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Re: [css4-images] element() behavior

From: Rik Cabanier <cabanier@gmail.com>
Date: Wed, 1 Aug 2012 11:12:41 -0700
Message-ID: <CAGN7qDBV0GEQN6ZKuZeWDtv5CLEZq6Hhyit2LvttPj_Jhmvf8A@mail.gmail.com>
To: Andrew Fedoniouk <news@terrainformatica.com>
Cc: www-style list <www-style@w3.org>
For instance:

<style>
#src { color: white; background: lime; width: 300px; height: 40px; }
#dst { color: black; background: element(#src); padding: 20px; margin: 20px
0; }
</style>

<div style="opacity:.5;">
  <p>more text</p>
  <p id='src'>I'm an ordinary element!</p>
  <p>even more text</p>
</div>
<p id='dst''>I'm using the previous element as my background!</p>


The text with id 'src' is rendered with opacity.
Internally, the alpha will cause the browser to create a stacking context
[1] (which is basically a bitmap).
The 3 text blocks are rendered into this bitmap and then alpha is applied
to it.

At no point will the browser have a transparent version of just the text
with id 'src' in memory, nor does it know what will happen to the text
while it's rendering it.
This means that the code behind element() has to find out how to do this.

1:
https://developer.mozilla.org/en/CSS/Understanding_z-index/The_stacking_context

On Tue, Jul 31, 2012 at 9:20 PM, Andrew Fedoniouk <news@terrainformatica.com
> wrote:

> This "the element has alpha from an ancestor"
> is not clear. What does it mean?
>
> --
> Andrew Fedoniouk.
>
> http://terrainformatica.com
>
> On Tue, Jul 31, 2012 at 9:05 PM, Rik Cabanier <cabanier@gmail.com> wrote:
> > I'm a bit puzzled by this statement in the spec [1] (highlight is mine):
> >
> > Implementations may either re-use existing bitmap data generated for the
> > referenced element or regenerate the display of the element to maximize
> > quality at the image's size (for example, if the implementation detects
> that
> > the referenced element is an SVG fragment); in the latter case, the
> layout
> > of the referenced element in the image must not be changed by the
> > regeneration process. That is, the image must look identical to the
> > referenced element, modulo rasterization quality.
> >
> >
> > Does this mean that if the element has alpha from an ancestor, the
> element()
> > will generate a bitmap with that alpha, or is the alpha ignored or
> blended?
> > What if an ancestor has a css filter (or blending/compositing), would the
> > element() return part of the filtered bitmap?
> > What if one of your child elements has alpha and is alpha blending with
> an
> > ancestor. Will element() return a bitmap with alpha, or a blended image?
> >
> > I can see the intent of the spec, but it seems hard to implement.
> > Maybe you could say that element() can only reference elements that
> > establish a context or elements that don't contain another context.
> > The browser could then use the rasterized image of the context or raster
> the
> > element at that point. This might be easier to define and certainly
> > implement.
> >
> > 1: http://dev.w3.org/csswg/css4-images/#element-notation
>
Received on Wednesday, 1 August 2012 18:13:09 GMT

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