Re: [css-compositing] why general blending is confusing

On Sat, Oct 12, 2013 at 3:20 AM, Robert O'Callahan <robert@ocallahan.org>wrote:

> On Sat, Oct 12, 2013 at 5:04 AM, Dean Jackson <dino@apple.com> wrote:
>
>> On 12 Oct 2013, at 7:00 pm, Robert O'Callahan <robert@ocallahan.org>
>> wrote:
>>
>> I'm not sure what problem you're pointing to. Are you saying the spec is
>> unclear
>>
>>
>> No.
>>
>
> Hmm OK. Your most recent message ("Should the image be blending with its
> sibling?") suggested you consider the spec unclear.
>
>
>> , or that it's too hard to implement in some situations (which
>> situations?)
>>
>>
>> It will depend on what the specification requires, but it will be hard to
>> implement
>> something that produces the same output for examples 3 and 4 in my test.
>>
>
> According to the spec they should produce different output. In example 4
> the CSS transform makes the blendingbg element a stacking context,
> therefore an isolated group, and therefore the 'content' element does not
> blend into the page background (it blends into the transparent black
> background of the isolated group instead).
>
> Mostly this, yes. I don’t think an author will understand when layers are
>> created,
>> and why behaviour changes dramatically when they are.
>>
>
> The implicit creation of an isolated group for every stacking context may
> be confusing, but it's much easier to implement, more performant, and
> probably less confusing in edge cases than any other proposal on the table.
>

Yes, this is exactly why we decided on the current wording.

Hopefully better tooling (which is already starting to land in browsers)
will help and educate users about when stacking contexts appear and what
they apply to.
I think graphics designers will grasp the concept of isolation since this
idiom has been available in graphics tools for many years now. Flash had
the same behavior where a blend mode on a movieclip would isolate and I
don't recall hearing many complaints.

For level 2 of the spec, I will put non-isolated blending back in (as a new
keyword for 'isolated') but we'll have to see if hardware and operating
systems are able to support it.


>
> However, if the WG are happy with this, and we can describe all of the
>> complicated ways a page may be composited (in a cross browser way),
>>
>
> As far as I know the spec is already clear enough.
>
> Rob
> --
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