[mpeg-OTspec] OTL feature suggestion : Required Contextual Forms <rclt>

[John Hudson]

Adam Twardoch wrote:

> So if the font maker considers the "calt" feature "required" for the
> "latn" script in her font, she can point the ReqFeatureIndex for
> "latn"/DefaultLangSys to the "calt" feature.

But a font may contain different kinds of contextual substitutions, some 
of them considered required and some of them not. The font maker may 
wish the user to be able to turn off the latter without affecting the 
required substitutions.

Of course, one way around this would be to put the required contextual 
substitutions into a different feature than <calt>, reserving the latter 
for substitutions that one doesn't mind being turned off, and tagging 
this other feature as the ReqFeatureIndex. But which feature? There is 
the issue of discrete feature processing ordering to consider, which 
makes <ccmp> not always appropriate because, depending on script, the 
input strings for the required contextual substitutions might not occur 
until long after <ccmp> has been processed. Presuming one wants the 
ReqFeatureIndex tagged feature to be used for contextual form 
substitutions analogous to those in <calt>, one would probably want a 
feature that was processed at the same time as <calt>.

Whichever way I look at it, an <rclt> feature looks desirable. Actually, 
since the architecture obliges us to distinguish default state at the 
feature level, one could argue that a number of existing features should 
have come in <rxxx> required, <sxxx> standard and <dxxx> discretionary 
flavours, as those for ligatures do.

John Hudson


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