Glyphs-Handbook-2013

A typical alignment zone
setup: top zones with positive
widths at ascender, cap
height and x-height; bottom
zones with negative widths
at baseline and descender.
More precisely, the maximum
size of an alignment zone is
constrained by the blueScale
value (see below), which implies
that no zone must be larger
than 240 ÷ (240 × blueScale – 0.98).
height, i.e., use the same amount of pixels vertically. The same
applies to ascenders of letters like f, h, or k, descenders of g,
p or y, and the heights of all capital letters. And of course, all
letters should share the same baseline when pixellated at a
low resolution.
But all these letters usually do not really align precisely.
For instance, the bottom of a lowercase o will extend slightly
below the baseline, while the serifs of an n may sit exactly
on it. Or the apex of an uppercase A may extend a little bit
beyond the height of an uppercase H. This dierence, usually
some ten units, is commonly referred to as ‘overshoot’.
Alignment zones are a way to tell the rasterizer about the
overshoots. Anything with a valid horizontal hint that
reaches into the same alignment zone will be aligned at low
resolutions. In other words, the overshoots will be suppressed
in small pixel sizes.
Alignment zones take two values: a position and a size.
The position is the vertical height of the zone, usually the
vertical metrics, like x-height or ascender. The position is
sometimes also referred to as the ‘flat edge’ of a zone. The size
is the thickness of the maximum overshoot that may appear
at that position. If the overshoot extends above the position
(e.g., x-height, cap height, ascender), the size value must be
positive. Such zones are referred to as ‘top zones’. If, however,
the overshoots extend below the position (e.g., baseline,
descender), the size must be negative and we call them
‘bottom zones’.
Alignment zones should be as small as possible, so do not
try to make them larger ‘to be on the safe side’. In any event,
a zone must not be larger than 25 units. You can have a
maximum of 5 top zones and 6 bottom zones. Zones must
not overlap. There must be a minimum distance of one unit
between them.
Glyphs Handbook, May 2013 68