Re: [Groff] grotty and \D'l 0 0'

[Werner LEMBERG]

> > Using grotty, this code
> >
> >   x \D'l 1m 0' x
> >
> >   x \D'l 0 1v' x
> >
> > produces
> >
> >   x --x
> >
> >   x |
> >     |x
> >
> > [You have to use the current CVS or version 1.19.0 and earlier -- the
> > versions inbetween handle this incorrectly.]
>
> My old and crufty 1.18.1 on slightly different input produces
>
>     $ cat w.tr
>     X
>     .br
>     X\D'l 1m 0'X
>     X\D'l 0 1v'X
>     $ groff -Tutf8 w.tr | cat -s
>     X
>     X-X X|
>          X
>
>     $

Exactly.  groff treats a two-cell line having a one-cell width.
Closer inspection would show that e.g.

  X\D'l 1m 0'X

is translated to

  X--^HX

> > As can be seen, a line of length 1m (1v) is drawn as *two*
> > character cells, not one, but still takes only a length (height)
> > of one character cell.  This result may be surprising but it is
> > the correct one so that it is completely analogous to the output
> > of other devices -- it took a long time until I came to this
> > conclusion :-)
>
> I can believe it took a long time.  I puzzled over a pixel drawing
> problem once for ages not knowing that accepted practice had already
> worked out the "right way".  However, I'm not sure your output
> above, or mine, is the "right" result.  :-)

Both are right.  You've obviously missed the `^H' backspace control
character.

> Looking at the PostScript output of w.tr, I think this is a better
> approximation.
>
>     X
>     X_X X
>          |X
>
> The horizontal line should be the baseline of the text,

Using `_' instead of `-' is not a good idea.  Just compare

   +____+            +----+
   |    |   versus   |    |
   +____+            +----+

> and only one character long because we assume a fixed-width font.

This is a wrong assumption.

> I suppose you're thinking of an em-dash being represented by two --
> characters in plain text so a 1m line should be two characters too.

No, this isn't the reason.  Consider this example:

  foo

  .sp -1
  \D'l 0 1v'
  .sp -1
  \h'3m'\D'l 0 1v'
  .br
  \D'l 3m 0'

  bar

which results in

  foo
  |  |
  +--+

  bar

Now imagine that \D'l 3m 0' is only three character cells long, and
\D'l 0 1v' only a single one.  You would get

  foo
  |  |
  ---

  bar

Which is not acceptable, I think.  We have to consider both line
lengths and line positions.

BTW, the current behaviour of grotty is fine for must purposes, exept
for single-line tables with vertical lines as field separators...

> >   x \D'l .1m 0' x
> >
> >   x \D'l 0 .1v' x
> >
> > produces
> >
> >   x +x
> >
> >   x +x
> >
> > The line lengths are rounded to the next multiple of the
> > horizontal and vertical device resolution, which is 0 here in both
> > directions -- the line more or less degenerates to a dot,
> > represented with `+'.
>
> Previous groffs, e.g. 1.18.1, used to do this though?

AFAIK, it was never possible with GNU troff to produce a single-cell
line in TTY output with \D'l...'.


     Werner