Inspect a variable font

A variable font can contain a range of weights or widths in one file. ALTO Font lets you list those axes and read glyph measurements or outlines at chosen coordinates.

These selections are read-only views. They cannot be exported or subsetted as fixed-weight fonts. To convert or subset with all axes preserved, use the original variable font.

List the available axes

Place a variable TrueType font at fonts/Variable.ttf, then run this script beside vendor:

<?php

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

use Alto\Font\Font;

$font = Font::fromFile(__DIR__.'/fonts/Variable.ttf');
$variations = $font->variations();

if (null === $variations) {
    echo "This is a static font.\n";
} else {
    foreach ($variations->axes as $axis) {
        printf(
            "%s: %g to %g, default %g\n",
            $axis->tag,
            $axis->minimum,
            $axis->maximum,
            $axis->default,
        );
    }
}

For a weight axis, a line might be wght: 100 to 900, default 400. The actual ranges depend on the file. Axes also expose their optional name and hidden flag; named presets are available through $variations->instances.

Compare a glyph at another weight

Continue the script with a font that has a wght axis and contains A:

<?php

if (null === $variations || !$variations->hasAxis('wght')) {
    throw new RuntimeException('Choose a variable font with a weight axis.');
}

$bold = $font->withVariations(['wght' => 700]);

printf("Default A advance: %g\n", $font->metrics('A')->advanceWidth);
printf("Selected A advance: %g\n", $bold->metrics('A')->advanceWidth);

Both values are in font design units. They may be equal if that glyph's advance does not vary. Use $bold->glyphOutline($bold->metrics('A')->glyphId) to read its selected outline as well. The original $font remains unchanged.

Values outside an axis range are clamped. Unknown axes and selections on static fonts raise InvalidFontException. Query the available axes before assuming that a font has width (wdth) or any other axis.

This uses the bundled Recursive-derived sample, with the other axes fixed as shown. The outline changes while A keeps the same 650-unit advance. Each column comes from an ALTO read-only view; no static font was exported. Glyphs are placed by their advances without text shaping or kerning. The illustration uses the bundled variable-font fixture.

Return to the source for conversion or subsetting

$bold->withoutVariations() returns the original variable source with all its axes. It does not freeze weight 700. A weighted finder query can also return a selected view, with the same writing restrictions.

Coordinate API

Pass an array to withVariations() as above, or build a VariationCoordinates object with defaults($variations) and with(). variationCoordinates() reports the selected coordinates.

Measurements and outlines use supported TrueType variation data (avar, gvar, and HVAR). Text shaping and rendering at those coordinates are the responsibility of the application using the font.