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

I don't think group opacity can be properly defined without
flattening. What if some planes in the group are non-opaque themselves
and later intersect with non-opaque planes outside of the group?

Specifically, how would you define the expected behavior for this
particular example? http://jsbin.com/tepixi/

On Wed, Sep 21, 2016 at 3:08 PM, Matt Woodrow <mwoodrow@mozilla.com> wrote:
> 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
> Firefox-with-prototype:
> http://people.mozilla.org/~mwoodrow/firefox-with-prototype-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
> demo.
>
> 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
>