optimize-fonts@1.0.0

optimize-fonts/scripts/font-metrics.py

@@ -0,0 +1,94 @@
+#!/usr/bin/env python3
+"""Print metric-override fallback @font-face blocks for a web font.
+
+The blocks make a local system font (Arial, Roboto) occupy the same space as the
+web font, so switching from the fallback to the web font on load does not shift
+layout (CLS).
+
+Usage:
+    python3 font-metrics.py <webfont.woff2> [--weight 500] [--name Montserrat] [--weights "400 800"]
+
+Requires Python 3 with fonttools and brotli:  pip install fonttools brotli
+"""
+
+import argparse
+import sys
+
+from fontTools.ttLib import TTFont
+
+# Advance-width metrics of common fallback fonts, measured over the same
+# character set as the web font (a-z A-Z 0-9 space), normalised to unitsPerEm.
+# The ascent/descent/lineGap values are not used: only the average width matters
+# for size-adjust. Keys are the local() src names.
+FALLBACKS = {
+    'Arial': {'local': 'Arial', 'suffix': '', 'avg': 0.5742885044642857},
+    'Roboto': {'local': 'Roboto', 'suffix': ' Android', 'avg': 0.5608646453373016},
+}
+
+CHARS = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 '
+
+
+def web_metrics(path, weight):
+    font = TTFont(path)
+    if weight is not None and 'fvar' in font:
+        from fontTools.varLib import instancer
+
+        font = instancer.instantiateVariableFont(font, {'wght': weight}, inplace=False)
+    head = font['head']
+    hhea = font['hhea']
+    cmap = font.getBestCmap()
+    hmtx = font['hmtx']
+    upm = head.unitsPerEm
+    widths = [hmtx[cmap[ord(c)]][0] / upm for c in CHARS if ord(c) in cmap]
+    return {
+        'ascent': hhea.ascent / upm,
+        'descent': abs(hhea.descent) / upm,
+        'lineGap': hhea.lineGap / upm,
+        'avg': sum(widths) / len(widths),
+    }
+
+
+def family_name(path):
+    font = TTFont(path, lazy=True)
+    for id_ in (16, 1):
+        name = font['name'].getDebugName(id_)
+        if name:
+            return name.split()[0]
+    return 'WebFont'
+
+
+def pct(value):
+    text = f'{value:.2f}'.rstrip('0').rstrip('.')
+    return f'{text}%'
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
+    parser.add_argument('webfont', help='path to the web font file (woff2/ttf/otf)')
+    parser.add_argument('--weight', type=int, default=500, help='weight to measure variable fonts at (default: 500)')
+    parser.add_argument('--name', default=None, help='font family name used in the fallback family names')
+    parser.add_argument('--weights', default='400 800', help='font-weight value for the fallback rules (default: "400 800")')
+    args = parser.parse_args()
+
+    web = web_metrics(args.webfont, args.weight)
+    name = args.name or family_name(args.webfont)
+
+    for key, fb in FALLBACKS.items():
+        size_adjust = web['avg'] / fb['avg']
+        if not size_adjust:
+            print(f'warning: empty width for {key}', file=sys.stderr)
+            continue
+        print('@font-face {')
+        print(f'    ascent-override: {pct(web["ascent"] / size_adjust * 100)};')
+        print(f'    descent-override: {pct(web["descent"] / size_adjust * 100)};')
+        print(f"    font-family: '{name} Fallback{fb['suffix']}';")
+        print(f'    font-weight: {args.weights};')
+        print(f'    line-gap-override: {pct(web["lineGap"] / size_adjust * 100)};')
+        print(f'    size-adjust: {pct(size_adjust * 100)};')
+        print(f"    src: local('{fb['local']}');")
+        print('}')
+        print()
+
+
+if __name__ == '__main__':
+    main()