MS Proposal: new feature tag

[Michelle Perham]

If no one has any questions or concerns here, I'd like to propose that Vlad add this to his working draft.
Michelle
From: Michelle Perham
Sent: Thursday, January 3, 2013 12:20 PM
To: 'opentype-migration-list at indx.co.uk'; OTspec (mpeg-OTspec at yahoogroups.com)
Cc: Andrew Glass (WINDOWS)
Subject: MS Proposal: new feature tag

Here is another proposal put together by Andrew Glass of Microsoft. He'd like to propose a new feature tag to support the Syriac Abbreviation Mark and other marks which stretch to enclose other characters.

Tag: "stch"
Friendly name: Stretching Glyph Decomposition
Registered by: Microsoft

Function: Unicode characters, such as the Syriac Abbreviation Mark (U+070F), that enclose other characters need to be able to stretch in order to dynamically adapt to the width of the enclosed text. This feature defines a decomposition set consisting of an odd number of glyphs which describe the stretching glyph. The odd numbered glyphs in the decomposition are fixed reference points which are distributed evenly from the start to the end of the enclosed text. The even numbered glyphs may be repeated as necessary to fill the space between the fixed glyphs. The first and last glyphs may either be simple glyphs with width at the baseline, or mark glyphs. All other decomposition glyphs should have width, but must be defined as mark glyphs.

Example: In Syriac, the character 0x070F is a control character that is rendered as a line above an abbreviation in Syriac script. The line should have a circle at each end and at the mid point. The decomposition sequence for this character should consist of a circle at the start of a line, a connecting line, a circle on a line for the mid point, a second connecting line, and a circle at the end of the line. The connecting lines will repeat in order to fill the space between the circle glyphs.

Recommended implementation: The stch table maps the character to a set containing an odd number of corresponding glyphs (GSUB lookup type 2). The rendering engine reorders the last glyph from the substituted set to the end of the set of characters being enclosed. The remaining glyphs from the substituted set are positioned at the start of the set of characters being enclosed. Odd-numbered glyphs in the decomposition set are positioned so that they are distributed evenly over the width of the text being enclosed. Even-numbered glyphs in the decomposition set are repeated by the rendering engine so the width of the space between fixed, odd-numbered glyphs is filled by the spacing, even-numbered glyphs.

Application interface: For GIDs found in the stch coverage table, the application passes the sequence of GIDs to the table, and gets back the GIDs for the multiple substitution.

UI suggestion: This feature should be on by default.
Script/language sensitivity: None.
Feature interaction: None.


Please respond with questions of comments. Thanks!

Michelle
Microsoft Typography Group
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