Re: [Groff] Problem with bottom page margin in MM

[Tadziu Hoffmann]

> I  have  found a problem with pagination in MM. In
> the following example, the bottom margin  is  dif-
> ferent  for  the  first  page  (which  ends with a
> filled line of text) and the  second  page  (which
> ends with a non-filled line of a footnote).
[snip]

My guess is that it's a bug in m.tmac.

If you look at macro address@hidden, you'll see that it spaces
to |\\n[pg*foot-trap]u+1v before printing the footnotes,
one line *below* the point where they "should" have been
printed.

So why is that "+1v" there?  Well, the biggest problem with
troff's traps is that they get sprung *after* a line has been
printed reaching or exceeding the trap position.  There's no
way to say that a particular margin should *not* be printed
into, i.e., to spring the trap *before* a line overstepping
the boundary would be output.  But if you're working with
high-res devices with different font sizes and line spacings,
it's likely that you don't reach the trap position exactly,
but possibly exceed it by up to almost a full baselineskip
(or more, if that last line includes extra vertical space).
Perhaps that "+1v" is there to avoid overprinting the last
line with the footnotes.

My usual solution in these cases is to set the trap position
at margin-1v+1u, slightly below the second-to-last line (and
hope that the line that then springs the trap doesn't have
extra vertical space or uses a different vertical spacing).

[However, I just tried this with groff -Tascii, and noticed
that a trap at position 481 units was sprung by a line output
to position 480 units.  Werner, is this intended?]

Anyhow, the upshot of all this is that if you're only working
with character-cell devices with fixed baseline spacing, just
remove the "+1v" and all should be fine.  If you're also using
high-res devices, you'll probably need to rethink mm's setting
of trap positions.  (I'll leave that to those who actually use
mm.)