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From: Gerben Wierda <gerben.wierda@rna.nl>
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Subject: METAPOST subpath rounding issue
Date: Thu, 7 May 2020 21:28:40 +0200
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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    >=20
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>=20

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

G=

--Apple-Mail=_6EE1B793-DFF4-4689-8670-E5389D567992
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<html><head><meta http-equiv=3D"Content-Type" content=3D"text/html; =
charset=3Dutf-8"></head><body style=3D"word-wrap: break-word; =
-webkit-nbsp-mode: space; line-break: after-white-space;" class=3D"">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.<div class=3D""><br class=3D""></div><div class=3D"">In =
very rare cases this crashes, because the subpath doesn=E2=80=99t work =
as expected.</div><div class=3D""><br class=3D""></div><div =
class=3D""><div class=3D""><font face=3D"Courier" class=3D""><span =
style=3D"font-style: normal;" class=3D"">&nbsp; &nbsp; &nbsp; firstPart =
:=3D subpath (0,halfWayTime) of workingConn;</span></font></div><div =
class=3D""><font face=3D"Courier" class=3D""><span style=3D"font-style: =
normal;" class=3D"">&nbsp; &nbsp; &nbsp; secondPart :=3D subpath =
(halfWayTime,pathTimeLen) of workingConn;</span></font></div></div><div =
class=3D""><br class=3D""></div><div class=3D"">may sometimes result in =
something like this:</div><div class=3D""><br class=3D""></div><div =
class=3D""><div style=3D"margin: 0px; font-stretch: normal; font-size: =
10px; line-height: normal; font-family: Monaco;" class=3D""><span =
style=3D"font-variant-ligatures: no-common-ligatures" class=3D"">metapost =
log&nbsp; &nbsp; &gt; &gt;&gt; Path at line 0:</span></div><div =
style=3D"margin: 0px; font-stretch: normal; font-size: 10px; =
line-height: normal; font-family: Monaco;" class=3D""><span =
style=3D"font-variant-ligatures: no-common-ligatures" class=3D"">metapost =
log&nbsp; &nbsp; &gt; (273,-427)..controls (259.50666666666666,-427) and =
(246.01333333333335,-427)</span></div><div style=3D"margin: 0px; =
font-stretch: normal; line-height: normal;" class=3D""><span =
style=3D"font-variant-ligatures: no-common-ligatures;" class=3D""><font =
face=3D"Monaco" size=3D"1" class=3D"">metapost log&nbsp; &nbsp; =
&gt;&nbsp; ..(<span style=3D"font-style: normal;" class=3D""><font =
color=3D"#c81b20" =
class=3D"">232.52000000000001</font></span>,-427)</font></span></div><div =
style=3D"margin: 0px; font-stretch: normal; font-size: 10px; =
line-height: normal; font-family: Monaco;" class=3D""><span =
style=3D"font-variant-ligatures: no-common-ligatures" class=3D"">metapost =
log&nbsp; &nbsp; &gt;&nbsp;</span></div><div style=3D"margin: 0px; =
font-stretch: normal; font-size: 10px; line-height: normal; font-family: =
Monaco;" class=3D""><span style=3D"font-variant-ligatures: =
no-common-ligatures" class=3D"">metapost log&nbsp; &nbsp; &gt; &gt;&gt; =
Path at line 0:</span></div><div style=3D"margin: 0px; font-stretch: =
normal; font-size: 10px; line-height: normal; font-family: Monaco;" =
class=3D""><span style=3D"font-variant-ligatures: no-common-ligatures" =
class=3D"">metapost log&nbsp; &nbsp; &gt; (<font color=3D"#c81b20" =
class=3D"">232.51999999999998</font>,-427)..controls =
(161.68000000000001,-427) and (90.840000000</span></div><div =
style=3D"margin: 0px; font-stretch: normal; font-size: 10px; =
