W3C home > Mailing lists > Public > www-style@w3.org > June 2014

Re: [compositing] background-blend-mode on the root element

From: Rik Cabanier <cabanier@gmail.com>
Date: Wed, 25 Jun 2014 09:16:13 -0700
Message-ID: <CAGN7qDB52KdmEP096wGPRrn-N84yXA2OxykSUSprrwct0K4eww@mail.gmail.com>
To: Alan Gresley <alan@css-class.com>
Cc: www-style list <www-style@w3.org>, "public-fx@w3.org" <public-fx@w3.org>
On Wed, Jun 25, 2014 at 6:53 AM, Alan Gresley <alan@css-class.com> wrote:

> On 21/06/2014 2:42 AM, Rik Cabanier wrote:
>
>> On Thu, Jun 19, 2014 at 11:29 PM, Alan Gresley <alan@css-class.com>
>> wrote:
>>
>>  On 20/06/2014 12:40 PM, Rik Cabanier wrote:
>>>
>>>  All,
>>>>
>>>> the background-blend-mode [1] property recently landed in Firefox and
>>>> Chrome and we noticed that a very common design is to use it to create
>>>> patterns with gradients. [2]
>>>>
>>>> The spec states:
>>>>
>>>> Each background layer must blend with the element’s background layer
>>>> that
>>>> is below it and the element’s background color. Background layers must
>>>> not
>>>> blend with the content that is behind the element, instead they must act
>>>> as
>>>> if they are rendered into an isolated group.
>>>>
>>>>
>>>> This works for most elements except for the root element which has a
>>>> different paint order [3]:
>>>>
>>>>
>>> It's not a different painting order but different painting method.
>>>
>>>
>>>  The background of the root element becomes the background of the canvas
>>>> and
>>>> covers the entire canvas, anchored (for 'background-position') at the
>>>> same
>>>> point as it would be if it was painted only for the root element itself.
>>>> The root element does not paint this background again.
>>>>
>>>>
>>> Note that the root element may not be <html> but something like <svg> or
>>> possibly <any-name-element> if using XML. This is why the spec mentions
>>> root element here since CSS can style different web languages.
>>>
>>>
>>>  All browsers implement painting of the root backdrop by drawing the
>>>> background color and images on top of a 100% opaque white backdrop.
>>>>
>>>>
>>> This is not quite true. In the Firefox setting for 'Content > Colors', IE
>>> setting 'Appearance > Colors' and Opera setting for 'Preferences >
>>> Webpages', I can change the background/backdrop of the root element to at
>>> least a minimum of 70 colors. Internally, each UA has a hidden element
>>> wrapping the root element. For old Opera (not sure about Blink), this
>>> was a
>>> black backdrop.
>>>
>>
>>
>> This is unspecified behavior.
>>
>
> What part of the above is unspecified behavior?
>
> For legacy reasons, I don't believe it is wise to remove user control of
> the appearance of the backdrop. Some users use these browsers for
> accessibility reasons since they can control colors.


I didn't say that it should be removed. :-)



>  Prior to blending, this did not make a difference but now we need to know
>> that there could be another color 'layer' for the root element.
>>
>
> Upon testing, I find that Firefox does *not* allow any blending of any
> background layers that are painted to the root element but Chrome does. A
> test.
>
> http://css-class.com/test/temp/blending-backdrop.htm
>
> Firefox shows red, lime and blue where Chrome shows cyan, fuchsia and
> yellow since the 'background-blend-mode' has the value of 'exclusion'.


You are correct that Firefox is different. I must have tested with the
wrong file since I can no longer reproduce the issue there.


>   Because of this, background images that would blend with transparency
>>>> (which doesn't change the color), now blend with white and display
>>>> differently.
>>>>
>>>>
>>> Agree. An image with semi-transparent red rgba(255,0,0,0.5), the blended
>>> color would paint an opaque 'salmon'. If a user had white text and black
>>> background/backdrop for accessibly reasons, the blended color would paint
>>> opaque 'maroon'.
>>>
>>
>>
>> Yes, or if you use any color with 'screen', the content completely
>> disappears.
>>
>
> Not in Firefox (see test below) but yes in Chrome.
>
> http://css-class.com/test/temp/blending-backdrop2.htm
>
>
>  My question is, should we keep it as specified and make a note in the spec
>>>> that the root element behaves differently, or should we fix the
>>>> implementation so background images blend on a transparent backdrop
>>>> followed by matting and clarify the paint order in css colors?
>>>>
>>>> I think authors would prefer the latter but they can work around it if
>>>> needed.
>>>>
>>>>
>>> For accessibly reasons, background images should blend on a transparent
>>> backdrop.
>>>
>>
>>
>> I agree that authors would prefer that the blending happens in a
>> transparent group. I'm unsure what that has to do with accessibility
>> though. Can you explain?
>>
>
> If you look at the test above, you will see that Firefox has already taken
> control of the painting of the root element. What I mean by accessibility
> is that if I user needs to have white text of a background to read, then a
> user should be able to override an author style sheet.
>
> See attached screenshot named 'colors-settings.jpg'. If I un-tick the box
> with the text 'Allow pages to choose their own colors, instead of my
> selection above', then I would expect to see all backgrounds of any element
> colored black and all text colored white.


Ok, so it seems that Firefox is following the model here where they treat
the body's background color normally.
WebKit and Blink seem to be adding another painting pass for the background
color which is interfering with the background blending.
Received on Wednesday, 25 June 2014 16:16:41 UTC

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