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Re: [css-device-adapt] Decoupling zoom and width/height

From: Kenneth Rohde Christiansen <kenneth.christiansen@gmail.com>
Date: Wed, 14 Nov 2012 15:29:09 +0100
Message-ID: <CAEC208vaSRqgPNZhQMiJVdQYcWKw191zKDGtcsoFi-q5pAx6-A@mail.gmail.com>
To: Rune Lillesveen <rune@opera.com>
Cc: www-style@w3.org
Why do we have to map it to the @viewport syntax? I believe for WebKit we
will just have a separate code path for the meta element, which ensures
that it keeps the same algorithm as always.

Cheers
Kenneth

On Wed, Nov 14, 2012 at 10:27 AM, Rune Lillesveen <rune@opera.com> wrote:

> On Fri, 02 Nov 2012 21:45:39 +0100, Florian Rivoal <florian@rivoal.net>
> wrote:
>
>  On Wed, 31 Oct 2012 08:58:14 +0100, Rune Lillesveen <rune@opera.com>
>> wrote:
>>
>>  The current constraining procedure has a coupling between zoom and auto
>>> widths/heights (6-9) and extending widths/heights to fill the viewport for
>>> a non-auto zoom (10-11).
>>>
>>> [...]
>>>
>>> I would like to decouple the zoom from width/height by letting auto
>>> width/height rely on non-auto width/height and initial viewport only.
>>>
>>> [...]
>>>
>>> Proposal, rule-wise:
>>>
>>> Replace step 6 and 7 with:
>>>
>>>    - If width and height are both ‘auto’, set width = initial-width
>>>
>>> and drop step 10 and 11.
>>>
>>> I have not worked out the details for the meta viewport to @viewport
>>> mapping in chapter 11 to keep the same meta viewport behavior, but it looks
>>> doable. I'll get back to that.
>>>
>>
>> This makes sense to me. Assuming a semantics preserving mapping with the
>> meta viewport is possible (which I don't doubt), I think we should do
>> this,
>> and that it is a good time to do it. The only two implementations I know
>> of are Opera's, which is willing to change, and Microsoft, whose support
>> is currently limited to parts not influenced by this change.
>>
>
> I needed to add a UA-internal value which I've called 'extend-to-zoom' to
> <viewport-length> to preserve the meta viewport mapping. I have implemented
> this, and my 128 viewport meta tests still pass.
>
>
> I've ended up with this constraining procedure, which should be what I
> proposed (+ moved the zoom resolution steps to the beginning):
>
> == Resolve min-zoom and max-zoom values ==
>
> 1. If min-zoom is not ‘auto’ and max-zoom is not ‘auto’, set max-zoom =
> MAX(min-zoom, max-zoom)
>
> == Constrain zoom value to the [min-zoom, max-zoom] range ==
>
> 2. If zoom is not ‘auto’, set zoom = MAX(min-zoom, MIN(max-zoom, zoom))
>
> == Resolve non-‘auto’ lengths to pixel lengths ==
>
> 3. Resolve relative and absolute lengths, percentages, and keywords
> (‘device-width’, ‘device-height’) to pixel values for the ‘min-width’,
> ‘max-width’, ‘min-height’ and ‘max-height’ descriptors.
>
> == Resolve initial width and height from min/max descriptors ==
>
> 4. If min-width or max-width is not ‘auto’, set width = MAX(min-width,
> MIN(max-width, initial-width))
>
> 5. If min-height or max-height is not ‘auto’, set height = MAX(min-height,
> MIN(max-height, initial-height))
>
> == Resolve width value ==
>
> 6. If width and height are both ‘auto’, set width = initial-width
>
> 7. If width is ‘auto’, set width = height * (initial-width /
> initial-height)
>
> == Resolve height value ==
>
> 8. If height is ‘auto’, set height = width * (initial-height /
> initial-width)
>
> -------------------------
>
> But there are quite a bit of changes to the meta viewport chapter:
>
> Changed UA styles in chapter 11:
> ------------------------------**--
>
> @viewport {
>   width: extend-to-zoom 980px;
>   min-zoom: 0.25;
>   max-zoom: 5;
> }
>
>
> Description of the new ‘extend-to-zoom’ value:
