Subsetting

A document that embeds a font, such as a PDF file, only needs the glyphs it draws. FontSubsetter makes a font program with these glyphs only:

static byte[] Subset(FontFace face, string text)
{
    // The glyphs the document uses; the closure adds the glyph 0 and the components of composite glyphs
    var glyphs = new HashSet<int>();
    foreach (var rune in text.EnumerateRunes())
    {
        glyphs.Add(face.MapCodePoint(rune.Value));
    }

    // A font program to embed in a document that refers to the glyphs by their identifiers, such as a PDF file:
    // the identifiers are kept, the other glyphs are empty, and there is no character map
    return face.Outlines is FontOutlineKind.TrueType
        ? FontSubsetter.SubsetTrueType(face, glyphs, keepHinting: false)
        : throw new NotSupportedException("SubsetCff gives the CFF table of a font with CFF outlines");
}

• The glyph identifiers are kept: the glyphs that are not kept are empty, so that what refers to a glyph by its identifier, such as a PDF content stream or a shaped text, is unchanged.
• GetClosure gives the glyphs a subset keeps: the requested glyphs, the glyph 0 (.notdef) and the components of the composite glyphs. Identifiers that are not glyphs of the face are ignored.

The result is a font program to embed, not a font to install: it has no character map and no layout tables. A method called with a face whose outlines are of the other kind throws an ArgumentException.