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

[Hans Hagen]

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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