> ------------------------------**----------------
>
> 11.3. ‘extend-to-zoom’
>
> In order to be able to implement the functionality from META viewport
> where the viewport width or height is extended to fill the viewing area at
> a given zoom level, we introduce a UA internal value to the list of
> <viewport-length> values called ‘extend-to-zoom’ It will be used in width
> and height declarations in the translation outlined in the section below.
>
> This new value is necessary in order to implement the mapping for two
> reasons. First, whether resolving the width/height needs to extend the
> pixel length to the visible width/height for a given zoom factor, depends
> on the current initial width/height. <meta name="viewport"
> content="width=400, initial-scale=1"> yields a width of 400px for an
> initial-width of 320px, and 640px for an initial width of 640px. This can
> not be expressed as normative min/max descriptors that would constrain
> correctly when the initial width changes like for an orientation change.
>
> Secondly, the extended width/height also relies on cascading viewport
> properties from different sources, including ‘min-zoom’ and ‘max-zoom’ from
> the UA stylesheet. For instance, if the UA stylesheet has max-zoom: 5, and
> the initial width is 320px, <meta name="viewport" content="width=10"> will
> resolve to 48px.
>
>
> How to resolve ‘extend-to-zoom’ to pixel length or auto:
> ------------------------------**--------------------------
> (Since this is non-normative, I've added the description to chapter 11,
> but when supported, it needs to be done as part of step 3 of the
> constraining procedure.)
>
>
> 11.3.1 Resolving ‘extend-to-zoom’
>
> The ‘extend-to-zoom’ value is resolved to pixel or auto lengths as part of
> step 3 of the constraining procedure. Since this is a non-normative
> descriptor value, the resolution is described here. Note that
> max-descriptors need to be resolved to pixel lengths before min-descriptors
> when ‘extend-to-zoom’ is a valid value.
>
> Let extend-zoom = MIN(zoom, max-zoom)
>
> For non-‘auto’ extend-zoom, let:
>
>  extend-width = initial-width / extend-zoom
>  extend-height = initial-height / extend-zoom
>
> For max-descriptors, ‘extend-to-zoom’ resolves as follows:
>
>  - If extend-zoom is ‘auto’, set max-width = ‘auto’ or max-height = ‘auto’
>  - If extend-zoom is non-‘auto’, set max-width = extend-width or
> max-height = extend-height.
>
> For min-descriptors, ‘extend-to-zoom’ resolves as follows:
>
>  - If extend-zoom is ‘auto’, set min-width = max-width or min-height =
> max-height
>  - If extend-zoom is non-‘auto’, set min-width = MAX(extend-width,
> max-width) or min-height = MAX(extend-height, max-height).
>
>
> Changes in translation of viewport meta properties to width/height
> descriptors:
> ------------------------------**------------------------------**
> -------------------
>
> The width and height viewport META properties are translated into ‘width’
> and ‘height’ descriptors, setting the ‘min’ value to ‘extend-to-zoom’ and
> the ‘max’ value to the pixel length from the viewport META property.
>
> For a viewport META element that translates into an @viewport rule with a
> non-‘auto’ ‘zoom’ declaration and no ‘width’ declaration:
>
> - If it adds a ‘height’ descriptor, add "width: auto;"
> - Otherwise, add "width: extend-to-zoom;"
>
>
> --
> Rune Lillesveen
>
>


-- 
Kenneth Rohde Christiansen
Senior Engineer, WebKit, Qt, EFL
Phone  +45 4093 0598 / E-mail kenneth at webkit. <http://gmail.com>org

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Received on Wednesday, 14 November 2012 14:29:58 GMT

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