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[css3-background] box-shadow spread radius and rounded corners

From: Sylvain Galineau <sylvaing@microsoft.com>
Date: Wed, 28 Apr 2010 00:31:37 +0000
To: "www-style@w3.org" <www-style@w3.org>
CC: Brian Manthos <brianman@microsoft.com>, Prabs Chawla <pchawla@microsoft.com>
Message-ID: <045A765940533D4CA4933A4A7E32597E214481D2@TK5EX14MBXC113.redmond.corp.microsoft.com>
Note: the WG has resolved to add box-shadow back to CSS3 Background & Borders [1] but as the 
editor's draft has not yet been updated, the following refers to the last document version for which 
it was defined [2].

According to the spec:

# The fourth length is a spread radius. Positive values cause the shadow to grow in all directions by the 
# specified radius. Negative values cause the shadow to shrink. The shadow should not change shape 
# when a spread radius is applied: sharp corners should remain sharp.

The requirement to not change the shape of the shadow seems very desirable from an author standpoint
e.g. the large spread shadow of an oval-shaped box should be oval as well.

This in turn implies scaling border-radii. One could. for instance, adjust the border-radii of the spread 
shadow by multiplying  them by  (1+((2 *spread-radius)/max(width, height)).  Alternatively, one 
could attempt to obtain better fidelity by adjusting horizontal and vertical border radii independently.

Given that preserving the overall shape is desirable and that the number of alternatives to achieve it should
be fairly small, is this something implementors are interested in defining interoperably as part of this edit ?

[1] http://lists.w3.org/Archives/Public/www-style/2010Apr/0273.html
[2] http://www.w3.org/TR/2008/WD-css3-background-20080910/#the-box-shadow
Received on Wednesday, 28 April 2010 00:32:11 GMT

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