googlefonts.github.io

Vertical metrics

🐸 Vertical metrics are used to determine the space between two lines of text. Some metrics are meant for Mac, other for Windows; and are interpreted differently according to web or desktop usage.

Throughout countless threads and discussions, GF decided to make the priority on cross-platform compatibility and therefore apply the following requirements. Read them carefully because once a family is onbaorded, vertical metrics are meant to remain always the same to avoid regression, as mentioned in Adding & upgrading fonts to Google Fonts.

Please note that the first metrics guidelines referred to in this guide are Latin focused and probably will not fully work for other complex scripts such as Burmese and Devanagari among others. CJK fonts (Chinese-Japanese-Korean scripts) are treated as special cases and should follow the instructions at the end of the document.
Background reading:
must→ Adding & upgrading fonts to Google Fonts
must→ Overall font files requirements

Table of contents

Requirements for all fonts (except CJK)

The following rules apply to all new font families, and should be enforced to upgraded font families when possible.

1. Vertical metrics must not be calculated by the font editor automatically.

GF doesn’t want this because they all do it differently.

2. The following vertical metric parameters must be set for each font in a family.

Ms Spec ttf spec Glyphs.app Master customParameter FontLab ufo3 fontinfo.plist
[OS/2] sTypoAscender typoAscender [OS/2] TypoAscender openTypeOS2TypoAscender
[OS/2] sTypoDescender typoDescender [OS/2] TypoDescender openTypeOS2TypoDescender
[OS/2] sTypoLineGap typoLineGap [OS/2] TypoLineGap openTypeOS2TypoLineGap
[hhea] Ascender hheaAscender [hhea] Ascender openTypeHheaAscender
[hhea] Descender hheaDescender [hhea] Descender openTypeHheaDescender
[hhea] LineGap hheaLineGap [hhea] LineGap openTypeHheaLineGap
[OS/2] usWinAscent winAscent [OS/2] WinAscent openTypeOS2WinAscent
[OS/2] usWinDescent winDescent [OS/2] WinDescent openTypeOS2WinDescent

For brevity, we will refer to the 3 sets of metrics as Typo, Hhea, Win.

3. Vertical metrics must be consistent across a family.

Each font in a family must share the same vertical metric values.

This rule can be avoided if a font is being upgraded and previously had inconsistent family metrics. If this is the case, the aim should be to visually match the line spacing of each font, but fix any clipping issues caused by incorrect WinAscent, WinDescent values.

4. WinAscent and WinDescent values must be the same as the family’s tallest/deepest yMin and yMax bounding box values.

The Microsoft OpenType specification. Recommends the following:

If any clipping is unacceptable, then the value should be set to yMax.

Overpass issue #33 demonstrates what happens in MS applications when the winAscent and winDescent are not set correctly.

By changing these values, the line height will be increased in MS applications. This is can lead to very loose line heights if the bounding box is exceedingly tall. This mainly occurs in families featuring Vietnamese, Devanagari and Arabic or other tall scripts. To counteract this, we enable the Use Typo Metrics flag and set the Typo values to match the previous Win values. By swapping the sets, we should retain the previous line heights in Windows as well as remove the clipping.

5. Use_Typo_Metrics must be enabled.

This will force MS Applications to use the Typo values instead of the Win values for line spacing. By doing this, we can freely set the Win values to avoid clipping and control the line height with the Typo values. It has the added benefit of future line height compatibility. When a new script is added, we simply change the Win values to the new yMin and yMax, without needing to worry if the line height have changed. Note that the Use_Typo_Metric flag is also called fsSelection bit 7 (related to how it is set in the OS/2 table).

6. If the family is being updated, the line height must visually match the previous release.

Some applications do not allow users to control the line height/leading of their fonts. Word processors and text editors are common culprits. It is essential their documents do not reflow.

7. Hhea and Typo metrics should be equal.

Hhea metrics are used in macOS, whilst Microsoft uses Typo when Use_Typo_Metrics is enabled. They should ideally be identical.

This rule can be avoided if a font is being upgraded and previously had inconsistent values.

8. LineGap values must be 0.

The LineGap value is a space added to the line height created by the union of the (typo/hhea)Ascender and (typo/hhea)Descender. It is handled differently according to the environment. This leading value will be added above the text line in most desktop apps. It will be shared above and under in web browsers, and ignored in Windows if Use_Typo_Metrics is disabled. For better linespacing consistency across platforms, (typo/hhea)LineGap values must be 0.

9. Uppercases should be centered if the font’s primary script has uppercases letterforms such as Latin, Greek and Cyrillic.

Web designers will thank you if you managed to have the same space above and under capitals: typoAscender - CapsHeight = abs(typoDescender). It will make easier for them the setting of padding in buttons for example.

10. typo/hheaAscender value should be greater than Agrave’s yMax when it makes sense.

Some Mac applications such as TextEdit will position the first line of text by, either, using the height of the A grave, or by using the font’s hheaAscender (whichever is taller). To keep the positioning consistent across a family, we require that the hheaAscender is greater than the tallest A grave in the family. See this issue for further info, https://github.com/googlefonts/fontbakery/issues/3170.

