"Complex" glyph substitution

At the TPAC WFWG meeting we discussed one difficult aspect of the glyph-keyed patch encoding problem: glyphs with more “complex” substitution patterns that the encoder punts on and just includes in the initial font. I mentioned that this was a significant problem for certain fonts when I was working on the IFTB prototype, which raised the question of whether I remembered what any of those fonts were.

Short answer: I don’t.

However, I was looking through some old slides and they do mention a pattern, which was that big aalt and nalt features tended to cause problems. I also recall issues with vertical layout in Japanese. So these are the things I recommend looking at first.

More generally, I think it would not take too long to build a simple evaluator using the tree built with my draft depend branch of HarfBuzz: https://github.com/skef/harfbuzz/tree/depend . Maybe that’s not much different from just encoding each of the relevant fonts and seeing how large the list of punts is, but evaluation code could be more specific about what is causing the problems without having to go in and look by hand.

I’m also probably in a position where I can run any or all of the fonts in Adobe’s library through an encoder, and while I obviously wouldn’t be able to share the actual fonts freely, I could characterize the issue and in some cases check with the foundry about providing limited access for research purposes.

Skef