img2bez

img2bez traces raster glyph images into bézier outlines and writes them directly into UFO font sources.

Most autotracers reproduce a glyph's silhouette and leave the outline structure to a human. img2bez aims for the structure a font source needs: points at extrema with horizontal or vertical handles, straight segments as lines, points at inflections, smooth joins harmonized toward G2, and the minimum number of points that render the shape. Traces run in milliseconds on a CPU. Built on kurbo and norad from the Linebender ecosystem.

Read the blog post for the design rationale and an interactive in-browser demo.

Install

Not yet published to crates.io. Install from GitHub for now:

cargo install --git https://github.com/eliheuer/img2bez            # the CLI
cargo add --git https://github.com/eliheuer/img2bez img2bez        # as a library

Quick start

# Trace a glyph image into a UFO source (created if it doesn't exist)
img2bez --input glyph.png --output MyFont.ufo --name A --unicode 0041

# With font metrics and grid snapping
img2bez --input glyph.png --output MyFont.ufo --name A --unicode 0041 \
  --target-height 1024 --y-offset -256 --grid 2

img2bez --help is the authoritative flag list, including the masters (interpolation-compatible variable-font masters) and new-font subcommands. Key flags: --profile wild|clean|photo (source-class preset), --accuracy (fit tolerance, font units), --mode smooth|line (shape constraint), --format ufo|glif|json|svg.

Library usage

use std::path::Path;

use img2bez::{trace_glyph_path, FontMetrics, Profile, TraceOptions};

let opts = TraceOptions::for_profile(Profile::Clean)
    .with_em_height(1088.0)
    .with_grid(2);
let metrics = FontMetrics::from_target_height(1088.0, -256.0)
    .with_advance(600.0);

let glyph = trace_glyph_path(
    Path::new("glyph.png"), "A", &['A'], &opts, &metrics,
)?;

// The same Glyph model serializes to GLIF, JSON, or SVG.
std::fs::write("A.glif", glyph.to_glif())?;

# Ok::<(), Box<dyn std::error::Error>>(())

Designed to embed: trace_glyph takes image bytes and returns a Glyph with no filesystem access; output is deterministic (cache by input hash); options are per call; a lean build (--no-default-features --features ufo) drops the CLI and renderer dependencies. WASM bindings run the identical pipeline in the browser; mcp/ exposes it to AI agents as an MCP server.

How it works

 Input image (grayscale)
      |
 1. Sub-pixel boundary extraction — marching squares at the
    threshold iso-level; the anti-aliasing carries ~0.1px accuracy
 2. Structure detection — corners, straights, tangent points,
    extrema, inflections from the curvature signal
 3. Constrained cubic fitting — tangent directions fixed (exactly
    H/V at extrema), handle lengths solved, candidate structures
    scored against the source pixels
 4. Font cleanup — contour direction, grid snapping, H/V handle
    correction, G2 harmonization, corner reconstruction
      |
 UFO / GLIF / JSON / SVG

Optional learned models (tiny decision heads, a few KB each, plain Rust inference) can replace individual structural judgment calls. They only choose between structures the procedural pipeline proposes — all drawing stays procedural — and they are off by default (IMG2BEZ_SITE_HEAD=1, or siteHead in the wasm config). The demo has a live rules-vs-learned toggle.

Tracing quality

The eval harness traces clean renders of a reference font and scores the result against the original UFO outlines (point count and placement, line/curve structure, H/V handles). Across basic Latin (a–z, A–Z, 0–9): mean structural score 0.964, 12 glyphs exact, 62/62 pass. Try your own images in the demo.

A structure-compare sheet: the source font's outlines (left), the trace of a 1024px rendering of them (right) — same 27 points, same structure.

Cargo features

Development

The tracer is developed against a measurable loop: render a reference font, trace it back, score the structural match. The harness and the structure-compare QA tool (side-by-side sheets like the one above, for any UFO) live in eval-harness/ (repository only, not in the published crate). Known gaps: docs/known-problems.md.

Minimum supported Rust version (MSRV)

img2bez has been verified to compile with Rust 1.88 and later.

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be licensed as above, without any additional terms or conditions.