[metapost] some question about pens

Stephan Hennig mailing_list at arcor.de
Wed Apr 2 22:42:56 CEST 2008

Taco Hoekwater schrieb:
> Stephan Hennig wrote:
>> (ii) For all examples in the left-hand column (rounded pen) the end
>> points of the strokes seems to have a slightly wrong direction, i.e., it
>> is not exactly "up" or "down".  A bug?
> This looks like ghostscript bug. The generated PS code seems correct,
> and when printing to a true PostScript engine (create actual hardcopy)
> or when viewed converted to PDF (use Adobe software), it looks fine.

The mptopdf output also looks OK in gs.

> Perhaps it is worth filing a bug report to the gs people. Xpdf is also
> a little bit off, but that is nothing compared to the massive gs
> deviation.

I wouldn't want to during the next four weeks, so, I wouldn't mind if
anybody else did that sooner.

>> (iii) For the upper and middle example in the right-hand column (squared
>> pen with rounded/squared linecap) I'd expect exactly the same results.
>> Or better, what exactly is the effect of linecap on squared pens?  The
>> manual is not very verbose about that.
> Taking into account the effect of the bug in (iv), this output is
> exactly what I would expect.


> (Jacko has proposed that MP should ignore
> linecap for polygonal pens, but that would break orthogonality).

The squared/butt linecap distinction is still useful for polygonal pens,

>> (iv) Why are there diagonal end points in the middle and lower example
>> in the right-hand column (squared pen, squared/butt linecap)?
> This is already in the tracker:
>    http://foundry.supelec.fr/tracker/?atid=129&group_id=13&func=browse
> Item 4, "Pensquare weirdness B".
> The general cause of the problem is clear to me, but fixing it is still
> a bit above my knowledge level. Basically, MP tracks the 'outside' of
> the pen, and at the reversal point of the path, it jumps 180 degrees
> (sort of) to the opposite side of the pen, across the diagonal.

Then this is the only true problem with my example code and it's a known
problem.  Thanks!

Best regards,
Stephan Hennig

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