File: groff.info,  Node: Page Geometry,  Next: Measurements,  Prev: Text,  Up: GNU troff Reference

5.2 Page Geometry
=================

'roff' systems format text under certain assumptions about the size of
the output medium, or page.  For the formatter to correctly break a line
it is filling, it must know the line length, which it derives from the
page width (*note Line Layout::).  For it to decide whether to write an
output line to the current page or wait until the next one, it must know
the page length (*note Page Layout::).

   A device's "resolution" converts practical units like inches or
centimeters to "basic units", a convenient length measure for the output
device or file format.  The formatter and output driver use basic units
to reckon page measurements.  The device description file defines its
resolution and page dimensions (*note DESC File Format::).

   A "page" is a two-dimensional structure upon which a 'roff' system
imposes a rectangular coordinate system with its upper left corner as
the origin.  Coordinate values are in basic units and increase down and
to the right.  Useful ones are therefore always positive and within
numeric ranges corresponding to the page boundaries.

   While the formatter (and, later, output driver) is processing a page,
it keeps track of its "drawing position", which is the location at which
the next glyph will be written, from which the next motion will be
measured, or where a geometric object will commence rendering.
Notionally, glyphs are drawn from the text baseline upward and to the
right.(1)  (*note Page Geometry-Footnote-1::) The "text baseline" is a
(usually invisible) line upon which the glyphs of a typeface are
aligned.  A glyph therefore "starts" at its bottom-left corner.  If
drawn at the origin, a typical letter glyph would lie partially or
wholly off the page, depending on whether, like "g", it features a
descender below the baseline.

   Such a situation is nearly always undesirable.  It is furthermore
conventional not to write or draw at the extreme edges of the page.
Therefore the initial drawing position of a 'roff' formatter is not at
the origin, but below and to the right of it.  This rightward shift from
the left edge is known as the "page offset".(2)  (*note Page
Geometry-Footnote-2::) The downward shift leaves room for a text output
line.

   Text is arranged on a one-dimensional lattice of text baselines from
the top to the bottom of the page.  "Vertical spacing" is the distance
between adjacent text baselines.  Typographic tradition sets this
quantity to 120% of the type size.  The initial drawing position is one
unit of vertical spacing below the page top.  Typographers term this
unit a vee.

   Vertical spacing has an impact on page-breaking decisions.
Generally, when a break occurs, the formatter moves the drawing position
to the next text baseline automatically.  If the formatter were already
writing to the last line that would fit on the page, advancing by one
vee would place the next text baseline off the page.  Rather than let
that happen, 'roff' formatters instruct the output driver to eject the
page, start a new one, and again set the drawing position to one vee
below the page top; this is a "page break".

   When the last line of input text corresponds to the last output line
that fits on the page, the break caused by the end of input will also
break the page, producing a useless blank one.  Macro packages keep
users from having to confront this difficulty by setting "traps" (*note
Traps::); moreover, all but the simplest page layouts tend to have
headers and footers, or at least bear vertical margins larger than one
vee.