On 8 May 2020, at 00:46, ntg@scorecrow.com wrote:

On 7 = May 2020, at 20:28, Gerben Wierda <gerben.wierda@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=E2=80=99t work as = expected.

     firstPart :=3D subpath = (0,halfWayTime) of workingConn;
     secondPart :=3D 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=E2=80=99t always = exactly do what I expect it to do (and many explanations, but not the = official manual) state. Again, this is rare.

I=E2=80=99ve done this to work around it but I wondered if = there was a better (reliable) solution

     save cutFirstPart; path = cutFirstPart; cutFirstPart :=3D firstPart maxcutbefore = fromPicOutline;
     save = cutSecondPart; path cutSecondPart; cutSecondPart :=3D 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 :=3D = cutFirstPart--cutSecondPart;
     else:
       resultConn :=3D = cutFirstPart & cutSecondPart;
     fi
A crude test of

path pb;
pb:=3D(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?

Because the = double exact points are also creating (different) problems in my = algorithm as they make the path have 'no direction' at that point = (direction is (0,0).

--
Bruce Horrocks
Hampshire, UK

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