Bug 285382 – Incorrect "extension" of CSS gradient with "longer hue" interpolation into regions beyond the first/last color stop

• Component: Layout and Rendering
• Status: RESOLVED FIXED

[Jonathan Kew]

See attached example and description.

The CSS spec[1] is clear that when coloring a gradient,

> Before the first color stop, the gradient line is the color of the first color stop, and after the last color stop, the gradient line is the color of the last color stop.

Therefore, if the first color stop is not at (or before) zero, there will be a region of solid color from the beginning of the gradient line to the first color stop, using the color of that first stop. Similarly, if the last color stop falls before the end of the gradient line, there will be a region of solid color at the end.

However, when using "longer hue" interpolation, webkit renders such cases incorrectly, applying unwanted interpolation in these regions that should have solid first or last color.

(See also Gecko issue https://bugzilla.mozilla.org/show_bug.cgi?id=1939948 and Blink issue https://issues.chromium.org/issues/387475844.)

[1] https://drafts.csswg.org/css-images-4/#coloring-gradient-line

[Jonathan Kew]

Created attachment 473772 [details]
testcase describing and illustrating the bug

[Radar WebKit Bug Importer]

<rdar://problem/142738948>

[Simon Fraser (smfr)]

This happens because we follow the "longer hue" logic at https://www.w3.org/TR/css-color-4/#hue-longer for the 0-25% range and the 75% to 100% ranges as well as the stops in between (code in `interpolateColorComponents()`).

I'm a little surprised that going from red to red with `longer hue` hits all the colors, but that's what other browsers do.

[Simon Fraser (smfr)]

This is tested by css/css-images/gradient/gradient-single-stop-longer-hue-hsl-002.html

[Simon Fraser (smfr)]

Pull request: https://github.com/WebKit/WebKit/pull/46303

[EWS]

Committed 295860@main (ae2fd930ce5b): <https://commits.webkit.org/295860@main>

Reviewed commits have been landed. Closing PR #46303 and removing active labels.