Thanks Fabrice and Hans. I now see I can simplify some lines of the code: pp := ((xpart a[i])/cf*cm, (ypart a[i])/cf*cm); now becomes pp:= a[i]; and pp := pp --- ((xpart a[i+1]/cf)*cm, (ypart a[i+1])/cf*cm); now becomes pp := pp --- a[i +1] ; However I still have the problem in the calculation of xcoord and ycoord which are used in the creation of path pp: 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; I'm adding a small random amount to the x and ypart of the pair a[i] to produce a path that is like a squiggle (gribouiller in French, according to Google). I have to use cf to convert to the x and yparts to centimeters since they seem to loose the centimeter units on separation. I can't see anyway round this, or am I missing something? Thanks Keith McKay On 26/11/2020 15:19, Fabrice L wrote: > Hi keith, > >> Le 26 nov. 2020 à 10:09, Keith McKay > > 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: > >  a := (1cm,2in) ; > > with no problem. The « cm » and « in » parts of the expression will > become numbers to translate this number on the right unit. > Fabrice. >> >> ___________________________________________________________________________________ >> If your question is of interest to others as well, please add an >> entry to the Wiki! maillist :ntg-context@ntg.nl >> /http://www.ntg.nl/mailman/listinfo/ntg-context >> webpage >>  :http://www.pragma-ade.nl >> /http://context.aanhet.net >> archive >>  :https://bitbucket.org/phg/context-mirror/commits/ >> wiki >>     :http://contextgarden.net >> ___________________________________________________________________________________ >> >