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Re: Additional value for the visibility property

From: David Hyatt <hyatt@apple.com>
Date: Wed, 09 Jul 2008 13:46:25 -0500
To: Andrew Fedoniouk <news@terrainformatica.com>
Cc: robert@ocallahan.org, "Ph. Wittenbergh" <jk7r-obt@asahi-net.or.jp>, W3C Style List <www-style@w3.org>
Message-id: <801F4EF1-14D9-4F1D-B920-86FA33547411@apple.com>

On Jul 9, 2008, at 1:26 PM, Andrew Fedoniouk wrote:

>
> Robert O'Callahan wrote:
>> On Wed, Jul 9, 2008 at 3:27 PM, Andrew Fedoniouk <news@terrainformatica.com 
>>  <mailto:news@terrainformatica.com>> wrote:
>>
>>    That is what I would like to clarify - how exactly it should be
>>    rendered.
>>
>>
>> I think it's clear that according to the spec it should be rendered  
>> the way Gecko and Webkit render it.
>>
>> If you're suggesting that the spec should be changed --- this has  
>> been specified and interoperably implemented in Gecko and Webkit  
>> for years, so I think you'd need a pretty strong argument.  
>> Personally it feels unnatural to me to render an element and its  
>> descendants as a single composition group but carve out an  
>> exception for descendants that happen to be out-of-flow. (Although  
>> I'm not actually sure what you're proposing, since there might be  
>> descendants which are out-of-flow but still have the element as  
>> their containing block ancestor.)
> Opacity is an attribute of some layer. Element and its in-flow  
> descendants is a layer. absolute positioned elements
> establish their own layers.
>
> It appears that Opera:
> 1) draws element and only in-flow children on the offscreen buffer  
> (layer)
> 2) each absolute positioned element - descendant of transparent  
> parent - inherits value of opacity and draws
> itself on separate offscreen buffer (layer).
> 3) these buffers are blended separately with respect of z-order.
>
> That appear as the only correct way of doing this.
>
> FF and WebKit share the same error. Take a look on these samples:
>
> http://terrainformatica.com/w3/opacity.htm
> http://terrainformatica.com/w3/no-opacity.htm
>
> These two files are the same except of transparency.
> Note that FF and WebKit simply ignore value of z-index when opacity  
> is applied.

FF and WebKit have the correct rendering.  Opacity is supposed to  
establish a stacking context (it's as though a z-index of 0 is  
specified) precisely to avoid the problem of requiring multiple  
buffers.  You should never have to use separate buffers.  The spec  
vaguely implies this when it mentions that an element including its  
children should blend as a unit.  To use separate buffers contradicts  
what the spec is saying.

This issue will be clarified in the spec.

See Issue 26:

http://csswg.inkedblade.net/spec/css3-color

http://lists.w3.org/Archives/Public/www-style/2008May/0147.html

dave
(hyatt@apple.com)
Received on Wednesday, 9 July 2008 18:47:06 GMT

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