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Re: [css-transforms] CSS3D breaks with opacity flattening

From: Matt Woodrow <mwoodrow@mozilla.com>
Date: Thu, 22 Sep 2016 10:08:32 +1200
To: "Tab Atkins Jr." <jackalmage@gmail.com>, Rik Cabanier <cabanier@gmail.com>
Cc: /#!/JoePea <trusktr@gmail.com>, Chris Harrelson <chrishtr@google.com>, Simon Fraser <simon.fraser@apple.com>, "public-fx@w3.org" <public-fx@w3.org>
Message-ID: <180afdbc-96f5-6def-9ba2-b1dcb5c6e367@mozilla.com>
On 21/09/16 10:17 PM, Tab Atkins Jr. wrote:

> On Wed, Sep 21, 2016 at 2:57 AM, Rik Cabanier <cabanier@gmail.com> wrote:
>> I have a counter-example here: http://codepen.io/adobe/pen/WGoKjA
>> If you have the following markup:
>> <div id="wrapper">
>>    <div id="addalpha">
>>      <div id="sq1">Square1</div>
>>      <div id="sq2">Square2</div>
>>    </div>
>>    <div id="sq3">Square3</div>
>> </div>
>> With your proposal, sq1 and sq 2 will be placed in the world, rendered to
>> texture and then have alpha applied.
>> However, sq3 is in between sq1 and sq2. How should this render?
> Or to show it off slightly better:
> <https://codepen.io/anon/pen/ALpgGd>. Left is with the group opacity
> applied to the #addalpha div, right is without that opacity (both have
> opacity applied to the individual squares).

Great, these are good examples. I assume addalpha was meant to have 
preserve-3d, otherwise we don't actually have any transforms being combined.

In this case, I feel that the author really has requested conflicting 
things, they want sq1 and sq2 to be rendered as an opacity group, but 
they also want sq3 to be rendered in the middle. It's not obvious that 
there is a correct solution to this, so it seems like we're down to 
picking the best fallback.

I fixed the missing preserve-3d, and changed the leaves background-color 
opacities to 1 (to make occlusion more visible) and we get these three 
different renderings:

Safari: http://people.mozilla.org/~mwoodrow/safari-sorted-planes.png
Chrome: http://people.mozilla.org/~mwoodrow/chrome-sorted-planes.png

You can see that Safari isn't doing group opacity, but gets the full 3d 
scene with sorting correct.

Chrome is applying the group opacity without the world transform 
applied, so loses sq2 entirely and gets what's left of the 3d scene 
sorted correctly.

Firefox-with-prototype is applying the group opacity, getting the 3d 
scene correct, but is sorting sq3 behind the other two.

It's my opinion that the Firefox prototype is the 'least bad' choice of 
these three, especially given that it's noticeably superior for my cube 

Sidenote: Without preserve-3d on addalpha, the Firefox prototype 
rendering is really broken and sq1 and sq2 are drawn in the wrong place 
entirely (you can see them if you switch to full page view). This is a 
silly bug I introduced, and not inherent to the approach.

- Matt
Received on Wednesday, 21 September 2016 22:09:07 UTC

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