On Jun 2, 2013, at 11:20 PM, "David Webber" <
[hidden email]> wrote:
> As I understand it, 'font features' or a 'glyph index' are features of the OpenType implementation, not of a font itself.
Glyph index is fundamental to any TrueType-based font implementation (thus also OpenType). It's the number by which the font engine, and the font itself, recognizes which glyph is which in every glyph-related operation.
This is different to a code point (a character), which is a text encoding thing. Any particular code point is mapped to a particular glyph index in a font. For example, in Unicode encoding, a code point U+0020, the space character, is normally mapped to a glyph representing space, but that glyph can be identified within the font with any index the font developer wants. This mapping, and there can be several of these for different text encodings for any given font, are all contained in the font.
This exact code point (character) versus glyph distinction makes it possible to have more glyphs in the font than any particular text encoding requires. I believe the API you're using is able to pick up any glyph from a font solely by its index, whenever it's mapped to a code point (character) or not. Still, this is pretty low-level. Font features are a higher level mechanism for picking up and shuffling around such unencoded, that is, not mapped to any code point, glyphs in the text stream. But yes, these are an addition to bare TrueType fonts, although they're supported on most, if not all, modern platforms. Especially on Microsoft's platforms, as it's the one who has developed, in collaboration with Adobe, the whole technology more than a decade ago.
I hope this clarifies the technical side of the discussion. Now, I'm not a music scholar, so I would better leave the different clef semantics problem for others to solve.
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