Use Bezier curves to improve quality

[ianthetechie]

Background:

There are two approaches to SDF glyph generation in wide use today.

1. Rasterize the glyphs directly (ex: using FreeType to get an alpha map) and compute the SDF from the bitmap. This is the approach taken by TinySDF: https://github.com/mapbox/tiny-sdf.
2. Compute the SDF using the bezier curves from the font, as in https://github.com/mapbox/sdf-glyph-foundry/blob/master/include/mapbox/glyph_foundry_impl.hpp (used by many projects including the old node frontend and the more modern https://github.com/maplibre/font-maker).

We currently use the first method as it is fast and easy to implement. However, there is a solid case for supporting the second method, as this would improve the rendering quality of Indic, Khmer, Burmese, etc.

[ianthetechie]

I took a deeper look into what it would take to accomplish this today. It's not going to be a trivial implementation, as the Mapbox implementation makes use of FT_Outline_Decompose.

The docs take a minute to grok, but basically it's walking over the outline struct with a bunch of function pointers and an inout that acts as global state. So it should now be obvious why freetype-rs doesn't have a wrapper for this 😂

This actually doesn't look too absurd to implement ourselves after reading over the FT_Outline docs, but it's going to take a fair bit of work. Not exactly something I could knock out in an evening reliably, but probably wouldn't take an entire week either.

It's still not a super high priority for me at the moment, but I'd welcome contributions to this effort by anyone interested.

[ianthetechie]

Huh.... Thanks to @louwers for sharing this link in the MapLibre Slack chat: https://developer.chrome.com/blog/memory-safety-fonts.

It looks like they have a pure Rust implementation of approximately the same API as FreeType when it comes to decomposition of an outline: https://docs.rs/skrifa/latest/skrifa/outline/index.html. If I'm understanding this correctly (terminology is confusing on the 3rd and 4th order bezier stuff), we now have a significantly more straightforward path to porting the SDF Glyph Foundry approach to font rendering, which would improve the quality for lower DPI screens.

[ianthetechie]

Another possibility: https://github.com/harfbuzz/ttf-parser