[Groff] Rules drawn with \D are rounded

[Peter Schaffter]

I've been contemplating adding some macros to the mom set to
ease the creation of simple graphical objects: rules, boxes and
elipses/circles ("ease" meaning "make easier for newcomers to
groff").

However, I've discovered something troubling: rules drawn with

    \D'l <co-ordinates>'

have rounded caps; the ends of the rules are not straight.

Equally, unfilled polygons drawn with

    \D'p <co-ordinates>

all have rounded corners, whereas filled polygons drawn with

    \D'P <co-ordinates>'

produces the expected "sharp" corners.

Typographically speaking, this is not an expected, or--in my
experience--justifiable default behaviour for rules and polygons.

The effect is scarcely noticeable with narrow rule weights
(i.e. < 1000 units), but when the rule weight increases the effect
becomes more and more pronounced.

So, the usual questions:

1.  What is the rational behind groff's drawing rounded caps on
    rules, and rounded corners on unfilled polygons?

2.  Can this behaviour be changed in user space?

3.  Could this behaviour be changed in the applicable groff binaries
    themselves?

Thanks.

-- 
Peter Schaffter