1840075 - Use the OpenType baseline table to implement textBaseline alignment and TextMetrics baseline attributes

Status: RESOLVED FIXED

(Core :: Graphics: Canvas2D, defect)

Description

[Jonathan Kew [:jfkthame]]

Currently, we "fake" hanging and ideographic baselines for these canvas2d APIs by just using arbitrary factors of the font ascent and descent: https://searchfox.org/mozilla-central/rev/d307d4d9f06dab6d16e963a4318e5e8ff4899141/dom/canvas/CanvasRenderingContext2D.cpp#4400-4407

For accurate results, we should read baseline positions from the font (when provided), and use those rather than these ad hoc guesses.

Comment 1

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1840075 - Adjust WPT 2d.text.measure.baselines tests to match current spec for canvas TextMetrics. r=#gfx-reviewers

These tests assume a TextMetrics.getBaselines() function, which does not exist in the current spec;
rather, it has specific attributes for alphabetic, hanging and ideographic baselines.
I think the test was based on an older proposal that didn't end up in the spec in that form.

(See https://html.spec.whatwg.org/multipage/canvas.html#textmetrics)

In addition, the values the tests expect don't correspond to what's in the CanvasTest font 'BASE'
table for the hanging and ideographic baselines (they seem to be the ascent and descent instead).
So fixing those, in preparation for actually implementing support.

Comment 2

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1840075 - Implement support for the OpenType BASE (baselines) table, and use it to back canvas2d TextMetrics attributes and textBaseline alignment. r=#gfx-reviewers

Depends on D181881

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/a908a6cc6603
Adjust WPT 2d.text.measure.baselines tests to match current spec for canvas TextMetrics. r=gfx-reviewers,lsalzman
https://hg.mozilla.org/integration/autoland/rev/f27a6b03f043
Implement support for the OpenType BASE (baselines) table, and use it to back canvas2d TextMetrics attributes and textBaseline alignment. r=gfx-reviewers,lsalzman

Comment 4

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

Created web-platform-tests PR https://github.com/web-platform-tests/wpt/pull/40730 for changes under testing/web-platform/tests

Comment 5

[Cristina Horotan [:chorotan]]

https://hg.mozilla.org/mozilla-central/rev/a908a6cc6603
https://hg.mozilla.org/mozilla-central/rev/f27a6b03f043

Comment 6

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

Upstream PR merged by moz-wptsync-bot