Le 26 nov. 2020 à 10:09, Keith McKay <mckaymeister@gmail.com> a écrit :

Hi,
I have been using xpart and ypart to extract these values from pairs of points in a path but I wasn't getting the correct result. I was expecting:
pair (2cm, 11cm) to give xpart 2 and ypart 11
However I was getting:
pair (2cm, 11cm) -> xpart 56.6929 and ypart 311.8096
After much head scratching and reading the MetaPost and MetaFun manuals, I realised that the units of xpart or ypart are in Postcript points or Big Points (bp), and 1 bp is 1/72 of an inch, and thus to get the values of xpart or ypart in cm I would have to use a correction factor. I had made the assumption that since the x and ypart in the pair was in cm that the result would be in cm, but I see now that this is not the case. Will I have to continue doing this or is there some magic within MetaFun which takes account of the units in a pair and outputs the result of x and ypart in the same units?
The MWE belows shows what I have been doing.
Thanks
Keith McKay
%%% MWE %%%
\setuppapersize [A5, landscape][A4, portrait]
\usecolors[crayola]
\starttext
\startMPpage
StartPage;
width := PaperWidth ; height := PaperHeight ; unit := cm ;
numeric squig;
pair a[]; a0 = (2cm,11cm); a1 = (4cm,10cm); a2 = (6cm,9cm); a3 = (8cm,8cm);
show xpart a[0], ypart a[0]; %Example of result from x and ypart before applying correction factor (cf) %
cf := 72/2.54; %Converts points/in to points/cm%
path pp;
for i = 0 step 1 until 3:
pp := ((xpart a[i])/cf*cm, (ypart a[i])/cf*cm);
for squig = 1 step 1 until 15:
xcoord := uniformdeviate(1) - 0.5 + (xpart a[i]/cf);
ycoord := uniformdeviate(1) - 0.5+ (ypart a[i]/cf);
pp := pp .. {curl 100}(xcoord*cm, ycoord*cm);
endfor;
pp := pp --- cycle;
if i < 3 :
pp := pp --- ((xpart a[i+1]/cf)*cm, (ypart a[i+1])/cf*cm);
fi;
f :=((1.4 - 0.6) * uniformdeviate(1)) + 0.8; % Factor to lighten/darken colour %
draw pp withpen pencircle xscaled 0.5mm yscaled .1mm rotated 45 withcolor (f[white,\MPcolor{BurntSienna}]);
endfor;
StopPage;
\stopMPpage
\stoptext
%%%%%%%%%%%%%%

MetaPost (MetaFun) is taking care of everything regarding units. As you ave discovered, everything is translated to a unique internal dimension. So you can write: