fontmerge

fontmerge merges two font source files together, copying selected glyphs from a donor font into a host font, along with any necessary OpenType layout features. It is a font-merging toolkit that powers the fontmerge command-line tool.

Typical use cases include:

• Adding a set of glyphs (e.g. a Latin or Cyrillic extension) to an existing font.
• Copying glyphs selected by name or by Unicode codepoint from one source into another.
• Transporting OpenType layout rules (features, lookups, mark classes) alongside the glyphs they reference.

Merging is performed on font sources (Glyphs, UFO/DesignSpace, Babelfont JSON, ...) via the babelfont crate rather than on compiled binaries, so you keep full editability after the merge.

Command-line usage

Install the CLI with:

cargo install fontmerge

Merge all of Donor.glyphs into Host.glyphs:

fontmerge Host.glyphs Donor.glyphs --output Merged.glyphs

Copy only specific glyphs by name:

fontmerge Host.glyphs Donor.glyphs --output Merged.glyphs --glyphs "A B C D"

Copy glyphs by Unicode codepoint (comma-separated, U+ prefix optional, ranges with a hyphen allowed):

fontmerge Host.glyphs Donor.glyphs --output Merged.glyphs --codepoints "U+0041-U+005A"

Exclude glyphs you don't want to bring across:

fontmerge Host.glyphs Donor.glyphs --output Merged.glyphs --glyphs "A B C" --exclude-glyphs "Aacute B"

By default, glyphs already present in the host font are skipped; use --existing-handling replace to overwrite them instead. Layout rules can be subset to the merged glyph set (default), closed over to pull in any extra glyphs referenced by rules (--layout-handling closure), or ignored entirely (--layout-handling ignore).

Using it as a library

The fontmerge crate is a mixed binary/library crate. Its library exposes the underlying merging routine so you can drive merges programmatically:

use babelfont::{load, SmolStr};
use fontmerge::{fontmerge, ExistingGlyphHandling, GlyphsetFilter, LayoutHandling};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // "Host" font receives glyphs; "donor" font contributes them.
    let mut host = load("Host.glyphs")?;
    let donor = load("Donor.glyphs")?;

    // Select which glyphs to copy from the donor font.
    let selection = GlyphsetFilter::new(
        vec![SmolStr::new("A"), SmolStr::new("B")],
        vec![],
        vec![],
        &mut host,
        &donor,
        ExistingGlyphHandling::Skip,
    );

    let merged = fontmerge(host, donor, selection, LayoutHandling::Subset, true)?;
    merged.save("Merged.glyphs")?;
    Ok(())
}

See the API documentation for details.

Status

fontmerge is in early development and its command-line interface and library API may change between releases.

License

fontmerge is available under the MIT or Apache-2.0 licenses, at your option. See LICENSE-MIT and LICENSE-APACHE.