Policies

Start with the defaults in Subsetting. Change one option at a time and compare the output in the application that uses it.

• Glyph numbering defaults to GlyphIdPolicy::Preserve. Try Compact when you need smaller output and the font supports it.
• Hinting defaults to HintingPolicy::Keep. Try Drop if smaller output matters more than preserving small-size rendering.
• Text layout defaults to LayoutPolicy::Preserve. Remove data only for controlled content that does not need it.

All three examples retain 104 glyphs and preserve layout. Default output is 19,784 bytes with 518 slots; compact output is 8,308 bytes with 104 slots. Dropping hinting reduces this sample to 8,280 bytes. The small additional saving does not establish that removing hinting is appropriate for your renderer. The measurements use the bundled Inter fixture.

Compact glyph numbers

Run this as a separate script beside vendor, with a source font and an existing output directory. The destination must be new.

<?php

require __DIR__.'/vendor/autoload.php';

use Alto\Font\Font;
use Alto\Font\Subset\GlyphIdPolicy;
use Alto\Font\Subset\SubsetOptions;
use Alto\Font\Subset\UnicodeSet;
use Alto\Font\Writer\WoffWriter;

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

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

printf("Saved inter-compact.woff (%d bytes)\n", filesize($destination));

Compact mode closes gaps in glyph numbering and rewrites the related data. Compare its file size with a default subset written in the same format. It can reject fonts containing structures that cannot be safely rewritten; see the support reference.

Keep the default if another part of your application stores original glyph identifiers. Compact mode also removes glyph names, and one renderer is known to depend on them: CoreText renders Recursive differently without its glyph names and extra glyphs, as the technical diagnosis records. Matching outlines and positioning do not guarantee matching pixels on every renderer; compare small text as well as large text before shipping.

Remove hinting

To test this option, replace the SubsetOptions call in your script with:

<?php

use Alto\Font\Subset\HintingPolicy;

$result = $font->subset(new SubsetOptions(
    $characters,
    hinting: HintingPolicy::Drop,
));

Removing hinting can reduce the file but may change sharpness or spacing on the target renderer. Keep it unless you have checked the sizes and devices your application uses. Hinting explains what the instructions do and which tables Drop removes.

Remove text layout data

Keep LayoutPolicy::Preserve for ordinary text. Layout data controls ligatures, joining, kerning, and mark placement; Layout explains the tables involved.

LayoutPolicy::SubstitutionsOnly keeps supported substitutions but removes OpenType positioning. LayoutPolicy::Drop removes both. These options can break prose or scripts that depend on shaping; reserve them for controlled content such as isolated symbols. Legacy kerning is retained independently, so Drop does not mean every kerning adjustment is removed.

Subset a variable font

Subset the original variable font. Supported axes remain available in the output. Exporting a fixed weight or restricting axis ranges is unsupported.

If you obtained a selected view from withVariations() or a weighted finder query, call withoutVariations() before subsetting. This returns to the full variable source; it does not keep only the selected weight.

Always review SubsetResult::$warnings. For precise table and offset limits, see Tables and Variable fonts.

To compare output sizes before shipping, see Sizes; Glyph counts explains why a subset keeps more glyphs than characters.