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Re: Overflow-clipping of filters with fixed-pos/abs-pos descendants

From: Robert O'Callahan <robert@ocallahan.org>
Date: Wed, 14 Jan 2015 10:20:52 +1300
Message-ID: <CAOp6jLaiechpP2X9MrEHHNJeSL=r4FNTEpMCXaUk4PdWYbHw9g@mail.gmail.com>
To: Simon Fraser <smfr@me.com>
Cc: "public-fx@w3.org" <public-fx@w3.org>
On Tue, Jan 13, 2015 at 10:09 PM, Robert O'Callahan <robert@ocallahan.org>
wrote:

> Conceptually, we avoid clipping the translucency group, and instead push
> all clipping down to its descendants, which allows us to apply different
> clips to different descendants.
>
> We can't take the same approach for 'filter', for the reasons described at
> the beginning of this thread. Although 'filter' and 'opacity' are
> "essentially the same" in some sense, 'opacity' is an important special
> case because the above approach works.
>

A better way to explain this is that 'opacity' has the nice property that
it commutes with clipping. I.e., applying opacity to an element E and
clipping the result always gives the same result as clipping element E and
then applying opacity.

In general, 'filter' doesn't have this property. I think 'mix-blend-mode'
currently does, although it wouldn't if we allowed the full set of
Porter-Duff operators.

Rob
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Received on Tuesday, 13 January 2015 21:21:20 UTC

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