[Mini RFC] TextLayer support for pre-packed font atlas

[Pessimistress]

Target Use Case

Currently, the TextLayer only supports packing font atlas locally on the client side. Although the process is usually pretty fast (~6ms for the default ascii character set, ~60ms if sdf:true), there are also some limitations:

• Auto packing relies on CSS font settings, which may silently fail if a web font is not yet loaded or fails to load. Additional steps are required to make it work in the documentation
• We have observed significant platform-dependent variation when it comes to text rendering, including the handling of baseline position, antialiasing, etc. As of now, TextLayer render tests are disabled because we are unable to get it to work consistently across dev environments.

Proposal

Allow TextLayer to use a pre-packed font atlas.

• Link to a tool in the documentation to generate and save a font atlas: prototype. A pre-packed font atlas is a JSON consisting:
  • source: the packed image as base64-encoded PNG
  • mapping: the character definitions
  • version: the deck.gl version used to generate the file
  • settings: the font settings used to generate the file
  • attribution: optional font licensing information
• Add a new prop fontAtlas to TextLayer which points to a statically hosted font atlas. If this prop is supplied, the FontAtlasManager will skip packing and use information from the file instead.

[ibgreen]

Some thoughts:

• default fonts - it is great that fonts can be generated in advance by users, however it is a somewhat messy step to integrate into an application build workflow. Can we find some fonts whose atlases we can host without getting into license troubles)
• separate font module (@deck.gl/fonts) or (@luma.gl/fonts) - the fonts functionality getting quite extensive (and overwhelming for someone just looking at the text layer). I feel that the font generation is a great candidate for breaking into a separate module, with a small clean general purpose interface. I think it could even go into luma.gl as it would not need to have any deck dependencies, but even in deck.gl it would make sense to be able to work this functionality separately.

Also, I suspect that there is extensive prior art, perhaps some projects have formats that could be supported?

• https://github.com/ShoYamanishi/SDFont
• I remember that the original SDF text layer author was looking into using IBM fonts to pre-generate SDFs.