Re: [css-text] letter-spacing tests

On 10/10/14 11:24 AM, John Cowan wrote:

>> As more font makers realise the
>> relative efficiency of handling Arabic without ligatures, instead
>> utilising contextually triggered variant letter glyphs,

> What is the difference between these two?

Contextual letter (or archegrapheme*) glyphs allow for a much smaller 
font glyph set, correspondingly fewer and simpler mark positioning GPOS 
lookups, and more thorough coverage of possible joining sequence behaviour.

As an example, an older Monotype implementation of Urdu using a ligature 
font required more than 20,000 glyphs, and still failed in some 
situations (the classic case was transliteration of foreign loan words; 
Urdu newspaper fonts needed to be updated with a whole-word ligature 
every time a new Soviet leader was announced).

By contrast, Urdu can be handled using contextual archegraphemes using 
only a few hundred glyphs.


The use of ligatures for Arabic type is an artefact of metal 
typesetting, and the difficulty of handling multiple vertical offsets of 
joining letters during composing. It was easier to cast sequences of 
joining letters as ligature sorts, even though this frequently led to 
problems in text composition. For example, suppose you have a letter 
sequence ABC, and you have ligatures for AB and BC, but not for the 
three letters; which ligature do you use? and how ugly is the result? 
One sees this sort of problem a lot in Arabic metal typesetting, and it 
was inherited into phototype and digital fonts (and even into Unicode 
via the presentation form blocks).

At its root, I would say, is a fundamental mis-analysis of Arabic 
writing, which looks at the script in terms of topographical variants 
(isolated, initial, medial and final) which sometimes form 'special' 
ligature shapes with each other. I think you get a more accurate picture 
-- and hence a more appropriate set of technical solutions -- if you 
look at the script in terms of style-specific graphotactic options 
arranged around a basic set of character-level joining behaviours. 
[Unicode largely gets the last bit right, by standardising joining 
behaviour properties rather than topographical variants, even though the 
latter are what tend to get illustrated.]

The mis-analysis looks at a discrete sequence of Arabic letters and says 
'This first one takes initial form, and this second one takes medial 
form, and this third one takes final form', and then says 'These 
particular medial and final letters take a special ligature shape that 
differs from their default connection'.

The better analysis looks at the same sequence and says 'Each of these 
letters has a particular appropriate form based on its neighbour(s).'


JH


* Particularly in script styles that involve significant amounts of 
vertical offset connections, e.g. nastaliq, it may make sense to 
decompose letters with disambiguating dots into archegrapheme base 
glyphs and dynamically positionable dot mark glyphs. This further 
reduces the number of glyphs needed in the font, and allows for 
contextual positioning of the dots relative to adjacent shapes.

Received on Friday, 10 October 2014 19:04:52 UTC