The problem with having the decision being made server-side is it's a
complex decision whose factors are fluid, involving more than simply
the screen resolution. Requesting 2x images also requires ~4x the
data, and when devices are on mobile networks, they might wish to
request 1x images instead of 2x, even though they have a screen
capable of displaying the 2x image. Or they might want to let the user
choose which images they prefer on mobile networks. And when they're
on Wifi, they might want the 2x image instead. In the future, when the
mobile networks are faster, that might no longer be the case, and the
right decision today will be wrong tomorrow. That's why the user agent
needs to choose, not the server.
-Tom P.
On Apr 27, 2012, at 7:53 PM, Arthur Clifford wrote:
Wouldn't it be better if the user agent sent back pixels-per-inch or
some other measure of bit-density information along with the User-
agent metadata and then server-side scripting could be adapted to
return imagery based on the user agent.
In fact if you used a server-side script, say getmedia.php, for the
src of your images you could currently check the user agent data and
figure out if it is an iphone/android/whatever and choose the
version of the requested asset to return. You would have full
control in getmedia.php over detecting the mime type of the
requested asset and depending how you store the asset could
determine whether any dynamic resampling of the asset is required;
or at least choose a high vs low-res version. including additional
parameters on the url like getmedia.php?&res=low or getmedia.php?
&res=hi or &res=4x or whatever would allow an explicit resolution to
be requested.
This just seems to me to be more of a server-side rather than client
side thing; and I don't necessarily think that all servers should be
expected to be media servers. If pixel-density is not currently User-
Agent provided header/metadata, then I think that would be a better
request. If one were to develop a server-side library for handling
the user-agent metadata with pixel density and do light-wieght media
serving with support for resolution specific settings, then those
who want that functionality could use such a library and those who
could care less could elect not to worry about it. In fact I would
be very surprised if somebody hasn't created such a library already.
Art C
On Apr 27, 2012, at 2:49 PM, Jason H wrote:
1. To my knowledge w3c has never specified file naming conventions.
Maybe extensions, as they are related to mime types, but this would
be a new function of filename itself that would affect rendeing
quality. What about images that already exist out there that are
named as such? No, we should not use the filename for metadata.
2. The user agent can use Javascript to do this already.
3. The JS method is backwards compatible and requires no changes.
To say it degrades gracefully, I think is the wrong direction.
Initial sources should be low-res, then upgrade accordingly.
From: Tom Penzer <[email protected]>
To: Jason H <[email protected]>
Cc: "[email protected]" <[email protected]>
Sent: Friday, April 27, 2012 4:57 PM
Subject: Re: Image Scaling for High-Pixel-Density (i.e. Retina)
Displays - Re: -webkit-image-set and <image>
The main advantages of doing it this way are:
1) You don't need to specify a different path for every scale of
every image if you follow a naming convention.
2) the user agent makes the decision of which asset to request
(based on factors such as screen resolution and network quality/
value of bandwidth)
3) It's backwards-compatible and degrades gracefully.
-Tom
On Apr 27, 2012, at 1:50 PM, Jason H <[email protected]> wrote:
I think the approach of attacking it from setting a scaling is
wrong.
The real approach is to set the final dimensions, and let the
software give the appropriate scaling based on the image metadata.
If you want a double-density image, you're just asking for 100x100
to be rendered in 50x50. The image tag already supports this. <IMG
HEIGHT=50 WIDTH=50 SRC="100x100.png"/> Far too often it is implied
that the display density is identical to the sampling (pixel
density) by not including the height and width attributes.
The real challenge as I see it, is having an efficient way of not
sending over-sampled data across the wire, unless you mean to.
Sending a double density image actually sends 4 times the data as
a native resolution image. I would suggest we leave it up to java
script to handle the pixel density matching, where it can
dynamically assign the assets to the proper size-density, if
needed. For the most part pick something not over sampled and let
the browser scale it up if need be. A server-component could
dynamically render and cache the common sizes of an image. But I
don't see a need to change HTML.
From: Tom Penzer <[email protected]>
To: [email protected]
Sent: Friday, April 27, 2012 3:47 PM
Subject: Image Scaling for High-Pixel-Density (i.e. Retina)
Displays - Re: -webkit-image-set and <image>
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 'filename.ext'.
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/[email protected]
") />', or to its css properties by doing: '.class {background-
image-2x: url("path/to/[email protected]");}'.
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
