Crate subsetter

Reduces the size and coverage of OpenType fonts with TrueType or CFF outlines for embedding in PDFs. You can in general expect very good results in terms of font size, as most of the things that can be subsetted are also subsetted.

Scope

Note that the resulting font subsets will most likely be unusable in any other contexts than PDF writing, since a lot of information will be removed from the font which is not necessary in PDFs, but is necessary in other contexts. This is on purpose, and for now, there are no plans to expand the scope of this crate to become a general purpose subsetter, as this is a massive undertaking and will make the already complex codebase even more complex.

In the future, klippa will hopefully fill the gap of a general-purpose subsetter in the Rust ecosystem.

Notes

A couple of important notes if you want to use this crate in combination with your own pdf writer:

• You must write your fonts as a CID font. This is because we remove the cmap table from the font, so you must provide your own cmap table in the PDF.
• Copyright information in the font will be retained.
• When writing a CID font in PDF, CIDs must be used to address glyphs. This can be pretty tricky, because the meaning of CID depends on the type of font you are embedding (see the PDF specification for more information). The subsetter will convert SID-keyed fonts to CID-keyed ones and an identity mapping from GID to CID for all fonts, regardless of the previous mapping. Because of this, you can always use the remapped GID as the CID for a glyph, and do not need to worry about the type of font you are embedding.

Example

In the example below, we remove all glyphs except the ones with IDs 68, 69, 70 from Noto Sans. Those correspond to the letters ‘a’, ‘b’ and ‘c’. We then save the resulting font to disk.

use subsetter::{subset, GlyphRemapper};

// Read the raw font data.
let data = std::fs::read("fonts/NotoSans-Regular.ttf")?;

// These are the glyphs we want to keep.
let glyphs = &[68, 69, 70];

// Create a new glyph remapper that remaps our glyphs to new glyph IDs. This is necessary because
// glyph IDs in fonts must be consecutive. So if we only include the glyphs 68, 69, 70 and remove all
// other glyph IDs, in the new font they will have the glyph IDs 1, 2, 3.
let mut remapper = GlyphRemapper::new();
for glyph in glyphs {
    remapper.remap(*glyph);
}

// Create the subset.
let sub = subset(&data, 0, &remapper)?;

// This is how you can access the new glyph ID of a glyph in the old font.
for glyph in glyphs {
    println!("Glyph {} has the ID {} in the new font", *glyph, remapper.get(*glyph).unwrap());
}

// Write the resulting file.
std::fs::write("target/Noto-Small.ttf", sub)?;

In the above example, the original font was 556 KB, while the resulting font is 1 KB.

Structs

• GlyphRemapper
  A remapper that allows to assign a new ordering to a subset of glyphs.
• Tag
  A 4-byte OpenType tag.

Enums

• Error
  Parsing failed because the font face is malformed.

Functions

• subset
  Subset the font face to include only the necessary glyphs and tables.