msoffice-kern

Derive a PowerPoint / MS Office–safe legacy kern table from a font's GPOS kerning, without touching GPOS.

PowerPoint and the GDI text paths in MS Office do not read GPOS kerning — they only kern by the old format-0 kern table. This package reads the full GPOS kerning of a compiled static TTF or OTF/CFF, selects a reduced set of pairs, and writes a single format-0 kern subtable. Modern applications keep using the untouched GPOS; Office gets a working fallback.

It is a single, well-tested, deterministic implementation of that conversion: golden byte tests lock the produced kern table, so the same input always yields the same bytes.

Installation

pip install msoffice-kern

Usage

from fontTools.ttLib import TTFont
from msoffice_kern import apply_legacy_kern

font = TTFont("Family-Regular.ttf")      # a compiled static TTF/OTF
result = apply_legacy_kern(font)         # mutates font in place; GPOS untouched
if result.applied:
    font.save("Family-Regular.ttf")
print(result.summary())

apply_legacy_kern(font, *, max_pairs=10900, min_abs_value=5, profile="latin-extended-text", strict=False) returns a LegacyKernResult. With strict=False (default) a font whose kerning cannot be derived — no usable GPOS kern, no cmap, malformed GPOS, or a variable font — yields applied=False instead of raising, so a batch never fails on one font; with strict=True the typed exceptions propagate. Variable fonts are always refused (VariableFontError): the legacy table could only encode the default instance, so derive it from exported static instances instead. max_pairs is validated against the format-0 hard limit of 10 920 pairs (MAX_FORMAT0_PAIRS, the subtable's 16-bit length capacity). The low-level primitives (kern_lookups, build_legacy_kern_pairs, reduce_pairs_to_max_count, replace_legacy_kern, …) are exported too.

Selection algorithm

When the candidate count exceeds the MAX_LEGACY_PAIRS (10 900) cap, pairs are chosen by:

• Frequency-weighted score — abs(value) × P(left) × P(right) × case_factor (European-language letter/punctuation frequencies; lowercase→UPPERCASE transitions penalised).
• Atomic class-pair selection — all expansions of one GPOS class×class record (A/Á/Â × T) are kept or dropped together. Whole groups are filled by score density (group score / group size); this is a heuristic, not a global optimum (selecting variable-size groups under a cap is a 0/1 knapsack).
• UPM-relative pruning — tiny pairs below round(min_abs_value × upm / 1000) are dropped first.
• Whitelist — Latin Extended text letters plus basic punctuation; symbols, currency and unencoded alternates stay out.

Diagnostic pairs (AT AV TA VA LT LV To Vo) and the strongest left/right pair per Latin Extended letter are always reserved.

Scope

This package does only the legacy kern derivation. Font naming/STAT/WWS metadata, autohinting, subsetting, WOFF and verification are out of scope.

Determinism

fontTools serialises the final kern table, so its version affects the byte output. The golden tests lock the produced bytes, and CI runs them on a matrix of fonttools versions across the supported range (>=4.61,<5) so the output stays byte-stable for the whole range.

Development

pip install -e ".[dev]"
pytest
ruff check src tests
mypy