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scripts/metrics.py
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238
scripts/metrics.py
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"""
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FontForge: Set Vertical Metrics
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───────────────────────────────
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Follows the Google Fonts vertical-metrics methodology:
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• OS/2 Typo → controls line spacing (with USE_TYPO_METRICS / fsSelection bit 7)
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• OS/2 Win → clipping boundary on Windows (must cover every glyph)
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• hhea → line spacing + clipping on macOS/iOS
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Run inside FontForge (File → Execute Script → paste, or fontforge -script).
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"""
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import fontforge
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import math
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f = fontforge.activeFont()
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# ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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# CONFIGURATION
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# ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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# Desired line height as a multiple of UPM.
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# 1.0 = no extra leading (glyphs may touch between lines)
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# 1.2 = 120% — a solid default for body text
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# 1.25 = matches the CSS default for most browsers
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# 1.5 = generous (double-spaced feel)
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LINE_HEIGHT = 1.0
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# Extra padding on Win and hhea metrics, as a fraction of UPM.
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# Prevents clipping of glyphs that sit right at the bounding-box edge.
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# 0.0 = trust the bounding boxes exactly
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# 0.01 = 1% pad (conservative)
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# 0.02 = 2% pad (safe for hinting artefacts / composites)
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CLIP_MARGIN = 0.01
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# ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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# HELPERS
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# ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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def _bbox(name):
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"""Return bounding box (xmin, ymin, xmax, ymax) or None."""
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if name in f and f[name].isWorthOutputting():
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bb = f[name].boundingBox()
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if bb != (0, 0, 0, 0): # skip empty glyphs (space, CR, …)
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return bb
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return None
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def measure_chars(chars, *, axis="top"):
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"""
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Measure a set of reference characters.
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axis="top" → return the highest yMax (ascenders, cap height)
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axis="bottom" → return the lowest yMin (descenders)
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Returns (value, display_char) or (None, None).
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"""
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idx = 3 if axis == "top" else 1
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pick = max if axis == "top" else min
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hits = []
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for ch in chars:
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name = fontforge.nameFromUnicode(ord(ch))
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bb = _bbox(name)
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if bb is not None:
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hits.append((bb[idx], ch))
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if not hits:
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return None, None
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return pick(hits, key=lambda t: t[0])
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def scan_font_extremes():
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"""Walk every output glyph; return (yMax, yMin, max_name, min_name)."""
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y_max, y_min = 0, 0
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max_nm, min_nm = None, None
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for g in f.glyphs():
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if not g.isWorthOutputting():
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continue
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bb = g.boundingBox()
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if bb == (0, 0, 0, 0):
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continue
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if bb[3] > y_max:
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y_max, max_nm = bb[3], g.glyphname
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if bb[1] < y_min:
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y_min, min_nm = bb[1], g.glyphname
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return y_max, y_min, max_nm, min_nm
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# ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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# STEP 1 — Measure design landmarks
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# ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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print("─── Design landmarks ───\n")
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cap_h, cap_c = measure_chars("HIOXE", axis="top")
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asc_h, asc_c = measure_chars("bdfhkl", axis="top")
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xht_h, xht_c = measure_chars("xzouv", axis="top")
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dsc_h, dsc_c = measure_chars("gpqyj", axis="bottom")
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for label, val, ch in [("Cap height", cap_h, cap_c),
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("Ascender", asc_h, asc_c),
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("x-height", xht_h, xht_c),
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("Descender", dsc_h, dsc_c)]:
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if val is not None:
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print(f" {label:12s} {int(val):>6} ('{ch}')")
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else:
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print(f" {label:12s} {'N/A':>6}")
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# ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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# STEP 2 — Full-font bounding-box scan
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# ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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print("\n─── Full font scan ───\n")
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font_ymax, font_ymin, ymax_name, ymin_name = scan_font_extremes()
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print(f" Highest glyph: {int(font_ymax):>6} ({ymax_name})")
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print(f" Lowest glyph: {int(font_ymin):>6} ({ymin_name})")
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# ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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# STEP 3 — Compute the three metric sets
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# ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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upm = f.em
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# Design top = tallest Latin ascender (fall back to cap height)
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design_top = asc_h if asc_h is not None else cap_h
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design_bot = dsc_h # negative value
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if design_top is None or design_bot is None:
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raise SystemExit(
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"ERROR: Could not measure ascender/cap-height or descender.\n"
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" Make sure your font contains basic Latin glyphs (H, b, p, etc.)."
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)
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# ── OS/2 Typo metrics ────────────────────────────────────────────────────────
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# These define line spacing when USE_TYPO_METRICS is on.
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# Strategy: ascender and descender sit at the design's actual ink boundaries;
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# lineGap absorbs all extra leading. This keeps the text vertically
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# centred on the line, which matters for UI / web layout.
