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

[Keith McKay]

/First « //uniformdeviate(1) - 0.5// » could be modified to « 
//uniformdeviate(0.5)//»./
/
/
/To add noise, you can use the randomized function of Metafun. For 
exemple, you can write:/
/
/
/draw pp randomized 1cm ;/
/
/
That's true Fabrice but in this use case I wanted a random number 
between -0.5 and +0.5, and I wasn't sure I would get that with randomized.
Thanks again
Keith McKay/
/
On 26/11/2020 17:20, Fabrice L wrote:
> Keith,
>
>> Le 26 nov. 2020 à 12:12, Keith McKay <mckaymeister at gmail.com 
>> <mailto:mckaymeister at gmail.com>> a écrit :
>>
>> 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
>
> First « uniformdeviate(1) - 0.5 » could be modified to « 
> uniformdeviate(0.5)».
>
> To add noise, you can use the randomized function of Metafun. For 
> exemple, you can write:
>
> draw pp randomized 1cm ;
>
> Fabrice.
>
>
>
>> On 26/11/2020 15:19, Fabrice L wrote:
>>> Hi keith,
>>>
>>>> Le 26 nov. 2020 à 10:09, Keith McKay <mckaymeister at gmail.com 
>>>> <mailto: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.
>>>>
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