Re: [css-transforms] rendering 3D-transformed sibling elements without preserve-3d

On Thu, Mar 12, 2015 at 5:48 AM, Simon Fraser <smfr@me.com> wrote:

>
> On Mar 10, 2015, at 9:27 pm, Robert O'Callahan <robert@ocallahan.org>
> wrote:
>
> Consider example 7 here:
> http://dev.w3.org/csswg/css-transforms/#3d-rendering-contexts
>
> AFAICT this doesn't match what Chrome and Firefox do (can't test IE). In
> those browsers, sibling elements can't intersect each other unless
> preserve-3d is involved somehow. In those browsers, in the absence of
> preserve-3d, 3D-transformed children are transformed, projected and
> composited in regular CSS order. I suspect that changing that will create
> major Web compat issues.
>
> Or am I misunderstanding something?
>
>
> Indeed this is a behavior change from my spec re-write which was done to
> more rigorously define what preserve-3d means, which altered the meaning
> from “start doing depth-sorting and intersection” to “make the children
> live in the same 3d space as the current element”, and those don’t mean the
> same thing.
>
> I’m not sure how to resolve these two competing uses of preserve-3d. Both
> have shipped, but I suspect that most content is authored to the
> WebKit/Blink behavior, which is the "make the children live in the same 3d
> space as the current element” behavior, and I can find tutorials that
> describe this behavior, but not that describe the  “start doing
> depth-sorting and intersection” behavior.
>

I'm worried about pages where there are sibling elements using 3D
transforms for perspective effects, but normal CSS painting order is
assumed. I believe that's what Webkit, Blink and Gecko all do in the
absence of preserve-3d. (Am I wrong?) I admit I don't have any examples of
such pages at hand, but does that not worry you?

If the spec stays as-is, with non-3D content rendered at z=0, then I think
we'll have even more problems with 3D-transformed elements unexpectedly
falling partially or wholly behind the background of their container.

Rob
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