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

[Fabrice L]

Hi keith,

> Le 26 nov. 2020 à 10:09, Keith McKay <mckaymeister at 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:

 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 at ntg.nl <mailto:ntg-context at ntg.nl> / http://www.ntg.nl/mailman/listinfo/ntg-context <http://www.ntg.nl/mailman/listinfo/ntg-context>
> webpage  : http://www.pragma-ade.nl <http://www.pragma-ade.nl/> / http://context.aanhet.net <http://context.aanhet.net/>
> archive  : https://bitbucket.org/phg/context-mirror/commits/ <https://bitbucket.org/phg/context-mirror/commits/>
> wiki     : http://contextgarden.net <http://contextgarden.net/>
> ___________________________________________________________________________________
> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.ntg.nl/pipermail/ntg-context/attachments/20201126/ba71676d/attachment-0001.htm>