bevy_terminal

bevy_terminal renders terminal-style character grids with Bevy's own text shaping, font fallback, glyph atlases, render device, and render world. Its only normal dependency is bevy. It knows nothing about any particular terminal UI library: producers write a renderer-neutral scene of cells, styles, colors and cursor state into a TerminalSurface, and the renderer turns that scene into a renderer-owned texture.

The Ratatui backend lives in the companion crate bevy_terminal_ratatui, which depends on this crate and on ratatui:

bevy_terminal ──> bevy
bevy_terminal_ratatui ──> bevy_terminal
                         └> ratatui

Scene model

• GridSize and CellPosition: small integer terminal coordinates.
• TerminalCell: a CellSymbol grapheme (opaque; 24 bytes, no allocation up to 22 UTF-8 bytes), a TerminalStyle, and a read-only CellOccupancy set by TerminalCell::new, wide or continuation_of.
• CellOccupancy::{Single, Wide { columns }, Continuation}: an ordinary glyph, the anchor of a glyph spanning several columns, or a column covered by the anchor to its left. Wide glyphs are always drawn inside their declared span and never overwrite the following cell.
• TerminalStyle: foreground, background and underline TerminalColors plus a StyleFlags bit set for bold, dim, italic, underline, slow/rapid blink, reverse, hidden and crossed-out.
• TerminalColor::{Default, Indexed(u8), Rgb(u8, u8, u8)}. Default resolves contextually against the TerminalTheme: theme foreground for text, theme background for backgrounds, and the resolved foreground for underlines.
• TerminalSnapshot: an owned copy of the grid, cursor and revision.

Writing a scene

use bevy::prelude::*;
use bevy_terminal::prelude::*;

let surface = TerminalSurface::new((40, 10));
// One lock, many cells, at most one published revision.
surface.update(|update| {
    let title = TerminalStyle::new()
        .fg(TerminalColor::BLACK)
        .bg(TerminalColor::LIGHT_CYAN)
        .with(StyleFlags::BOLD);
    for (column, symbol) in "hello".chars().enumerate() {
        update.set_cell((column as u16, 0), &TerminalCell::from(symbol).with_style(title));
    }
    update.set_cell((0, 2), &TerminalCell::wide("界", 2)); // writes its continuation cell too
    update.set_cell((3, 2), &TerminalCell::new("┌"));
    update.set_cursor_position((5, 0));
    update.set_cursor_visible(true);
});

App::new()
    .add_plugins((DefaultPlugins, TerminalPlugin))
    .add_systems(Startup, move |mut commands: Commands| {
        commands.spawn(Camera2d);
        commands.spawn((
            Terminal::new(surface.clone()),
            ImageNode::default(),
            Node { position_type: PositionType::Absolute, left: px(16.0), top: px(16.0), ..default() },
        ));
    })
    .run();

TerminalSurface::update is the only way to write: it takes the lock once, applies the closure and publishes at most one revision (none if nothing changed), returning whether it did. Positions are anything Into<CellPosition> — (x, y) tuples or CellPosition; sizes anything Into<GridSize>. SurfaceUpdate offers set_cell, set_cursor_position, set_cursor_visible, clear, clear_row, clear_range (row-major, inclusive), resize, scroll_up and scroll_down. Every mutation compares against the retained cell and marks only real changes dirty, so redrawing identical content publishes nothing and the renderer's unchanged-frame fast path stays intact.

Rendering

Add TerminalPlugin once, then spawn a Terminal component per terminal together with a TerminalRenderConfig (required; a default is inserted if you omit it) and, to show it, an ImageNode:

# use bevy::prelude::*;
# use bevy_terminal::prelude::*;
# let surface = TerminalSurface::new((80, 24));
# let mut app = App::new();
app.add_plugins(TerminalPlugin);
app.world_mut().spawn((
    Terminal::new(surface),
    TerminalRenderConfig {
        cell_size: Vec2::new(11.0, 20.0).into(),
        font: FontFaces::regular(bevy::text::FontSource::Monospace),
        font_size: FontSizing::FitCellWidth,
        ..default()
    },
    ImageNode::default(),
    Node { position_type: PositionType::Absolute, left: px(16.0), top: px(16.0), ..default() },
));

The plugin attaches to every Terminal entity:

• TerminalTexture — the renderer-owned Handle<Image> (stable for the terminal's lifetime; resizes reallocate the image in place), its physical size, logical_size, raster_scale and the effective font_size. TerminalReady { entity } is triggered on the entity once the texture exists.
• TerminalStats — per-frame counters (changed rows, snapshot cells, quads, draw batches, shape-cache misses, timings; Display prints a one-line summary).