line-height: normal; font-family: Monaco;" class=3D""><span =
style=3D"font-variant-ligatures: no-common-ligatures" class=3D"">metapost =
log&nbsp; &nbsp; &gt; 000003,-427)</span></div><div style=3D"margin: =
0px; font-stretch: normal; font-size: 10px; line-height: normal; =
font-family: Monaco;" class=3D""><span style=3D"font-variant-ligatures: =
no-common-ligatures" class=3D"">metapost log&nbsp; &nbsp; &gt;&nbsp; =
..(20,-427)</span></div></div><div class=3D""><span =
style=3D"font-variant-ligatures: no-common-ligatures" class=3D""><br =
class=3D""></span></div><div class=3D"">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 &amp; it =
fails:</div><div class=3D""><br class=3D""></div><div class=3D""><div =
style=3D"margin: 0px; font-stretch: normal; font-size: 10px; =
line-height: normal; font-family: Monaco;" class=3D""><span =
style=3D"font-variant-ligatures: no-common-ligatures" class=3D"">metapost =
log&nbsp; &nbsp; &gt; ! Paths don't touch; '&amp;' will be changed to =
'..'.</span></div><div style=3D"margin: 0px; font-stretch: normal; =
font-size: 10px; line-height: normal; font-family: Monaco;" =
class=3D""><span style=3D"font-variant-ligatures: no-common-ligatures" =
class=3D"">metapost log&nbsp; &nbsp; &gt; &lt;to be read =
again&gt;&nbsp;</span></div></div><div class=3D""><br =
class=3D""></div><div class=3D"">Which means subpath doesn=E2=80=99t =
always&nbsp;<i class=3D"">exactly</i> do what I expect it to do (and =
many explanations, but not the official manual) state. Again, this is =
rare.</div><div class=3D""><br class=3D""></div><div class=3D"">I=E2=80=99=
ve done this to work around it but I wondered if there was a better =
(reliable) solution</div><div class=3D""><br class=3D""></div><div =
class=3D""><div class=3D""><font face=3D"Courier" class=3D""><span =
style=3D"font-style: normal;" class=3D"">&nbsp; &nbsp; &nbsp; save =
cutFirstPart; path cutFirstPart; cutFirstPart :=3D firstPart =
maxcutbefore fromPicOutline;</span></font></div><div class=3D""><font =
face=3D"Courier" class=3D""><span style=3D"font-style: normal;" =
class=3D"">&nbsp; &nbsp; &nbsp; save cutSecondPart; path cutSecondPart; =
cutSecondPart :=3D secondPart maxcutafter =
toPicOutline;</span></font></div><div class=3D""><font face=3D"Courier" =
class=3D""><span style=3D"font-style: normal;" class=3D"">&nbsp; &nbsp; =
&nbsp; if ((xpart point 0 of cutSecondPart) &lt;&gt; (xpart point =
infinity of cutFirstPart))</span></font></div><div class=3D""><font =
face=3D"Courier" class=3D""><span style=3D"font-style: normal;" =
class=3D"">&nbsp; &nbsp; &nbsp; &nbsp; or ((ypart point 0 of =
cutSecondPart) &lt;&gt; (ypart point infinity of =
cutFirstPart)):</span></font></div><div class=3D""><font face=3D"Courier" =
class=3D""><span style=3D"font-style: normal;" class=3D"">&nbsp; &nbsp; =
&nbsp; &nbsp; resultConn :=3D =
cutFirstPart--cutSecondPart;</span></font></div><div class=3D""><font =
face=3D"Courier" class=3D""><span style=3D"font-style: normal;" =
class=3D"">&nbsp; &nbsp; &nbsp; else:</span></font></div><div =
class=3D""><font face=3D"Courier" class=3D""><span style=3D"font-style: =
normal;" class=3D"">&nbsp; &nbsp; &nbsp; &nbsp; resultConn :=3D =
cutFirstPart &amp; cutSecondPart;</span></font></div><div class=3D""><font=
 face=3D"Courier" class=3D""><span style=3D"font-style: normal;" =
class=3D"">&nbsp; &nbsp; &nbsp; fi</span></font></div></div><div =
class=3D""><br class=3D""></div><div class=3D"">G</div></body></html>=

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