# [NTG-context] Units of xpart and ypart of a pair in MetaFun

Hans Hagen j.hagen at xs4all.nl
Thu Nov 26 16:22:15 CET 2020

```On 11/26/2020 4:09 PM, Keith McKay wrote:
> 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.

cm is not really a unit but a multiplier

cf := 72/2.54; %Converts points/in to points/cm%

message(cm);
message(cf);

> 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
>
> %%%%%%%%%%%%%%
>
>
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>
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Hans Hagen | PRAGMA ADE
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