Measurements format

A data format to store definitions of font-level and glyph-level measurements.

Data structure
Python example
JSON format

Data structure

A measurement establishes a link between two points and returns the distance between them.

The order of the points matters: a measurement can be positive or negative.

Font-level measurements

A font may contain multiple font-level measurements.
Font measurement names must be unique.
The order of the font measurements matters.

attribute	description
name	name of the measurement	required
glyph 1	name of the glyph containing the 1st measurement point	required
point 1	index or shortcut of 1st measurement point	required
glyph 2	name of the glyph containing the 2nd measurement point	required
point 2	index or shortcut of 2nd measurement point	required
direction	direction of measurement	required
parent	parent measurement	optional
description	description of this measurement	optional

The attribute glyph 2 can be made optional. If it is omitted, we assume glyph 2 to be the same as glyph 1.

Glyph-level measurements

A glyph may contain multiple glyph-level measurements.
Glyph measurement names are related to font-level measurements.
Glyph measurement names must not be unique.
Glyph measurement identifiers are created from the index or name of its two points.
The order of glyph measurements follows the order of font measurements.

attribute	description
name	name of the measurement
point 1	index or name of 1st measurement point
point 2	index or name of 2nd measurement point
direction	direction of measurement

Direction keys

The direction of measurement must be one of the following characters:

characters	description
x	horizontal measurement
y	vertical measurement
a	angled measurement

The order of the points in a measurement matters: the sign of the measured value indicates its direction. Some measurements are by definition negative, for example the descender value or a bottom overshoot.

Point IDs

Points can be identified by a number (point index) or a letter (reference point).

Point indexes

Contour points are identified by their index (an integer).

Reference points

Font-level vertical metrics are also available using the following shortcut characters:

character	description	y-position
A	ascender	`font.info.ascender`
B	baseline	`0`
C	cap height	`font.info.capHeight`
D	descender	`font.info.descender`
X	x-height	`font.info.xHeight`

In addition to the standard (latin) vertical metrics values above, custom reference points can be defined – for example alignment zones for other scripts, or additional alignment points for latin.

Python example

Font-level measurements

The key for font-level measurements is the name of the measurement.

fontMeasurements = {
    'XTUC' : {
        'glyph 1'   : 'H',
        'point 1'   : 11,
        'glyph 2'   : 'H',
        'point 2'   : 8,
        'direction' : 'x',
        'parent'    : 'XTRA',
    },
    # more font-level measurements here ...
}

Glyph-level measurements

The key for glyph-level measurements is an identifier created from the two point IDs.

glyphMeasurements = {
    "a" : {
      f'{ptID1} {ptID2}' : {
          'name'      : 'XTRA',
          'direction' : 'x',
      },
      # more glyph-level measurements here ...
    },
    # more glyphs here ...
}

The current glyph-level measurement format has one limitation: a pair of points can only have one measurement attached to it. It should be possible to have both x and y measurements for same pair of points though (for example, the width and height of a serif). The format can be updated in the future to address this issue.

Custom reference points

There is no standard or character limit for custom point names. Short letter codes are preferable for quicker typing and reading.

customPoints = {
    # additional vertical metrics for latin
    'O' : {
        'y' : 640,
        'description' : 'old style figures top',
    },
    # custom vertical metrics for arabic
    'AS' : {
        'y' : 1600,
        'description' : '[arabic] sky',
    },
    'AT' : {
        'y' : 800,
        'description' : '[arabic] tooth',
    },
    'AE' : {
        'y' : -400,
        'description' : '[arabic] earth',
    },
    # more custom points here ...
}

JSON format

Measurements can be stored in standalone JSON files using the format below.

The same set of measurement definitions can be used to measure all sources in a designspace.

{
  "font": {
    "XTUC": {
      "direction": "x",
      "glyph 1": "H",
      "point 1": "11",
      "glyph 2": "H",
      "point 2": "8",
      "parent": "XTRA"
    },
    /* more font-level measurements here ... */
  },
  "glyphs": {
    "n": {
      "0 20": {
        "direction": "x",
        "name": "XOLC"
      },
      "11 12": {
        "direction": "x",
        "name": "XOLC"
      },
      "13 19": {
        "direction": "x",
        "name": "XTLC"
      },
      /* more glyph-level measurements here ... */
    },
    /* more glyphs here ... */
  },
}