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

[Hans Hagen <j.hagen@xs4all.nl>]

On 11/26/2020 6:12 PM, Keith McKay wrote:
> 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?


	for squig = 1 step 1 until 15:
		xcoord := (xpart a[i]/cf);
		ycoord := (ypart a[i]/cf);
         xcoord := xcoord randomized (xcoord/50);
         ycoord := ycoord randomized (ycoord/50);
		pp := pp  .. { curl 100 } (xcoord*cm, ycoord*cm);
	endfor;


> 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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>>
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