1838089 - WPT failure in css/css-transforms/scale/svg-scale-013.html and -014.html

Status: RESOLVED FIXED

(Core :: SVG, defect)

Description

[Daniel Holbert [:dholbert] (PTO until 2/24)]

We fail these two WPT tests:
https://wpt.fyi/results/css/css-transforms/scale/svg-scale-013.html
https://wpt.fyi/results/css/css-transforms/scale/svg-scale-014.html

Direct links to tests:
http://wpt.live/css/css-transforms/scale/svg-scale-013.html
http://wpt.live/css/css-transforms/scale/svg-scale-014.html

The failure looks like a rounding / antialiasing issue, arguably not really a bug. The test has a rect with fractional size and fractional position, which gets scaled down, and the test is expecting the math to work out precisely correctly for that rect to cover up a red area. We're off by a very small amount, whether due to pixel-snapping or rounding error.

We should fix the tests to avoid triggering this rounding error (i.e. avoid depending on precise pixel snapping implementation details), probably by adjusting the scales to not involve fractional pixels. (Or alternately: by making the rect slightly larger so that it's got a bit of wiggle room for rounding error.)

Comment 1

[Daniel Holbert [:dholbert] (PTO until 2/24)]

I filed https://github.com/web-platform-tests/interop/issues/359 to track the test-change upstream.

Comment 2

[Daniel Holbert [:dholbert] (PTO until 2/24)]

Attachment: Bug 1838089: Change WPTs svg-scale-013 and -014 to use whole-number pixel values, to avoid depending on precise pixel snapping behavior. r?#layout-reviewers

These tests were making unjustified subpixel-positioning assumptions about
rects with positions and sizes of 133.333px; and the tests were also not
accounting the fact that 1.5 * 133.333px is technically not-quite-200px.

This patch addresses these issues by using 1.25 * 160px instead -- testing the
same thing, but with a whole-number pixel value that doesn't require a
truncated decimal approximation.

This addresses https://github.com/web-platform-tests/interop/issues/359

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/2f7a472cb1c6
Change WPTs svg-scale-013 and -014 to use whole-number pixel values, to avoid depending on precise pixel snapping behavior. r=TYLin

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/40561 for changes under testing/web-platform/tests

Comment 5

[Iulian Moraru]

https://hg.mozilla.org/mozilla-central/rev/2f7a472cb1c6

Comment 6

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

Upstream PR merged by moz-wptsync-bot