Re: Units of xpart and ypart of a pair in MetaFun

[Keith McKay <mckaymeister@gmail.com>]

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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 <mckaymeister@gmail.com 
>> <mailto:mckaymeister@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.
>>
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