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typo_ascender = int(round(design_top))
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typo_descender = int(round(design_bot)) # negative
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typo_extent = typo_ascender - typo_descender # total ink span (positive)
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desired_lh = int(round(upm * LINE_HEIGHT))
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typo_linegap = max(0, desired_lh - typo_extent)
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# ── OS/2 Win metrics ─────────────────────────────────────────────────────────
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# Clipping boundaries on Windows. Must cover every glyph or Windows clips them.
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# usWinDescent is a *positive* distance below the baseline (unlike Typo/hhea).
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margin = int(math.ceil(upm * CLIP_MARGIN))
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win_ascent = int(math.ceil(max(font_ymax, design_top))) + margin
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win_descent = int(math.ceil(max(abs(font_ymin), abs(design_bot)))) + margin
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# ── hhea metrics ──────────────────────────────────────────────────────────────
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# macOS/iOS always uses hhea for *both* line spacing and clipping (it ignores
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# USE_TYPO_METRICS). To keep line height consistent across platforms, we fold
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# the Typo lineGap into hhea ascent/descent so hhea_lineGap can be 0.
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# Then we take the max with the font bbox to also prevent Mac clipping.
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half_gap = typo_linegap // 2
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extra = typo_linegap - 2 * half_gap # +1 rounding remainder → ascent side
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spacing_asc = typo_ascender + half_gap + extra
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spacing_dsc = typo_descender - half_gap # more negative
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hhea_ascent = max(spacing_asc, int(math.ceil(font_ymax)) + margin)
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hhea_descent = min(spacing_dsc, int(math.floor(font_ymin)) - margin) # negative
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hhea_linegap = 0
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# ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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# STEP 4 — Apply to font
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# ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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# FontForge's own ascent / descent (used for UPM split in the head table)
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f.ascent = typo_ascender
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f.descent = abs(typo_descender)
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# OS/2 table
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f.os2_typoascent = typo_ascender
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f.os2_typodescent = typo_descender
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f.os2_typolinegap = typo_linegap
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f.os2_winascent = win_ascent
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f.os2_windescent = win_descent
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# hhea table
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f.hhea_ascent = hhea_ascent
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f.hhea_descent = hhea_descent
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f.hhea_linegap = hhea_linegap
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# USE_TYPO_METRICS — fsSelection bit 7
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# FontForge exposes this differently across versions. We try three known paths.
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typo_metrics_set = False
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# Method 1: dedicated boolean (FontForge ≥ 2020-ish)
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if hasattr(f, "os2_use_typo_metrics"):
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f.os2_use_typo_metrics = True
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typo_metrics_set = True
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# Method 2: direct fsSelection manipulation (if exposed)
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if not typo_metrics_set and hasattr(f, "os2_fsselection"):
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f.os2_fsselection |= (1 << 7)
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typo_metrics_set = True
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# Method 3: via the OS/2 version (some builds gate the flag on version ≥ 4)
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if not typo_metrics_set:
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if hasattr(f, "os2_version") and f.os2_version < 4:
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f.os2_version = 4
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if not typo_metrics_set:
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print("⚠ Could not set USE_TYPO_METRICS programmatically.")
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print(" → In Font Info → OS/2 → Misc, tick 'USE_TYPO_METRICS'.\n")
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# ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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# STEP 5 — Report
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# ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
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typo_line = typo_ascender - typo_descender + typo_linegap
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hhea_line = hhea_ascent - hhea_descent + hhea_linegap
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win_total = win_ascent + win_descent
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print("\n─── Applied metrics ───\n")
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print(f" UPM: {upm}\n")
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print(f" {'':18s} {'Ascender':>9s} {'Descender':>10s} {'LineGap':>8s} {'Total':>6s}")
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print(f" {'─'*18} {'─'*9} {'─'*10} {'─'*8} {'─'*6}")
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print(f" {'OS/2 Typo':18s} {typo_ascender:>9d} {typo_descender:>10d} {typo_linegap:>8d} {typo_line:>6d}")
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print(f" {'hhea':18s} {hhea_ascent:>9d} {hhea_descent:>10d} {hhea_linegap:>8d} {hhea_line:>6d}")
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print(f" {'OS/2 Win':18s} {win_ascent:>9d} {'-'+str(win_descent):>10s} {'n/a':>8s} {win_total:>6d}")
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print(f"\n Effective line height: {typo_line} ({typo_line/upm:.2f}× UPM)")
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print(f" Design ink span: {typo_extent} ({typo_extent/upm:.2f}× UPM)")
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print(f" Clipping headroom: +{win_ascent - int(round(font_ymax))} above, "
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f"+{win_descent - int(round(abs(font_ymin)))} below")
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if cap_h is not None:
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print(f"\n Cap height: {int(cap_h)}")
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if xht_h is not None:
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print(f" x-height: {int(xht_h)}")
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print("\nDone. Review in Font Info → OS/2 and Font Info → General.")
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