2029869 - Synthesize vertical font metrics/baselines from the horizontal metrics/baselines

Status: RESOLVED FIXED

(Core :: Layout: Block and Inline, task)

Description

[Sajid Anwar [:sajidanwar]]

Bug 2014156 introduced HarfBuzz's font baseline synthesis for both horizontal and vertical fonts. But during my testing, I found that using HarfBuzz for synthesis of vertical baselines was insufficient for a couple reasons:

• HarfBuzz's synthesis of vertical baselines for normally horizontal glyphs loses the horizontal baseline information, making it impossible to get sane alignment for some baselines. For example, vertical synthesis always returns an alphabetic baseline of zero (which will almost certainly be wrong since vertical font metrics use a centered ascent/descent) and a hanging baseline of 0.6em. These defaults make it so that horizontal glyphs can never actually align to the alphabetic, hanging, or x-middle baselines in vertical writing modes.

• Some common fonts are wildly misbehaved w.r.t. their vertical baselines, rendering the synthesized values meaningless. For example, the Noto Sans CJK SC font has a defined ideographic-under (ideo in OpenType) baseline value of 0. Per the CSS Inline 3 specification, the synthesized ideographic-under is 1em away at 1000, and the synthesized central is halfway at 500. But these are nonsensical given the font's vertical ascent/descent is 500/-500.

As a result, the most compelling option seems to be instead to synthesize vertical baselines from horizontal ones, by mapping the horizontal baselines within the horizontal maximum height to the equivalent location in the horizontal maximum height. Essentially, v_base = ((h_base + h_descent) / h_height * v_height - v_descent.

Comment 1

[Sajid Anwar [:sajidanwar]]

Oh, for sensible x-middle baselines (for alignment-baseline) and cap baselines (for text-box-edge), these metrics should also be synthesized from the horizontal metrics since they only make sense anyway in the context of horizontal glyphs.

Comment 2

[Sajid Anwar [:sajidanwar]]

Attachment: Bug 2029869 - Synthesize vertical font metrics/baselines from horizontal metrics/baselines. r=#layout-reviewers

Uses the font's existing horizontal metrics and baselines to synthesize the
vertical ones. Additionally updates multiple reftests whose expectations or
assertions were implicitly dependent on specific vertical font values when they
didn't need to be. Specifically:

• Updates the misleading expectation for css/css-writing-modes/full-width-003.html.
  This reftest checks for support of the text-combine-upright: digits <number>
  property by asserting a mismatch between the test and the reference, where the
  test uses text-combine-upright and the mismatch reference does not. Browsers
  with no support for the digits syntax are supposed to fail as the rendering
  should be the same. Firefox Linux was passing despite not supporting digits
  due to slight rendering differences caused by a discrepancy in vertical font
  metrics synthesis. The improved synthesis in Bug 1403440 removes this
  discrepancy, now causing the reftest to (correctly) fail.

• Updates some existing canvas veritical writing mode reftests. Previously, the
  reference renderings for these tests were rotated horizontal canvas text using
  default fonts. Now, these tests are referenced against HTML text in vertical
  writing modes using the Ahem font for consistent layout and baseline
  calculations. See https://github.com/whatwg/html/issues/11449 for ongoing
  discussions around formal specifications for canvas writing modes.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/8699ee2727ec
https://hg.mozilla.org/integration/autoland/rev/da01a99a7079
Synthesize vertical font metrics/baselines from horizontal metrics/baselines. r=layout-reviewers,jfkthame

Comment 4

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/7301b087c642
https://hg.mozilla.org/integration/autoland/rev/4c01058fabf0
Synthesize vertical font metrics/baselines from horizontal metrics/baselines. r=layout-reviewers,jfkthame

Comment 5

[Web Platform Test Sync Bot [:wpt-sync] (Matrix: #interop:mozilla.org)]

Created web-platform-tests PR https://github.com/web-platform-tests/wpt/pull/59986 for changes under testing/web-platform/tests

Comment 6

[Cristian Tuns]

https://hg.mozilla.org/mozilla-central/rev/4c01058fabf0

Comment 7

[Web Platform Test Sync Bot [:wpt-sync] (Matrix: #interop:mozilla.org)]

Upstream PR merged by moz-wptsync-bot