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

From: Simon Fraser <smfr@me.com>
Date: Tue, 27 Apr 2010 20:07:53 -0700
Cc: Sylvain Galineau <sylvaing@microsoft.com>, "www-style@w3.org" <www-style@w3.org>, Brian Manthos <brianman@microsoft.com>, Prabs Chawla <pchawla@microsoft.com>, Bert Bos <bert@w3.org>
Message-id: <1B710B79-9FC1-4770-BCBA-858C5702D0BF@me.com>
To: fantasai <fantasai.lists@inkedblade.net>
On Apr 27, 2010, at 6:32 PM, fantasai wrote:

> On 04/27/2010 05:31 PM, Sylvain Galineau wrote:
>> 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 ?
> 
> Hm, I didn't realize this was ambiguous.
> 
> The simplest definition would be:
>  * When border-radius is zero, the shadow's radius is also zero.
>  * When border-radius is nonzero, the shadow's radii are increased (or decreased)
>    by the spread value. For circular curves, this effectively moves all points on
>    the curve outward by the same amount the sides move outward.
> 
> However, that might not give a correct "spread" effect for ellipses: I suspect the
> concept of pushing the curve "outward" does not work for elliptical curves because,
> IIRC, elliptical tangents are not perpendicular to radii. (I suspect pushing the
> curve outward--in the direction perpendicular to the curve at each point on the
> curve--would result in something that is not an ellipse.)

I don't understand what the 'spread' value is trying to do. Can someone post links to some pictures?

Simon
Received on Wednesday, 28 April 2010 03:08:29 GMT

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