Re: a boundary-pushing challenge for drawing commands

[Tadziu Hoffmann]

> How does the formatter know which parts to fill?

In the case of groff (I can't say anything about the original
troff), I don't think the formatter actually knows or cares,
but simply passes the path on to the device postprocessor,
which just incorporates it into the device-dependent output,
to be dealt with by the actual output device.

For Postscript, the Postscript Language Reference Manual
has a short description of how filling conceptually works
(Section 4.5.2, "Filling").  There's actually two different
algorithms that can be selected, "non-zero winding rule"
or "even-odd rule", which can give different results under
certain circumstances, such as self-intersecting paths or
paths consisting of multiple sub-paths.  However, in the
case of your bowtie, both render the same.