GSoC 2020 update, June 15

[Owen Lamb]

Hi all,

Apologies for the lateness of this update. Last week was mostly research
into stylistic alternates and how to support OpenType features in a SMuFL
font, so not much was done in the way of code changes aside from creating a
proof-of-concept stylistic alternate mapping.

It appears that the HarfBuzz library, already a LilyPond dependence thanks
to Pango, can read an OT font's table of features. I sent a help request to
the HarfBuzz mailing list, but it appears not to have gone through yet,
since I'm not a member of the list. Now that the weekend's over and my
message still hasn't appeared on the HarfBuzz archive, I've joined the list
and sent the message again.

Rather late in the week, I also came to the realization that, in order to
support *any* SMuFL font's optional features, I'll also have to read its
metadata JSON file if it has one, before defaulting to its OpenType
features if it doesn't. (Thanks, Werner, for guiding me through that
business!)

So, my game plan for this week is as follows: while I correspond with
HarfBuzz, I will generate a JSON file for Emmentaler. Then, I will have
open-type-font.cc try to load a font's JSON file, and if that fails its
GSUB table, into some sort of Scheme object (perhaps an alist?), which can
then be read (or written--who knows if a user might want to mess with font
metadata!) by LilyPond. I will probably need to create another
font-specific Scheme alist so that specific font features, such as
ligatures, alternates, or style sets, can be turned on or off at any point,
telling HarfBuzz (or whatever glyph-replacing method we go with) which of
them to use at any given moment.

(A SMuFL font's metadata file also includes settings such as staff line
thickness, etc. for drawing primitives in their correct proportions, so
I'll eventually need to make LilyPond respect those as well. But one step
at a time!)

Once again, if you have any worries/suggestions, please let me know. I plan
to send my next update on time, on Friday.

Thanks,
Owen