Use TinySDF to generate missing glyphs in the client

[wipfli]

Sometimes a font file does not contain a glyph. Say for example that you have a font which covers a-z and A-Z but nothing else. Then if you want to render Zürich, the default behavior of MapLibre GL JS is to show Zrich I think. So it just skips the missing ü. In such cases where a glyph is missing from the font, we could generate it dynamically in the client with TinySDF.

[wipfli]

What do you think? cc @1ec5 and @ZeLonewolf because I think you spent quite some time putting a font together which has maximal coverage for Americana...

[ZeLonewolf]

I don't think I'm smart enough on font issues yet to offer an opinion here.

[1ec5]

Your assumption is correct: osm-americana/openstreetmap-americana#850 has some examples of what happens when the glyph PBFs lack required glyphs.

From an end-user perspective, your proposal is good for basic usability, assuming that the text-field value isn’t garbage. From a developer perspective, I don’t see a backwards compatibility issue at a glance, but it deserves a deeper look. Font selection is already so complex that preserving compatibility with Mapbox software might be somewhat important.

The traditional fontstack model always seemed unreasonable to me, especially that the fontstack would get flattened on the server side into a single set of PBFs, requiring the style to specify a pan-Unicode, last-resort fallback font as part of the fontstack. A last-last-resort fallback on the client side would reduce the dependency on fontstacks, getting us closer to the point where we can ditch that concept and, along with it, the faulty assumptions in maplibre/maplibre-native#778 (reply in thread).

As a side benefit, if developers who roll their own glyph PBFs can assume a version of MapLibre that has the proposed behavior, then they can roll much smaller PBFs that contain only the characters from specialty fonts and none of the fallback characters from boring pan-Unicode fonts. But a bit of caution is warranted: whenever a combining character’s font mismatches the surrounding text’s font, text layout always gets wonky.