behdad
commented
5 years ago @mhosken and I proposed this in 2016: https://github.com/OpenType/opentype-layout/blob/master/proposals/glyph_filtering.md
Microsoft was strongly opposed to it on (imo unfounded) "security" basis.
Lookups can currently specify, via the lookup flag, to ignore base glyphs, ligatures, or marks as complete sets, or ignore a specified subset of marks, or ignore marks that are not in a specified subset of marks.
In many cases, it's necessary to ignore specific glyphs that by their nature wouldn't be OpenType marks, such as spacing medial consonants, post-base vowels, or pre-base vowels that haven't been reordered yet. The documentation for the Universal Shaping Engine discusses a "technique" (really a hack) whereby such glyphs are labeled as marks, which then leads to other complications, in particular their width being set to 0.
There should be a way to ignore specified subsets of glyphs of any kind, or, inversely, to only look at glyphs within a specified subset of glyphs of any kind.
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