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Image Scaling for High-Pixel-Density (i.e. Retina) Displays - Re: -webkit-image-set and <image>

From: Tom Penzer <tpenzer@mailcan.com>
Date: Fri, 27 Apr 2012 12:47:50 -0700
Message-Id: <ABE9E2D8-F2E7-4B6F-A6BE-31DB4D46537F@mailcan.com>
To: public-html-comments@w3.org
Hi everybody,

I'm seeking feedback for my (hopefully relatively painless in practice  
compared to the alternatives - i.e. -webkit-image-set and html5  
<image>) proposal to solve the problem of 2x-res (double-resolution)  
images with our current HTML and CSS standards for devices with high- 
resolution displays, such as 3rd Generation iPads and 4th generation  
iPhones and newer.

We add the following elements:

1) The new 'meta' attribute 'image-scaling' with arguments listed in  
the format {'scaling factor', 'scaling filename key'}, where the  
filename key is the often-standardized string added to the filename  
for 2x assets, i.e. '_2x' (it might even be possible to specify a  
different filename extension for the 2x asset by detecting whether the  
'scaling filename key' string contains a period i.e. 'xxx.xxx'). Sub- 
attributes to the 'image-scaling' attribute would include the optional  
boolean (defaulted to 'true') attribute 'assume-present', and  
potentially the optional attribute 'image-scaling-path' for cases  
where sites store their various scaled image assets in different  
directories than their 1x images.

2) A new series of optional attributes to the img tag named after the  
scaling factor, i.e. '2x', '4x', etc., (possible values include  
'true', 'false', a string for the double-res filename key, or 'url()'  
to specify a completely different path for the asset corresponding to  
that scaling factor)

3) A series of new optional CSS properties named after the scaling  
factor, i.e. 'background-image-2x', 'border-image-2x' and 'list-style- 
image-2x' (possible values for these include 'true', 'false', a string  
for the double-res filename key, or 'url()').

A simple example usage of these new capabilities would be the following:

<meta image-scaling="{2,'_2x'}" />

The effect of adding this single line to the page would be that a user  
agent that wishes to display double-res images would then attempt to  
access 'filename_2x.ext' whenever it encounters an img tag like '<img  
url=("filename.ext") />', or a CSS property like '.class {background- 
image: url("filename.ext");}', '.class {border-image:  
url("filename.ext");}' or '.class {list-style-image:  
url("filename.ext");}'. For all these, in the case that the  
'filename_2x.ext' file does not exist, a second request is made for  

If the bulk of the 2x-resolution images are located in a different  
directory than the 1x assets, the meta tag could be extended as such:

<meta image-scaling="{2,'_2x'}" image-scaling-path="{2,'2x_images/'}" />

Then, any 2x img or css-image assets would be requested from  
'2x_images/filename_2x.ext' instead of 'images/filename.ext'.

If a particular 2x img tag asset or css-image asset has a '@2x' double- 
resolution filename key instead of '_2x' for some reason (maybe you're  
integrating with some 3rd party off-site content with a different 2x  
naming convention), you could add a '2x' attribute to its img tag,  
such as '<img 2x="@2x" />', or to its css properties, such as '.class  
{background-image-2x: "@2x";}'.

If a particular 2x-resolution img tag asset or css-image asset is not  
located in the same directory as the 1x asset, or if the filenames and/ 
or file formats are not identical to the 1x asset, a separate path  
could be specified by doing this: '<img 2x=url("path/to/ 
filename_@2x.ext") />', or to its css properties by doing: '.class  
{background-image-2x: url("path/to/filename_@2x.ext");}'.

In the case that a majority, but not all img and css-image assets are  
available in 2x resolution, the img assets that lack a 2x version  
would include the a tag such as, '<img 2x=false />, or a css property  
such as '.class{background-image-2x: false;}'.

In the case that a majority, but not all img and css-image assets are  
unavailable in 2x resolution, you would add the 'assume- 
present="{2,false}' attribute to the meta 'image-scaling' attribute,  
such as '<meta image-scaling="{2,'_2x'}" assume-present="{2,false}" / 
 >', and use the '2x' attribute to flag img assets with a double- 
resolution asset available, such as '<img 2x=true />, and the css with  
'.class {background-image-2x: true;}'.

In the case that no double-resolution image assets are available, the  
meta 'image-scaling' attribute can be simply omitted.

By using this approach, we avoid the need to specify the same list of  
filenames varying only by scaling factor filename key for every single  
image asset, which is a bunch of busy work that just seems extremely  
redundant and clumsy to me. We are also able to achieve the same level  
of performance for those willing to put in the extra work to flag  
assets that deviate from the default setting (to minimize requests),  
and we allow the flexibility to be lazy or wrong, and have the user  
agent make two requests in those cases. This solution is also  
completely backwards-compatible with existing browsers.

As a corollary to this, a similar approach could be used to add  
support for different image formats without losing backwards- 
compatibility, and again saving many precious developer-years. Imagine  
<meta image-formats="{jpeg2000, '.jp2'}" assume- 
present="{jpeg2000,boolean}" />

-Tom Penzer
humble web coding noob
Received on Friday, 27 April 2012 19:58:52 UTC

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