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Re: [CSSWG] Minutes and Resolutions 2009-02-04: box-shadow and border-image

From: David Hyatt <hyatt@apple.com>
Date: Thu, 12 Feb 2009 16:24:55 -0600
Cc: robert@ocallahan.org, Brad Kemper <brad.kemper@gmail.com>, fantasai <fantasai.lists@inkedblade.net>, "Tab Atkins Jr." <jackalmage@gmail.com>, "www-style@w3.org" <www-style@w3.org>
Message-id: <511E1DBB-149A-494B-A3F7-07BDF6865973@apple.com>
To: David Hyatt <hyatt@apple.com>
On Feb 12, 2009, at 4:22 PM, David Hyatt wrote:

> On Feb 12, 2009, at 4:15 PM, Robert O'Callahan wrote:
>> On Fri, Feb 13, 2009 at 10:52 AM, Brad Kemper  
>> <brad.kemper@gmail.com> wrote:
>> On Thu, Feb 12, 2009 at 12:22 PM, Robert O'Callahan <robert@ocallahan.org 
>> > wrote:
>> On Fri, Feb 13, 2009 at 8:43 AM, Brad Kemper  
>> <brad.kemper@gmail.com> wrote:
>> 1. How to deal with soft image edges, where there is variable  
>> opacity? Currently, WebKit seems to clip shadows to the vector edge  
>> of the border box, and not draw them behind areas of variable  
>> opacity (such as backgrounds). It seems like for a raster image  
>> that was 50% opaque, it would have draw the shadow behind the whole  
>> image at 50% of its normal opacity.
>> That's the way to go. What you would do is construct a mask the  
>> size of the border-box whose alpha values in the border are the  
>> border-image's alpha values, and whose alpha values in the padding- 
>> box are 1. Then you apply blur to that mask (or not)
>> ..and then clip out the padding-box from that, which ordinarily  
>> would not have a shadow from the border cast onto it (or does this  
>> shadow still get drawn behind the padding box?)...
>> You build the element's mask, then you build the shadow, then you  
>> cut the mask out of the shadow by painting alpha=0 with the  
>> "source" Porter-Duff operator using the element's mask as the mask.  
>> The alpha values of the cut-out shadow will be the shadow alpha  
>> value multiplied by (1 - the mask alpha value). So no shadow will  
>> draw under the padding-box.
> It's a given that the shadow itself when offset will contain only  
> transparent pixels in its own offset padding are

"Pre-blur" that is.  Obviously blurring it could result in pixels in  
the offset padding area. ;)

Received on Thursday, 12 February 2009 22:25:38 UTC

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