[metapost] wrong bounding box with pencircle

Boguslaw Jackowski B_Jackowski at GUST.org.pl
Sat Jun 22 19:30:44 CEST 2013


> in the attached example bounding boxes are not as expected for a squared
> and a butt linecap.  (The line is drawn with a pencircle.)  Is this a
> known issue?

I believe that it is a long-standing issue. For the first time, as far
as I remember, it was raised by Antonin Strejc <STREJC at vc.cvut.cz>
(email from 22 Jul 1998 to: metafont at ens.fr; subject
"MetaPost draw & bounding box"). Antonin's example was

     pickup pencircle scaled 10;

Then, the problem several times "payed a visit" to the MF/MP discussion
list. In particular, the problem was disscussed in the thread
"some question about pens", triggered by Stephan Hennig 01 Apr 2008. :-)

In this thread, Hartmut Henkel <hartmut_henkel at gmx.de> pointed out that

   metapost doesn't calculate the bounding box right in this case, see:
   prologues := 3;
   path hc;
     hc := halfcircle scaled 200;
     linecap := squared;
     draw hc shifted (150,300) withpen pencircle scaled 50;
     draw bbox(currentpicture) withcolor red;
     currentpicture := currentpicture shifted -llcorner(currentpicture);
   if you comment out the linecap line, you get a tight bounding box.

and Dan Luecking <luecking at uark.edu> provided an example showing that 
miter-limitted paths can get wrong bounding boxes:

     miterlimit := 11;
     linejoin := mitered;
     draw (0,0)--(72,7)--(0,14) withpen pencircle scaled 2bp;

(the right vertex protrudes beyond the bounding box).

I got accustomed so much to this "feature" of MP that
I did not not emphasize it in my example which I sent
a few days ago:

    % file err-pen2.mp
    beginfig(100) draw (0,0)--(5cm,0) withpen pensquare scaled 3mm; endfig; % WRONG (oblique ends)
    beginfig(101) draw (0,0)--(5cm,0) withpen pencircle scaled 3mm; endfig; % OK
    % EOF

In the beginfig(101) line of the code, the resulting bounding box
is NOT OK which I took for granted. :-)

All in all, there is much to be done with MP strokes. On the theoretical
part too. (Maybe here also using bitmaps as an auxiliary mean could
be a help?)

Cheers -- Jacko

  Bogus\l{}aw Jackowski: B_Jackowski at GUST.ORG.PL
  Hofstadter's Law: It always takes longer than you expect, even
                    when you take into account Hofstadter's Law.

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