[metapost] internal pen question

[Hans Hagen]

On 12/10/2020 12:38 AM, Walt Burkhard wrote:
> Hello,
> 
> What size variable is set when using the statement
>                        pickup pencircle scaled 1 ;
> I would like to be able to examine the  current "size" of the pen 
> without  actually writing anything.   I am only intending to use this 
> variable when
> I know the pen is a circle too.

actually fullcircle is defined as being the pencircle (which is an 
approximation of a circle)

you scale to 1 so the dimensions are 1 bp

\startMPpage
     pickup pencircle scaled 1 ;
     message(llcorner fullcircle); % negative values .5
     message(llcorner image (draw origin));
     draw makepath currentpen ;
     draw fullcircle withcolor "red" ;
     currentpicture := currentpicture scaled 10cm;
     % etc
\stopMPpage

its gets a bit less clear when you use a real penshape (different x/y 
scale and rotation or so); mp then has to determine the boundingbox and 
there was a time when that was somewhat bugged because for some pens it 
can be tricky

because mp targets postscript circular pens are using native ps drawing 
while special pens result in two (transformed) paths where again some 
inaccuracy can creep into a boundingbox calculation (the already decades 
old mp to pdf converter in (pdf)tex has to do some magic to keep 
accuracy ok)

when you use special pens you sometimes have to increase the boundingbox 
a little to prevent clipping (for the circular pen that is centered in 
the origin half the pen traverses outside a path); nowadays we have a so 
called high resolution boundingbox but that was not in the original 
postscript (high res bboxes were actually comments picked up by drivers 
that were aware of them) so in those times one had definitely to make 
sure that rounding went the right direction

(there is some pen infrastructure in plain mp but that comes from mf and 
relates to fontdrawing and is probably never really used, at least i 
never did)

Hans


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                                           Hans Hagen | PRAGMA ADE
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