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Re: [css4-images] The image-set() function (for responsive images)

From: Brian Kardell <bkardell@gmail.com>
Date: Tue, 28 Feb 2012 18:15:42 -0700
Message-ID: <CADC=+je+y7mjBiDOxKF1_8YNNZVECP1z1Tej98Zvqj7EkVp64g@mail.gmail.com>
To: "Edward O'Connor" <eoconnor@apple.com>
Cc: www-style@w3.org
I am not commenting on the value of the specific proposal, just noting that
it feels like you really want a media() function.
On Feb 21, 2012 5:59 PM, "Edward O&apos;Connor" <eoconnor@apple.com> wrote:

> Hi all,
>
> It's all too easy for authors to make mistakes when adapting their sites
> for high-resolution displays such as the iPhone's Retina display.
> Consider the following stylesheet:
>
>  
>  selector {
>    background: url(foo-lowres.png) center;
>  }
>  
>  @media mq {
>    
>    selector {
>      background: url(foo-highres.png) center / 100px 100px;
>    }
>    
>  }
>  
>
> The references to the low- and high-resolution variants of foo.png are
> far apart from one another in the stylesheet, and the (potentially
> complicated) selector has been duplicated. On a large site with many
> image assets, this causes stylesheets to become very large and less
> maintainable. Here are just a few of the problems with the current
> situation:
>
> * Bugs due to non-locality: One developer fixes a bug in the selector,
>  but only in the low-resolution case. Another developer changes an
>  image reference to refer to a different icon, but only in the
>  high-resolution case.
>
> * Both assets may be loaded by the browser, which may degrade
>  performance in a constrained bandwidth environment.
>
> * Authors can't specify both assets inside a style="" attribute.
>
> I'd like to propose a new function for the Images module. This function
> will allow developers to provide, in a compact manner, multiple variants
> of the same image at differing resolutions. Using @media pushes the two
> asset references apart from one another, whereas such a function keeps
> related asset references together. It also helps keep selectors DRY.
> We've called it image-set(), and it takes one or more image specifiers.
> Image specifiers consist of an asset reference and a scale factor:
>
>  image-set( imagespec [ ',' imagespec ]* )
>  imagespec ::= <image> S {num} 'x'
>
> The above example could be rewritten using image-set() like so:
>
>  selector {
>    background: image-set(url(foo-lowres.png) 1x,
>                          url(foo-highres.png) 2x) center;
>  }
>
> By using image-set() here, the author is saying that foo-highres.png is
> twice the resolution of foo-lowres.png. UAs which support image-set()
> could then use the 2x image on a high-resolution display, and the 1x
> image on a low-resolution display. UAs aren't required to fetch the
> assets in order to determine which should be displayed, so we avoid
> redundant asset loading.
>
> Some Q&A:
>
> * What's the intrinsic size of an image-set()? Does it vary depending on
>  which image is picked? Does the UA apply the scale factor to derive
>  the intrinsic size of the image?
>
>  The intrinsic size of the image-set() can be computed from the
>  intrinsic size of the actual image asset chosen and that asset's
>  associated scale factor.
>
>  Suppose that foo-lowres.png is 100x100 and foo-highres.png is 200x200
>  in the above example. If the UA chooses foo-lowres.png, it computes
>  the intrisnic size as (100/1)x(100/1) = 100x100. If the UA chooses
>  foo-highres.png, it computes the intrisnic size as (200/2)x(200/2) =
>  100x100.
>
> * Why a scale factor and not a full-blown media query?
>
>  Media queries are a claim about the state of the UA, whereas here
>  we're making a claim about (the relationship between) the image assets
>  themselves. It would be confusing to use similar syntax for such
>  different things. Also, UAs should have the ability to choose between
>  the given variants based on a variety of factors. For instance, a UA
>  could use the lower res asset when a user has zoomed out. No existing
>  media query distinguishes between the page being zoomed-out and being
>  zoomed in. And even if such a media query existed, UAs should be free
>  to choose between variant assets regardless of which media queries
>  happen to match.
>
> * The problem of stylesheet verbosity when using media queries isn't
>  limited to image assets. Shouldn't we have a general mechanism for
>  keeping media-specific property values together in rule sets?
>
>  In talking with content producers here, we've found that the
>  non-locality of asset references is a real source of Web author pain.
>  I think a focused feature to help with that pain is sensible. That
>  said, we should also explore how to help authors avoid selector
>  repetition and the other pains of @media in general.
>
> * Why not enhance the image() function instead of inventing a new
>  function?
>
>  Unlike image(), these image specifiers are unordered. This isn't about
>  fallback. There is no preferred variant. UAs are free to use whichever
>  image it believes would be best. For instance, consider the page zoom
>  example I mentioned earlier. Authors could even use image() and
>  image-set() together to handle more exotic cases, e.g.:
>
>    image(image-set(url(foo.png) 1x, url(foo@2x.png) 2x) rtl,
>          image-set(url(bar.png) 1x, url(bar@2x.png) 2x) ltr);
>
> * What about other cases where authors would like to specify alternative
>  image assets, such as when the UA is on a low- or high-bandwidth
>  connection?
>
>  This is definitely something worth exploring. In the future we could
>  extend the asset descriptors to cover such cases. Something like this,
>  maybe:
>
>    selector {
>      background: image-set(url(foo-lowres.png) 1x low-bandwidth,
>                            url(foo-highres.png) 2x high-bandwidth);
>    }
>
>  I don't have a proposal for how to describe bandwidth here, though,
>  and I'd love to hear ideas. I don't think addressing the multiple-
>  resolutions case needs to wait for a solution to the bandwidth case.
>
> * What about content images?
>
>  First off, this is www-style, so the design of an HTML feature for
>  responsive <img>es is out of scope. :) That said, I can imagine the
>  image-set() microsyntax being used in an attribute like so:
>
>    <img alt="A description of foo"
>         src=foo-lowres.png
>         set="foo-lowres.png 1x, foo-highres.png 2x">
>
>  I'll post something to the whatwg thread referencing this proposal.
>
>
> Ted
>
>
Received on Wednesday, 29 February 2012 01:16:10 GMT

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