7 May 2020, at 20:28, Gerben Wierda <gerben.wierda at rna.nl> wrote:
> 
> I have a METAPOST algorithm that splits a path at a certain time in two, does something with both ends (not the ends where they were split) and then rejoins them.
> 
> In very rare cases this crashes, because the subpath doesn’t work as expected.
> 
>       firstPart := subpath (0,halfWayTime) of workingConn;
>       secondPart := subpath (halfWayTime,pathTimeLen) of workingConn;
> 
> may sometimes result in something like this:
> 
> metapost log    > >> Path at line 0:
> metapost log    > (273,-427)..controls (259.50666666666666,-427) and (246.01333333333335,-427)
> metapost log    >  ..(232.52000000000001,-427)
> metapost log    > 
> metapost log    > >> Path at line 0:
> metapost log    > (232.51999999999998,-427)..controls (161.68000000000001,-427) and (90.840000000
> metapost log    > 000003,-427)
> metapost log    >  ..(20,-427)
> 
> As can be seen in these (rare) cases the two calls to subpath result in a different point resulting from both. so, when I later try to rejoin them with & it fails:
> 
> metapost log    > ! Paths don't touch; '&' will be changed to '..'.
> metapost log    > <to be read again> 
> 
> Which means subpath doesn’t always exactly do what I expect it to do (and many explanations, but not the official manual) state. Again, this is rare.
> 
> I’ve done this to work around it but I wondered if there was a better (reliable) solution
> 
>       save cutFirstPart; path cutFirstPart; cutFirstPart := firstPart maxcutbefore fromPicOutline;
>       save cutSecondPart; path cutSecondPart; cutSecondPart := secondPart maxcutafter toPicOutline;
>       if ((xpart point 0 of cutSecondPart) <> (xpart point infinity of cutFirstPart))
>         or ((ypart point 0 of cutSecondPart) <> (ypart point infinity of cutFirstPart)):
>         resultConn := cutFirstPart--cutSecondPart;
>       else:
>         resultConn := cutFirstPart & cutSecondPart;
>       fi

A crude test of 

  path pb;
  pb:=(5.5cm,0cm)--(5.5cm,0cm)--(10.5cm,0cm);
  draw pb;

gives no errors so why not just join using -- all the time and save the test?

--
Bruce Horrocks
Hampshire, UK