[MPEG-OTSPEC] Defining the text shaping working group’s scope

John Hudson john at tiro.ca
Thu Jul 30 18:23:18 CEST 2020

On 29072020 8:56 pm, suzuki toshiya wrote:
>> 1. Technology Documentation
>> Describes the underlying technologies script implementations rely on. 
>> For example, OpenType, shaping engines.
> Indeed! Detailed information about the existing implementations of 
> OpenType is really really important for the font-related software 
> engineers. I remember, in the past, there was a rumor of a discussion 
> whether the documents of the script-dependent shaping mechanism of 
> OpenType (currently provided by Microsoft) should be a part of ISO 
> specs (or ISO TR), or not, but I cannot remember the conclusion. If 
> the Text Shaping WG is created under the current JTC1/SC29/WG11 font 
> AHG, the WG will make such ISO document? If it is created under 
> Unicode, the WG will make such document as a part of Unicode standard 
> (or UTR)? Either way, I'm sure that it would be an admirable decision. 

Documenting script shaping engine behaviour is important, but I think we 
need to start at a lower level, and provide the missing OTL 
implementation specification and a reference implementation. Where 
different shaping environments (i.e. a shaping engine in combination 
with the software within which it is operating) produce different 
results, this tends to be because of differing interpretation of the OTL 
table data and understanding of what to do with that data.

Script shaping produces the real world test cases for OTL 
implementation, so helps us determine what needs to be tested and what 
kinds of behaviour need to be specified—beginning with how to do script 
itemisation and run segmentation correctly, and proceeding to things 
like tracking ordering of glyphs in which the glyph count is being 
altered multiple times by GSUB—but we need to be able to generalise the 
rules to apply at an abstract glyph processing level independent of 
specific scripts and languages.

The early years of OTL script specification and development were 
characterised by a focus on script-specific shaping, instead of 
generalisation. This is how we ended up with multiple registered OTL 
features that perform the same function, e.g. ccmp and akhn: the feature 
sets were specified as something particular to a class of scripts, 
instead of generalised at the level of glyph processing behaviour.



John Hudson
Tiro Typeworks Ltd    www.tiro.com
Salish Sea, BC        tiro at tiro.com

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