OTL feature suggestion : Required Contextual Forms <rclt>

[John Hudson]

Each OpenType Layout feature is optimally implemented by layout engines 
and applications in one of three possible states:

1. On by default and impossible to turn off (required feature, e.g. <rlig>);

2. On by default but possible to turn off (standard feature, e.g. <liga>);

3. Off by default but possible to turn on (discretionary feature, e.g. 
<dlig>.

In practice, some software makers opt, given limited support for feature 
UI, to treat e.g. standard features as required for some scripts, but 
the ideal is fairly clearly defined in the feature description (and can 
be even more clearly defined with appropriate annotations).

[If I were designing the OpenType Layout architecture from scratch, I 
think I would be inclined to try to treat these states as lookup-level 
flags, rather than feature-level properties, which among other benefits 
would decrease the number of features needed. But that's not the 
architecture we've got, so consider this an speculative aside.]

In the past couple of years, I've noticed an increasing number of fonts 
using contextual substitutions for design-specific basic shaping, e.g. 
variants to improve spacing as in my own Gabriola font or as described 
in Eben Sorkin's TypeCon 2008 presentation. Most recently, there was a 
query about making such fonts in the Typophile Build forum.

At present, such substitutions are typically implemented in the 
Contextual Forms <calt> layout feature. However, this feature is 
intended to be implement using state 2: standard feature, on by default, 
can be turned off. Because these substitutions are intrinsic to the 
correct or desired normal display of the typeface design -- and because 
of the possibility that the same font may include other, standard 
substitutions that are not similarly intrinsic to the design --, I am 
increasingly of the view that we need a Required Contextual Forms <rclt> 
layout feature, which would be on by default and impossible to turn off 
(state 1).

I'm happy to draft a feature description, but am writing to the OT and 
OFF lists first to garner feedback on the idea.


John Hudson



-- 

Tiro Typeworks        www.tiro.com
Gulf Islands, BC      tiro at tiro.com

Car le chant bien plus que l'association d'un texte
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