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

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.

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