Shaping Ligatures in Monospace Fonts

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Style Pass

2025-01-09 22:30:02

For some time I’ve been working towards building a graphical code editor from scratch. I’m still in the explorative phase of this project which involves creating many small, conceptual pieces to better understand the various problem spaces. The problem space I’m working through at the moment is text rendering, with the current focus being shaping.

Shaping is the process of converting text (UTF-8 code points in our case) to a sequence of glyphs with positional information to be rendered. It can get very complex and computationally expensive. For more info take a look at the HarfBuzz Manual.

You might think shaping for a code editor with a monospace font should be trivial (hint: nothing with text rendering is trivial). The text is almost all English with glyphs from the Basic Latin Unicode block. That means there are rarely diacritics or other complex font structures. So we should be able to match every code point to a single glyph, and each glyph has the same width as we’re working with a monospace font.

With these simplifications you can go a long way with text rendering for a basic code editor, that is, until you want to handle emojis (which also relies on a font fallback subsystem) or ligatures.