Create a font subset

A subset keeps the characters required by an application and the glyphs needed to display them. Start with the conservative defaults before enabling compact glyph IDs or removing layout data.

Create a subset from text

<?php

use Alto\Font\Font;
use Alto\Font\Subset\SubsetOptions;
use Alto\Font\Subset\UnicodeSet;

$font = Font::fromFile(__DIR__.'/fonts/Inter-Regular.ttf');
$characters = UnicodeSet::fromText('ALTO Font 0123456789');
$result = $font->subset(new SubsetOptions($characters));

The default options preserve glyph IDs, hinting, and OpenType layout tables. Unused glyph slots remain present but empty. This is the safest starting point for an unfamiliar font.

Inspect the result

subset() returns a SubsetResult rather than only the transformed font:

<?php

printf(
    "%d codepoints mapped to %d retained glyphs\n",
    $result->mappedCodepointCount,
    $result->retainedGlyphCount,
);

foreach ($result->warnings as $warning) {
    fwrite(STDERR, $warning."\n");
}

Property	Meaning
`font`	The immutable subset font passed to a writer
`requestedUnicodes`	The normalized set requested by the caller
`mappedCodepointCount`	Requested codepoints found in the source cmap
`originalGlyphCount`	Glyph count before subsetting
`retainedGlyphCount`	`.notdef`, mapped glyphs, compound components, and layout dependencies retained
`sfntSize`	Materialized SFNT size, not the final WOFF or WOFF2 size
`warnings`	Explicit transformations or losses that the caller should record

With preserved glyph IDs, font->face()->glyphCount still includes empty slots. It can therefore be greater than retainedGlyphCount.

Write the subset

The result is an ordinary Font and can be passed to any writer:

<?php

use Alto\Font\Writer\WoffWriter;

new WoffWriter()->write(
    $result->font,
    __DIR__.'/output/inter-subset.woff',
);

Read Conversion for output behavior and Compression when the target is WOFF or WOFF2.

Choose a different character source

Use UnicodeSet::fromText() for application strings and translation corpora. Use CSS ranges, explicit codepoints, or composed ranges when the selection is defined independently from text. See Build Unicode sets.

Optimize further

Compact glyph IDs, hint removal, and layout removal can reduce output size but change more font data. See Choose subset policies before enabling them.

Subsetting currently supports TrueType glyf outlines. CFF, color, bitmap, and unmodeled glyph-indexed structures fail explicitly instead of producing a partially valid font. Supported variable and layout data follow the selected policies.