fastframe-text

Follow the desktop’s font rendering settings in egui.

egui’s defaults match no desktop exactly. Linux desktops ask for slight hinting with sub-pixel positions and draw coverage linearly (cairo, FreeType); egui’s dark theme thickens glyphs and nothing tells it what the desktop wants. A hard-coded “full grid fit on whole pixels” looks too sharp, with uneven kerning. This crate reads what the desktop asks for and applies it to every font egui draws.

Usage

let rendering = fastframe_text::detect();

let mut fonts = egui::FontDefinitions::default();
// ... insert the app's own fonts ...
rendering.apply_to(&mut fonts);
ctx.set_fonts(fonts);

// After the app has set its own visuals (replacing them resets this),
// for both themes:
ctx.all_styles_mut(|style| rendering.apply_to_visuals(&mut style.visuals));

detect() blocks briefly on Linux (a D-Bus call, then fc-match if the portal has no answer), so call it once at startup, outside any tokio runtime.

With the watch feature, follow changes while the app runs:

let current = fastframe_text::detect();
let ctx = ctx.clone();
fastframe_text::watch::watch(current, move |rendering| {
    // Hand `rendering` to the interface thread, rebuild the fonts with
    // `rendering.apply_to(..)`, call `ctx.set_fonts`, reapply the visuals,
    // and repaint.
    ctx.request_repaint();
})?;

Whole pixels

Text placed by hand should start on a whole physical pixel, or egui resamples the galley and it looks soft. At 133% a point is 1.33 pixels, so whole points are often not whole pixels:

let x = fastframe_text::snap_to_pixels(rect.center().x - galley.size().x / 2.0, ctx.pixels_per_point());

For Pos2, Vec2 and Rect, egui::emath::GuiRounding::round_to_pixels does the same.

The model

pub struct TextRendering {
    pub hinting: Hinting, // None | Slight | Medium | Full
    pub antialias: bool,
    pub subpixel_positioning: bool,
    pub coverage: Coverage, // Linear | ThemeDefault
}

TextRendering::default() is what Linux desktops render: slight hinting, antialiased, sub-pixel positions, linear coverage.

Mapping to egui

Per font (FontTweak, through apply_to):

Per theme (Visuals::text_options, through apply_to_visuals):

apply_to touches only the three hinting fields, so each font’s scale, offsets and variation coordinates stay as the app set them.

Why this mapping

Measured on Hyprland at 133% and 160%, pixel for pixel against pango and cairo with hintslight, grayscale antialiasing and sub-pixel positions:

• Slight is egui’s default target. With linear metrics preserved, the autohinter only snaps vertically, which is what slight hinting is. For fonts without TrueType instructions (Inter has no fpgm, prep or cvt), light: true renders identically, so the crate keeps egui’s tested default.
• Sub-pixel positions stay on. Placing glyphs on whole pixels, with a full grid fit, measured two to four times the spacing error: the uneven kerning that made that setting look wrong. Medium and Full still grid-fit horizontally (preserve_linear_metrics: false) for those who ask for it: sharper stems, less even spacing.
• Coverage is linear on Linux. egui’s dark theme maps coverage through 2c - c², which drew text 11 to 18% heavier than GTK. Linear coverage (Off, egui’s light-theme value) matched GTK’s ink weight within 2%. macOS keeps egui’s per-theme choice, since CoreText renders heavier than FreeType; Windows keeps it too until someone measures DirectWrite.

The sub-pixel (LCD) order a desktop may request is ignored: egui renders grayscale coverage only, so rgba antialiasing counts as grayscale.

Where settings come from

Each Linux source answers only if it holds a known value; a source that is missing (no session bus, no portal, no fc-match) is skipped. Portal calls time out after a second. Coverage is not a desktop setting: it comes from the platform. The portal does not describe sub-pixel positioning, so it stays on, as in GTK 4. Only the portal is watched for changes; edits to fontconfig files are not.

fontconfig is read through fc-match rather than a binding because no fontconfig crate is common to the apps’ dependency trees and linking the C library would add a build dependency.

egui version

The crate depends on egui 0.36.1 or later from crates.io, without default features. The per-font hinting target and sub-pixel binning (emilk/egui#8262) and the font colour transfer function are in the stock 0.36.1 release, so no fork is required; apps that patch egui to crmne/egui apps-0.36 work too.