filmcraft/crates/text/README.md

filmcraft-text

FilmCraft's text engine (layer L1: depends on filmcraft-color, skrifa, harfrust, unicode-bidi and unicode-linebreak). It draws graphic-clip text and shapes, caption burn-in and the Timecode / Clip Name effects. Pure Rust; it builds for wasm32-unknown-unknown.

Module	What
fonts	Font database. Bundled faces (always present, also on the web): Inter Regular / Medium / SemiBold / Bold / Italic, JetBrains Mono Regular, Noto Serif Regular — all SIL OFL 1.1, unmodified, in assets/fonts/ with .attribution sidecars. More faces come from FontSources: DirectorySource::system() scans the platform font folders on first need (only the name and OS/2 tables are read, via sfnt); on wasm the scan finds nothing and hosts can call add_font_data. resolve(family, style) picks a face, synthesises bold/italic when the family lacks them, and falls back to Inter (flagged missing). fallback_for finds a face for characters the chosen face lacks: bundled faces first, then the optional craft-fonts faces (CRAFT_FONTS, embedded by build.rs when built with CRAFT_FONTS_DIR; Mincho for serif text, Gothic otherwise, nearest weight), then loaded system faces.
layout	Paragraph layout: per-character font fallback → bidi levels (UAX #9) → items (runs of one face, direction and size) shaped with harfrust (OpenType kerning, ligatures, marks, complex scripts) → tracking → line breaking at UAX #14 opportunities (area text) → visual reordering per line → alignment (left / centre / right / justify), leading (added to the natural line height), baseline shift, all caps / synthesised small caps (78 %), underline, strikethrough (OS/2 strikeout metrics; half the x-height when a font leaves them out). Every character boundary gets a caret stop (ligatures are split evenly), so editors can place carets, hit-test and draw selections. Layouts are cached by text + style (512 entries, LRU).
layout_rich	Per-character style runs (StyleRun: a byte range with its own TextStyle): runs split shaping items (font, size, tracking, caps, faux styles), baseline shift, underline and strikethrough apply per run, a line holding a larger run is pushed down from the line above (baseline distance = max(base line height, previous descent + this line's ascent + leading)), and every glyph / underline / strikethrough carries its style index so renderers can colour runs (render::draw_run).
raster	Our anti-aliased rasteriser (non-zero winding, 5 sub-scanlines, exact horizontal coverage, active-edge list), contour paths with transforms, rounded rectangles, ellipses, polygons and outline emboldening. Coverage is linear, so compositing in linear light is correct.
render	Glyph outlines (cached per face + glyph) and glyph masks (cached per size in ⅛ px, ¼-px horizontal phase, synthetic style). Axis-aligned uniform transforms use the mask cache with sub-pixel x positioning; rotated / skewed / non-uniform transforms fill the transformed outlines of the whole run in one pass. Synthetic bold grows outlines by 2.5 % of the size; synthetic italic slants by 12°.
mask	Outer / centre / inner strokes from an exact Euclidean distance transform (Felzenszwalb–Huttenlocher) with sub-pixel refinement, a three-pass box blur (≈ Gaussian) for shadows, offsets.

Coordinates

Pixels, y down. Point text: the origin is the alignment point on the first baseline (left edge, centre or right edge). Area text (ParagraphStyle::width): the origin is the box's top-left.

Performance

A three-line, centred, 96 px bold title at 1080p (perf::three_line_title_is_fast), release build on an M-series Mac: about 1.6 ms cold (shaping, outline extraction, glyph rasterisation) and 0.15 ms warm (layout cache + glyph cache). The test fails if the warm path exceeds 40 ms.

Not yet

OpenType feature toggles beyond kerning/ligatures, variable-font axes, hinting, colour (emoji) glyphs, and hyphenation. Words longer than an area-text line overflow instead of breaking mid-word.