If the entity has an ImageNode, the plugin keeps its image and its Node width/height in sync with the texture; where the node goes (absolute position, a flex/grid parent, …) is ordinary Bevy UI layout. Without an ImageNode the terminal is headless — only the texture is produced, for custom composition, image export or benchmarks — and TerminalRenderScale::Automatic resolves to 1.0.

Terminals can be spawned and despawned at any time; the images are released with the entity. Mutating TerminalRenderConfig rebuilds only that terminal. Terminals share the GPU pipeline and scratch buffers but keep independent surfaces, configurations, textures and statistics.

TerminalRenderConfig holds the explicit cell_size, the FontFaces (regular plus optional bold/italic/bold-italic; missing faces fall back bold-italic → bold → italic → regular, and synthesize decides whether the fallback face is asked for the bold weight / italic style), the FontSizing (FitCellWidth by default: the regular font's advance is measured and the font sized so one advance equals the cell width; Px(size) uses an explicit size), the TerminalTheme, CursorConfig { style, color, blink_hz }, BlinkConfig { slow_hz, rapid_hz } and RasterConfig { scale, hinting } (scale: TerminalRenderScale::Automatic follows the primary window when presented and resolves to 1.0 headless, Fixed(scale) rasterizes at a known scale; hinting is FontHinting::Disabled by default so measured metrics stay exact).

The renderer reads the surface once per changed frame, copies only dirty cells into its retained snapshot, and rebuilds only changed rows. Every glyph, including box-drawing, block, shade and braille characters, is shaped and rasterized from the configured font by Bevy text, cached in one renderer-owned atlas and clipped to its cell; nothing is drawn procedurally.

Resizing a surface (SurfaceUpdate::resize) preserves the overlapping cells. TerminalSurface::pixel_size reports the logical pixel size the renderer derived from its configuration (once one exists), so producers can expose window metrics without knowing the renderer's internals.

Cell sizing, texture format, features

• CellSizing::Logical(Vec2) (default) fixes the cell and, with FontSizing::FitCellWidth, sizes the font to it. CellSizing::FromFont { line_height } (CellSizing::FROM_FONT = 1.2) derives the cell from a FontSizing::Px size: width = the regular font's measured advance, height = the larger of size × line height and the font's measured line box (the rows a full block covers). TerminalTexture::cell_size reports the logical cell in use, TerminalTexture::grid_for / render::grid_for / render::grid_for_window compute the grid that fits a size, and render::raster_scale_for_window the physical/logical ratio for TerminalRenderScale::Fixed.
• Vertical fit: the renderer measures the primary font's line box (its full block's fully covered rows) and the ink of ASCII and accented-capital probes in every face, then shifts all glyphs by one whole-pixel offset so blocks keep covering the cell, ASCII (descenders included) is never clipped and accents fit when there is room. With FitCellWidth/FromFont the cell height grows to the line box; with FontSizing::Px in a Logical cell the configured sizes are exact and glyphs beyond the cell are clipped. Runs wider than their cell (fallback families, wide italics) drop their faintest columns; overhanging runs are pushed inside. The font is sized to the physical cell width so fractional raster scales do not open seams.
• The texture is Rgba8UnormSrgb with straight alpha; TerminalTheme::background alpha is honored (backgrounds replace, glyphs blend), so translucent terminals composite correctly.
• Cargo features: ui (default) enables ImageNode presentation and following the UI scale — without it terminals are headless and Automatic scale is 1.0; system_fonts (default) enables system font discovery — without it supply font assets (or FontSource::Handle(Handle::default()) with bevy/default_font). Font fallback for missing glyphs is Bevy's system-wide fallback; there is no per-terminal fallback list in Bevy 0.19.

Examples

cargo run -p bevy_terminal --example scene
cargo run -p bevy_terminal --example scene_export

scene renders two independent surfaces (styles, box drawing, blocks, a wide glyph, colors and a moving cursor) written directly through the surface API, using the repository's Iosevka Fixed faces when run from a checkout and the vendored JetBrains Mono Regular/Bold/Italic/BoldItalic faces under assets/fonts/jetbrains-mono (OFL) otherwise. scene_export writes the same textures headlessly under target/bevy-terminal-qa/.

License

Licensed under the MIT license.