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

> On Sep 14, 2016, at 4:24 PM, /#!/JoePea <trusktr@gmail.com> wrote:
> 
> > Authors can work around this by applying opacity on their leaf elements individually, or by using rgba() colors etc.
> 
> That's not a valid workaround because in many cases there needs to be opacity on parent elements.
> 
> For example, imagine a game object which is a vehicle. The parent contains sub children positioned relatively to the parent (using tranforms) in order to place the doors, wheels, and body of the vehicle (maybe it is a truck, car, or motorcycle). Now, imagine in this game that the cars are alive (like in the movie "Cars", and one of them is a ghost. To achieve this, we apply an opacity to the parent element, which makes the whole car transparent. Or, imagine that there is a "ghost" car in a racing game in order to show where you were during your last attempt, so that you can attempt to beat your last time. We apply opacity to the whole car, and expect for thecar to remain 3D, not be flattened.
> 
> In cases like with the car examples, we cannot simply apply opacity to leaf-most elements. Furthermore, rgba() colors apply only to things like background, border, text colors, etc, but not to the whole element, so that's not a workaround.
> 
> The only workaround is to NOT use nested elements, but that prohibits the use of CSS transform caching that we get with the nested approach. It is imperative that the nested approach works along with opacity without flattening, otherwise this change is detrimental to 3D in the web, and is completely unintuitive. Making a 3D object flat because it has opacity simply does not make sense (it doesn't matter if that's what spec says).
> 
> For reference, the "nested" approach is the form of CSS3D that IE10/11 does not support, with preserve-3d.
> 
> Grouping is fine, as it makes much sense for all children to inherit the opacity level and have it multiplied with their own opacity (just like how positional transformations work), but additional flattening is simply not a good design choice.

Unfortunately you can't have one (grouping) without the other (flattening), at least with the 3D implementation in WebKit, and I suspect other UAs too. The way that preserve-3D works is to hoist descendant layers up to effectively become siblings of their parent/ancestor elements in the same 3D-rendering context, so they call get positioned and depth-sorted in the same 3D coordinate space. There's no way you can do group opacity in this model without rendering some subtree of transformed elements into a texture (which is flattening).

Simon

> 
> Please consider modifying the spec so that opacity does not destroy 3D scenes. Also please consider the same for mix-blend-mode, and similar effects.
> 
> Best regards,
> - Joe
> 
> /#!/JoePea
> 
> On Wed, Sep 14, 2016 at 3:58 PM, Simon Fraser <simon.fraser@apple.com <mailto:simon.fraser@apple.com>> wrote:
>> On Sep 14, 2016, at 3:51 PM, /#!/JoePea <trusktr@gmail.com <mailto:trusktr@gmail.com>> wrote:
>> 
>> The following is a big problem for people making 3D scenes, and I really think this needs to be rethought:
>> 
>> https://bugs.chromium.org/p/chromium/issues/detail?id=646993 <https://bugs.chromium.org/p/chromium/issues/detail?id=646993>
>> 
>> Not only does it break existing apps, but it also makes it difficult to create 3D HTML scenes that involve opacity. The intuitively expected behavior (as listed in that issue) is that opacity applies to children but does not cause unintuitive flattening.
>> 
>> If browsers follow the latest spec, then we are making it more difficult to do and discouraging 3D in the web, which I don't think is what we want.
>> 
>> The faulty part of the spec is here: https://drafts.csswg.org/css-transforms/#grouping-property-values <https://drafts.csswg.org/css-transforms/#grouping-property-values>
>> 
>> It states that opacity should cause grouping, which is what causes the flattening.
>> 
>> Is there an alternative way to achieve opacity less than 1.0 without flattening of 3D scenes, that I may have missed? Unless there's an alternative, this behavior is a bad idea for authors of 3D HTML scenes.
> 
> CSS opacity is defined to be "group opacity", which means rendering everything into a bitmap, and then paint that bitmap with alpha. This gives a different result to painting individual elements with opacity, when those elements overlap.
> 
> The CSS transforms spec is conforming to this group opacity behavior, which, in implementations, requires flattening at rendering time.
> 
> Authors can work around this by applying opacity on their leaf elements individually, or by using rgba() colors etc.
> 
> Simon
> 
> 
>