Khaled HosnyMar 7

RE: https://typo.social/@fontra/116183016588316849

The blog post is a very interesting write-up, I suggest everyone interested in the font making to read it.

The technical part at the end is the most interesting to me. I’m particular proud of the very neat trick that allows #Fontra to emulate glyph positioning while maintaining interaction between it and manually written feature code.

For context. Font editors will generate positioning features from kerning and anchor data in the font.

Sometimes one wants to add some other feature code before or after the generated ones (e.g. move a dot contextual after being positioned by the default anchor to avoid a clash that happens only in a certain context). The order of the generated and manual code is important, since one might depend on the output of the other.

Show threadKhaled Hosny

However, #Fontra does not generate positioning feature code during live preview, as it does the glyph positioning itself, while the rest of the shaping is done by #HarfBuzz (as explained in the blogpost).

So how would it do this emulation in the middle of lookup application by HarfBuzz?

HarfBuzz has a so-called message callback that allows caller to get a message about what HarfBuzz is doing and the HarfBuzz glyph buffer before each shaping step (this is what https://www.corvelsoftware.co.uk/crowbar/ uses to debug OpenType layout).

Crowbar

What did my font do?

Mar 7 at 2:26pmWeb
Show threadKhaled HosnyMar 7

Here comes the neat trick; #Fontra registers a message callback function, which gets called before and after each lookup is applied, and whenever #HarfBuzz is about to apply a lookup that the generated code would have been inserted before, Fontra steps in and modifies the glyph buffer and apply its own positioning logic. Then later lookups will be applied by HarfBuzz and modify the glyph positions as it would have happened with a fully compiled font.

OK, it is a super nerdy and maybe a very few people can get why I’m so excited about it, but I’m really proud about it.

Show threadJust van RossumMar 7
@khaled I’m so grateful for this collaboration, and I’m glad to see you are as excited as I am!
Show threadKhaled HosnyMar 7
@justvanrossum It is always pleasure working with you