commit 909793b85dd2eb0bd37954ec50fbc86d547be50a
author Florin Malita <fmalita@chromium.org> Mon Dec 24 16:03:22 2018
committer Commit Bot <commit-bot@chromium.org> Mon Dec 24 16:03:22 2018
tree d293b7e3bc72cd8465270915f71007d0ee5d09e7
parent 7fd49d1f5087977e7da796441189394a9d0a014d

Update shadow blur-radius computations

According to both CSS [1] and Canvas2D [2], the implementation is expected
to perform a Gaussian blur with a standard deviation of half blur-radius.

(SVG wisely chose to define the shadow parameters directly in terms of
sigma [3])

Currently, for historical reasons, Blink uses a significantly smaller scale
factor when computing sigma: ~0.28 instead of 0.5 -- thus shadows in Blink
are about half the expected size.

This change updates the sigma computation to match specs, and aligns Blink's
shadow rendering with FF & WebKit.

Newly passing (manual) test:
http://test.csswg.org/suites/css3-background/nightly-unstable/html4/box-shadow-blur-definition-001.htm

[1] https://www.w3.org/TR/css-backgrounds-3/#shadow-blur
[2] https://html.spec.whatwg.org/multipage/canvas.html#when-shadows-are-drawn
[3] https://drafts.fxtf.org/filter-effects/#element-attrdef-fedropshadow-stddeviation

Bug: 179006
Test: wpt/css/css-backgrounds/box-shadow/box-shadow-blur-definition-001.xht
Change-Id: Idb6e01f39f86bafa7b3c351bd1bff2f7f7ce1f92
Reviewed-on: https://chromium-review.googlesource.com/c/1377519
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618825}