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Adaptive images

From: Dominique Hazael-Massieux <dom@w3.org>
Date: Mon, 30 May 2011 15:43:39 +0200
To: public-html@w3.org
Message-ID: <1306763027.2519.83.camel@altostratustier>
Hi,

CSS Media Queries (combined with <meta name=viewport>) provide a
reasonable way to create layouts that adapt to the size of the screen of
the device.

But one aspect that remains difficult to manage in such an approach is
handling of content images.

Indeed, HTML currently allows to specify only one version of an image as
an input source, which for raster images means a single resolution of
image. Sending very large images to devices were the layout (or the
screen resolution) wouldn't be able to fit them well is in most cases a
waste of bandwidth. Conversely, sending very small images to very large
screen doesn't take advantage of the underlying device capabilities.

I've gotten requests many times for a mechanism by which an author could
point to several sources for a given image that would be selected by the
client depending on the screen size.

There are server-side techniques to do that (e.g. tinySrc.net), but they
rely on databases of devices that have to be maintained and which don't
necessarily scale to all usage.

A client-side mechanism similar to what media queries allow for layout
would likely prove very useful.

Some ideas on how this could be done:

1. New @srclist attribute point to list of sources
        <img src=default.jpg srclist=alternativeSizes alt="Picture of
        Unicorn">
        <sourcelist id=alternativeSizes>
        <source src="big.jpg" media="min-width: 600px" width="600"
        height="400">
        <source src="small.jpg" media="max-width: 600px" width="320"
        height="320">
        </sourcelist>

(<sourcelist> could maybe be replaced by another existing element?
<map>? <datalist>? SVG's <switch>?)

2. New image format that points to alternative resolutions
        <img src="image.list" alt="Picture of Unicorn>
        
        image.list:
        Content-Type: image/list
        
        URI: big.jpg
        media: (min-width: 600px)
        
        URI: small.jpg
        media: (max-width: 600px)

(maybe the images list could also be embedded in a <script
type='image/list'>?)

Advantage of #1 is that the author still gets to define the actual size
of the images, which means no reflowing when the image get downloaded.

#2 allows to avoid redefining over and over the alternative versions of
an image.

I guess that only #1 actually requires modifications to HTML; #2 would
probably be more a matter of writing an IETF draft.

Any suggestions or feedback on this?

I'm planning on adding this message to the HTML.next [1] page once it's
archived.

Dom

1. http://www.w3.org/wiki/HTML/next
Received on Monday, 30 May 2011 13:43:55 GMT

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