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[fxtf-drafts] [filter-effects] "standard deviation" versus "blur radius" in blur() and dropshadow()

From: Amelia Bellamy-Royds via GitHub <sysbot+gh@w3.org>
Date: Mon, 22 May 2017 17:39:48 +0000
To: public-fxtf-archive@w3.org
Message-ID: <issues.opened-230468252-1495474787-sysbot+gh@w3.org>
AmeliaBR has just created a new issue for https://github.com/w3c/fxtf-drafts:

== [filter-effects] "standard deviation" versus "blur radius" in blur() and dropshadow() ==
The SVG `<feGaussianBlur>` filter primitive uses standard deviations to define the amount of blur.  In contrast, `box-shadow` and `text-shadow` use a "blur radius".  The Backgrounds and Borders spec has since been updated to clarify that the [blur radius is twice the standard deviation](https://drafts.csswg.org/css-backgrounds/#shadow-blur).

The Filters spec, however, throws around the two terms as if they are directly interchangeable.

The parameter passed to the [`blur()` shorthand filter function](https://drafts.fxtf.org/filters/#funcdef-filter-blur) is described as both "standard deviation" and as "radius"  (also [in the longhand definition](https://drafts.fxtf.org/filters/#blurEquivalent)).

Even worse (because it affects normative requirements), the [`drop-shadow()` shorthand filter definition states that](https://drafts.fxtf.org/filters/#funcdef-filter-drop-shadow) "Values are interpreted as for box-shadow", but then [gives a longhand equivalent](https://drafts.fxtf.org/filters/#dropshadowEquivalent) that uses the radius as `stdDeviation` in `<feGaussianBlur>`.

The longhand for `drop-shadow()` should probably be re-defined in terms of [`<feDropShadow>`](https://drafts.fxtf.org/filters/#feDropShadowElement), but that doesn't avoid the issue: `<feDropShadow>` still uses `stdDeviation`, while the shorthand uses the radius.

A consistent terminology is required throughout.  I would recommend using words like "the standard deviation or half-radius of the blur" to emphasize the relationship.

It might also be helpful to add a longer informative note explaining the difference between the standard deviation `blur()` parameter and the shadow blur radius.


Please view or discuss this issue at https://github.com/w3c/fxtf-drafts/issues/172 using your GitHub account
Received on Monday, 22 May 2017 17:39:55 UTC

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