denise-text

Fonts, a bounded glyph cache, line layout and word wrapping for Denise, a direct-rendering UI toolkit in Rust for embedded Linux and systems without a desktop environment.

One TextEngine holds every font an application uses and one GlyphAtlas that caches what has been rasterised. Measurement and drawing both go through it, so a label measured during layout and drawn a moment later rasterises its glyphs exactly once.

use denise::{Color, Point};
use denise_render::Pen;
use denise_text::{TextEngine, TextStyle};

# fn demo(canvas: &mut Pen<'_>) {
let mut text = TextEngine::new();
let style = TextStyle::built_in(16);

let extent = text.measure(style, "Kjærlighet på Øy");
text.draw(canvas, style, Point::new(20, 20), "Kjærlighet på Øy", Color::WHITE);

// And the lines a paragraph becomes at a given width, borrowing the input.
let lines: Vec<&str> = text.wrap(style, "en to tre fire fem seks", 80);
# let _ = (extent, lines);
# }

Three tiers, and what each costs

Measured as the increase in a stripped, statically linked aarch64-unknown-linux-musl binary:

A style that names no font carries FontId::DEFAULT, which is a redirection rather than a face. TextEngine::set_default_font points it at a registered source, and everything drawn from an unnamed style follows — so an application registers a face and says one more line, rather than threading a TextStyle through every widget it builds.

FontId::DEFAULT is the built-in bitmap until something says otherwise, which is what a board with no fonts installed gets and why the bitmap face is always registered first.

For scale: the whole of Denise, DRM, evdev and the widgets is about 840 KB, so the shaping tier is four times the rest of the toolkit put together. It is there because some panels genuinely need it, and off by default because most do not — a temperature readout and a Norwegian name do not need a shaper.

Wrapping

Greedy, returning slices that borrow the input, and explicit \n always breaks. A word wider than the line overflows on a line of its own rather than being cut between characters: breaking mid-word means knowing where a grapheme ends, and half of an æ is nothing.

Where this sits

Depends on denise and denise-render. denise-ui re-exports TextStyle and passes the tier features through, so an application picks its text tier once.

#![forbid(unsafe_code)].

Status

M5 complete, M6 in progress. The glyph cache is benchmarked in CI. Part of Denise — see the repository README and docs/design.md for the whole picture.

MIT licensed.