[metapost] Problem with generating EPS from MetaFont source

[Dan Luecking]

[Forgot to copy the list]

Michael Ummels <michael.ummels at rwth-aachen.de> wrote:

 >
 > I am trying to use MetaPost to generate EPS files for each
 > character in a
 > MetaFont as described in Appendix B of the MetaPost manual.
 > However, some
 > strokes do not come out correctly or are missing at all in the
 > resulting EPS
 > files. I would be glad if somebody could tell me what is going wrong.
 >
 > To replicate my results, run "mpost '&mfplain \mode:=localfont;
 > mag=100.375;input test'" on the file test.mf, which is given below.
 > In the resulting EPS
 > files test.2 and test.3, strokes are missing/incorrect, whereas
 > test.0 and
 > test.1 are fine.
 >
 > Interestingly, the problem does not occur when using pencircle
 > instead of
 > pensquare.

Metapost has longstanding bugs in the handling of polygonal pens.
It tries to calculate the path of the outermost corner, which can switch
from one corner to another along a curve. Unfortunately, this is a rather
unstable mathematical problem and metapost frequently gets it wrong.
Sometimes an infinitessimal rotation of the pen causes it to come out
right.

You would be advised to calculate these paths yourself and  fill the
resulting outline.

I believe Taco may know more about these issues.

Taco: I have wondered a lot about metapost's problems with polygonal
pens. If someone could point out the code that handles it (and perhaps
a description of the algorithm), I would like to take a whack at figuring
out at least where and/or why it goes wrong.


Dan


Daniel H. Luecking
Department of Mathematical Sciences
Fayetteville, Arkansas
http://www-cs-faculty.stanford.edu/~knuth/iaq.html