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Re: Issues with box-shadow spread

From: Brad Kemper <brad.kemper@gmail.com>
Date: Tue, 30 Nov 2010 21:35:18 -0800
Cc: "www-style@w3.org list" <www-style@w3.org>
Message-Id: <65FE96BC-EBC1-4F74-84F8-EBC3E6B164B9@gmail.com>
To: Behdad Esfahbod <behdad@behdad.org>

On Nov 30, 2010, at 9:15 PM, Behdad Esfahbod wrote:

> Hi all,
> 
> Re:
> 
> http://www.w3.org/TR/2010/WD-css3-background-20100612/#the-box-shadow

I do have to update that picture, because the blur doesn't match the spec any more.


> In reading the CSS Backgrounds and Borders Module Level 3 W3C Working Draft 12
> June 2010 it occurred to me that the spread distance value of a box-shadow
> property is not correctly specified.
> 
> Currently, the spec says that:
> 
> """The fourth length is a spread distance. Positive values cause the shadow
> shape to expand in all directions by the specified radius."""
> 
> However, in the Example XXVIII case, the "Spread Applied" contour does not
> follow the word of the spec.  If you check the lower left, if one was to
> follow the word of the spec, one would get a round corner, but what we see is
> a acute corner.  Ie. the lower-left corner of the Spread Applied contour is
> simply farther away from the lower-left corner of the box than the specified
> box-shadow spread value.

You must have missed this line:

"For corners with a zero border-radius, however, the corner must remain sharp—the operation is equivalent to scaling the shadow shape."

> What seems to be the *intent* of the spec is that, in Postscript terms, the
> spread box is the union of the box and the result of the stroke operation,
> with line-join=miter and an infinite miter-limit.  I can't describe it in a
> simpler way.  Negative values of the spread can be prescribed as the box with
> the stroke area removed instead of added.

That is more or less accurate in PostScript terms.
Received on Wednesday, 1 December 2010 05:35:54 GMT

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