11. The sum of the font’s vertical metric values (absolute) should be 20-30% greater than the font’s UPM.

By default, DTP applications such as Indesign will set the line height to be 20% greater than the font’s size (10pt size / 12pt leading). For consistency, we recommend setting the vertical metric values so their sum is in a similar range, for example:

UPM = 1000
(typo/hhea)Ascenders = 900
(typo/hhea)Descenders = -300
(typo/hhea)LineGap = 0

Total:
ascender + abs(descenders) + linegap = 1200

Exceptions are usually made if the font’s primary script isn’t Latin, Greek or Cyrillic. Some scripts such as Devanagari contain very tall and shallow glyphs. It may make more sense for the sum of the metrics to exceed 130% to avoid interline glyph collisions.

Note

Please keep in mind that this calculation is to be set according to the specificities of each font.

Concrete cases:

Setting vertical metrics usually falls into the following two categories:

  1. Calculating the vertical metrics for a new family
  2. Recalculating the vertical metrics for an upgraded family

1. Calculating the vertical metrics for a new family

Set these values to be the same across all masters to ensure that output instances have equal vertical metrics:

Expected result: vertical metrics should be around 120-130% of UPM. Anything greater, and the metrics may look too loose.

Example

A new Latin family has the following qualities:

  1. Set the default values, following the schema above:
typoAscender = 965 #(UPM * 1.2 - CapsHeight) / 2 + CapsHeight which is greater than Agrave, perfect.)
typoDescender = -235 #(UPM * 1.2 - CapsHeight) / 2 which is equal to deepest letterform)
typoLineGap = 0
hheaAscender = 965 #(typoAscender)
hheaDescender = -235 #(typoDescender)
hheaLineGap = 0 #(typoLineGap)
winAscent = 1116 #(Font bbox yMax)
winDescent = 320 #(*absolute value* of Font bbox yMin ie. a positive integer)
  1. Be sure to copy these same metric values to all of the masters in the family
  2. Enable Use_Typo_Metrics

2. Recalculating the vertical metrics for an upgraded family

Many font families receive upgrades, either by the original author or a 3rd party. When character extensions occur which modify the font’s bounding box, the vertical metrics will need to be recalculated.

Imagine we have a previous v1.000 release. We’re now adding Vietnamese. The version number has been bumped to V2.000. The font’s bbox yMax has changed from 1000 to 1100 and yMin from -200 to -300.

v1.000 had the following family vertical metrics:

typoAscender = 800
typoDescender = -200
typoLineGap = 200
hheaAscender = 800
hheaDescender = -200
hheaLineGap = 200
winAscent = 1000
winDescent = 200

Notice each set set adds up to 1200, if the negative integers are converted to positive. This should lead to consistent metrics across each platform

There are two cases which can occur.

I. Use_Typo_Metrics was already enabled in the v1.000 release.

The Win values simply need to reflect the new yMin and yMax values.

  1. v2.000 vertical metrics:

    typoAscender = 800
    typoDescender = -200
    typoLineGap = 200
    hheaAscender = 800
    hheaDescender = -200
    hheaLineGap = 200
    WinAscent = 1100. # Font bbox yMax
    WinDescent = 300. # Font bbox yMin (positive integer)
    
  2. Repeat process for each weight/style if values are not unique in v1.000

II. Use_Typo_Metrics was not enabled in the previous release.

The Typo Metrics need to inherit the v1.000 Win values. The WinAscent and WinDescent also need to reflect the new yMin and yMax values.

  1. v2.000 vertical metrics:

    typoAscender = 1000. #Old WinAscent
    typoDescender = -200. #Old negative WinAscent
    typoLineGap = 0 #Win Metrics has no LineGap parameter so we set this to 0
    hheaAscender = 1000 #set as new TypoAscender)
    Hhea Descender = -200
    hheaLineGap = 0 #consistent with typoLineGap
    winAscent = 1100 #Font bbox yMax
    winDescent = 300 #Font bbox yMin positive integer
    
  2. Repeat process for each weight/style if values are not unique in v1.000
  3. Enable Use_Typo_Metrics

If the font was previously hosted on fonts.google.com, you can test the upgraded vertical metrics visually match by using GF Regression. You can also use gftools gen-html with the command gftools gen-html diff --font-before font1.ttf --font-after font2.ttf if you want to compare two fonts locally.

CJK Vertical Metrics

CJK vertical metrics are based on Source Han Sans and Noto CJK fonts.

The following vertical metric values must be applied to all CJK fonts

Attrib Value Example using 1000upm font
OS/2.sTypoAscender 0.88 * font upm 880
OS/2.sTypoDescender -0.12 * font upm -120
OS/2.sTypoLineGap 0 0
hhea.ascender Set to look comfortable (~1.16 * upm) 1160
hhea.descender Set to look comfortable (~0.288 * upm) -288
hhea.lineGap 0 0
OS/2.usWinAscent Same as hhea.ascent 1160
OS/2.usWinDescent abs(value) of hhea.descent 288
OS/2.fsSelection bit 7 (Use_Typo_Metrics) Do not set / disabled 0

Our decision to follow the Adobe schema was based on Dr. Ken Lunde’s comments and his release notes on Source Han Sans: