Font Height

fontheight is a tool that provides recommendations on setting font vertical metrics based on shaped words.

Motivation

Vertical metrics frequently decide clipping boundaries, but are not used consistently across platforms: e.g. Windows uses OS/2 WinAscent/WinDescent, whereas for system fonts Android uses TypoAscent/TypoDescent and a combination of custom heuristics.

It is often desirable to derive metrics from shaped words as opposed to individual glyphs, as words may reach greater extents:

Early versions of this specification suggested that the usWinAscent value be computed as the yMax for all characters in the Windows “ANSI” character set. For new fonts, the value should be determined based on the primary languages the font is designed to support, and should take into consideration additional height that could be required to accommodate tall glyphs or mark positioning.

⬆️ OS/2 — OS/2 and Windows Metrics Table, OpenType Specification 1.9.1

For this reason, vertical metrics must be chosen with a combination of design (e.g. aesthetic, legibility) and engineering (e.g. clipping) considerations in mind. For the latter, fontheight evaluates the extents of a corpus of shaped text across each writing system that a font intends to support.

Usage & installation

Fontheight comes in three flavours:

1. A commandline tool
2. A Rust library
3. A basic Python API

fontheight as a commandline tool

Installation

Currently, both installation methods involve compiling fontheight from its source code, meaning you need to have a Rust toolchain installed and cargo available in your terminal. If you're new to Rust, check out https://www.rust-lang.org/tools/install for instructions on getting it installed and configured. You only need a stable compiler for Font Height's crates.

From GitHub:

STATIC_LANG_WORD_LISTS_LOCAL=1 cargo install --locked --git https://github.com/googlefonts/fontheight fontheight-cli

From crates.io (note: the build script for static-lang-word-lists requires network access, see its README for why):

cargo install --locked fontheight-cli

Note: after installing, the command you run is fontheight (not fontheight-cli).

Usage

Usage: fontheight [OPTIONS] [FONT_PATH]...

Arguments:
  [FONT_PATH]...  The TTF(s) to analyze

Options:
  -n, --results <RESULTS>       The number of words to log [default: 5]
  -k, --words <WORDS_PER_LIST>  The number of words from each list to test [default: 25]
  -h, --help                    Print help
  -V, --version                 Print version

Most of the word list shipped with fontheight are sorted by greatest vertical extremes to try and help reduce the number of words which need to be checked to produce a useful report. However, not all word lists have been sorted at this time, and so for greater reliability you may wish to use a greater value for --words, or use ⚠️ Not yet available --all.

fontheight's Python API

⚠️ Not yet available

The API will be available under the package fontheight.

Usage

See the method signatures & data types below, approximately written in Python. k_words is equivalent to --words in the CLI, and n_exemplars is equivalent to -n/--results.

import fontheight

# Entrypoints

fontheight.get_min_max_extremes_from(path: os.pathLike, k_words: int, n_exemplars: int) -> list[fontheight.Report]

fontheight.get_min_max_extremes(font_bytes: bytes, k_words: int, n_exemplars: int) -> list[fontheight.Report]

# Returned data types

@dataclass(frozen=True)
class fontheight.Report:
    location: dict[str, float]
    word_list_name: str
    exemplars: fontheight.Exemplars

@dataclass(frozen=True)
class fontheight.Exemplars:
    lowest: list[fontheight.WordExtremes]  # sorted, lowest lows first
    highest: list[fontheight.WordExtremes]  # sorted, highest highs first

@dataclass(frozen=True)
class fontheight.WordExtremes:
    word: str
    lowest: float
    highest: float

fontheight's Rust crate

On crates.io as fontheight.

For documentation, please refer to docs.rs.