[HarfBuzz] Font fallback strategies

[Grigori Goronzy]

Hi,

I'm currently investigating how to do better font fallbacks for my
project. At the moment, fallback selection is nondeterministic (for a
given codepoint) and doesn't make much effort to select a single font
per same-script text run.

Are there any general recommendations for fallback selection and
splitting runs? The OpenType specification doesn't say anything about
this (or I didn't look hard enough).

Moreover, I wonder how to accurately determine what codepoints are
supported by a given font. Is it safe to assume a codepoint or a
sequences of codepoints as supported under all possible GSUB
substitutions if they appear in the cmap?

Best regards
Grigori