Path: utzoo!utgpu!news-server.csri.toronto.edu!dgp.toronto.edu!avi
Newsgroups: comp.fonts
From: avi@dgp.toronto.edu (Avi Naiman)
Subject: Re: Types of kerning
Message-ID: <1990Sep7.204316.26956@jarvis.csri.toronto.edu>
Summary: Sector Kerning and Optical Spacing.  Philosophy on Character Spacing.
Organization: CSRI, University of Toronto
References: <38743@shemp.CS.UCLA.EDU> <4cac478e.20b6d@apollo.HP.COM>
Date: 8 Sep 90 00:43:16 GMT
Lines: 86

In article <4cac478e.20b6d@apollo.HP.COM> laporta@apollo.HP.COM
(John X. Laporta) writes:

>By the way, are you aware that there are three types of kerning:

>    overall kerning - an identical (positive/negative) amount is
>        added to the distance between each pair of characters.
>    track kerning - like overall kerning, but the values differ
>        for different ranges of point sizes within a given font.
>    pairwise/triplet kerning - a lookup table offers kerning
>        increments/decrements for specific pairs of characters
>        and for the pairs in specific triplets. Kerning triplets
>        are often used when the font has no ligatures (e.g. for
>        "ffl").

I'm familiar with at least two other types of kerning:

Sector Kerning.  Each character in a font is split into a number of
horizontal sectors.  In each sector, a penetration value is specified
on each side of the character, indicating to what extent other
characters are allowed to penetrate into its 'space'.  To determine
the kerning for a specific pair of characters, in each sector the
right-hand penetration value of the left-hand character is added to
the left-hand penetration value of the right-hand character.  The
minimum of these sums is the maximum penetration allowed.  Kerning
is a function of this penetration amount, as well as other factors
such as overall kerning, track kerning, and letter spacing.  Note that
nothing requires each sector to be the same height.  For example, more
detail can be placed around the x-height for setting body text.
Furthermore, sector kerning can be used across fonts (assuming the
sector boundaries are the same for the two fonts -- which they need
not be), and the penetration values of more than two characters can be
considered at the same time (e.g., the spacing of a pair of characters
can be sensitive to the other characters in the word).  If the number
of sectors is a multiple of 4 and the penetration values are unsigned
bytes, kerning can be determined quite efficiently with unsigned long
addition (this is the approach taken in a commercial version of troff
which supports sector kerning).

Optical Spacing.  This isn't so much a 'kerning' technique as an overall
character spacing technique developed by David Kindersley, a typographer,
together with Neil Wiseman, a computer scientist.  Their approach is
to position a right-hand character such that the linear distance between
its fourth moment and that of the left-hand character is constant.  For
example, the fourth moment of 'A' is fairly low, while the fourth moment
of 'V' is quite high.  Therefore, the *spacing* for "AVA" will achieve
what others refer to as *kerning*.  Note that characters which are not
symmetric about a vertical axis may have a fourth moment displaced right
or left from the horizontal center of the character.  See the following
references for more details:

%L Kindersley 76
%A David Kindersley
%T Optical Letter Spacing for New Printing Systems
%I Sandstone Press
%C New York
%D 1976

%L Kindersley and Wiseman 79
%A David Kindersley
%A Neil E. Wiseman
%T Computer-Aided Letter Design
%J Printing World
%D October 1979
%P 12-17

Although their approach is far from universally accepted, there are two
important aspects to note.  First, it is EXTREMELY DANGEROUS for those
not trained in the art of typography (and don't think it's a science)
to hack together kerning tables.  [This warning applies to MOST people
found on the net!]  After the basic letterform shapes of a typeface (or
font -- i.e., a typeface's rendition at a specific size realized in a
specific medium) have been designed, a considerable amount of time is
spent tuning, re-tuning, and fine-tuning the spacing (and/or kerning).
To my knowledge, noone has yet found a satisfactory technique to determine
character spacing automatically (including Kindersley).

The second aspect to note is that 'kerning' is basically an exception
handler.  If the character spacing could be computed directly from the
shapes of a pair of characters (or all the characters in a word, line,
or even page), there would be no need to 'kern'.  The question of optimal
(and automated) spacing remains an open research topic, and is best left
to qualified investigators working with competent typographers.
-- 
Avi Naiman
avi@dgp.toronto.edu