Choose subset policies

SubsetOptions controls three independent decisions. The defaults preserve the source font's structure and are recommended for the first result.

Decision	Default	Advanced choices	Main trade-off
Glyph IDs	`GlyphIdPolicy::Preserve`	`Compact`	Smaller glyph space, but more tables must be rewritten
Hinting	`HintingPolicy::Keep`	`Drop`	Smaller output, but low-resolution rendering may change
Layout	`LayoutPolicy::Preserve`	`SubstitutionsOnly`, `Drop`	Smaller output, but positioning, ligatures, or shaping may change

Preserve the source behavior

<?php

$result = $font->subset(new SubsetOptions($characters));

Preserve mode keeps original glyph IDs and leaves unused slots empty. Compound components and GSUB dependencies are retained. GPOS and GDEF remain valid because glyph IDs do not change.

Compact glyph IDs

<?php

use Alto\Font\Subset\GlyphIdPolicy;

$result = $font->subset(new SubsetOptions(
    $characters,
    glyphIds: GlyphIdPolicy::Compact,
));

Compact mode renumbers retained glyphs and removes PostScript glyph names by writing post format 3 when required. It rewrites supported cmap, metrics, compound, GSUB, GPOS, GDEF, gvar, and HVAR structures. Any glyph-indexed table that cannot be rewritten causes an explicit rejection.

Remove hinting

<?php

use Alto\Font\Subset\HintingPolicy;

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

This removes glyph instructions and the related cvar, cvt , fpgm, prep, hdmx, LTSH, and VDMX tables.

Reduce layout data

LayoutPolicy::SubstitutionsOnly keeps supported GSUB substitutions and their required GDEF data while removing GPOS positioning. LayoutPolicy::Drop removes GSUB, GPOS, and GDEF entirely.

Dropping layout is appropriate only for controlled content such as digits, icons, or isolated symbols. It is unsafe for general prose or scripts that depend on shaping and mark positioning.

Variable fonts

Supported variable TrueType subsets preserve all axes. Compact mode remaps per-glyph gvar blocks and HVAR mappings while preserving supported axis data. This is not static instancing or axis-range reduction.

A view created with withVariations() cannot be subsetted. Return to the variable source with withoutVariations() first.

Compact mode rejects VVAR, VARC, vertical metrics, kerning, color, bitmap, mathematical tables, unsupported layout formats, and other glyph-indexed structures it cannot remap. Private meta data is removed because its glyph references cannot be remapped safely.

Always inspect SubsetResult::$warnings and validate the final output in the environment that will shape and render it.