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[css-filters-2] Clarify 'backdrop-filter' in circular case

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Thu, 13 Aug 2015 13:44:20 -0700
Message-ID: <CAAWBYDCSbxKba2FpwzExRBqoV3GX4F4==rYSh2VNusJqux2mZA@mail.gmail.com>
To: "public-fx@w3.org" <public-fx@w3.org>
In our recent Intent To Implement for backdrop-filter, Enne brought up
a case that is seemingly circular:

<body>
  AAA
  <div style="opacity: .5">
    BBB
    <div style="backdrop-filter: blur(5px)">foo</div>
  </div>
</body>

What does BBB look like blur filter?  Is it half-opaque, as if you'd
already done all compositing of its ancestors?  If that's the case,
then when you composite the page for real, it'll get double-opacity'd,
right?

Or is it fully opaque, like you'd pretended the opacity effect doesn't
yet apply for the purpose of calculating the backdrop?  Presumably
this would mean recursively unwrapping any effects up to the root?

Or is there something else going on?  The spec isn't written clearly
enough for me to figure this out, but *maybe* the backdrop only goes
to the nearest isolation context, so the backdrop in this case would
just be BBB?

~TJ
Received on Thursday, 13 August 2015 20:45:08 UTC

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