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Community & Business Groups

Universal Images Community Group

Our goal is to provide tools and specifications for creatives and agencies to create and distribute multi-format images. These are images which contain metadata that lets them adjust to different sizes, depending of the format of the output device or the layout of the website it is used in. These images can be used to have a web-server automatically create the alternative versions needed for a responsive website.

Note: Community Groups are proposed and run by the community. Although W3C hosts these conversations, the groups do not necessarily represent the views of the W3C Membership or staff.

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Responsive Metadata XMP Standard

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A Proof of Concept

This is a followup to the previous post that suggested using a metadata standard to define the rules for art direction. The advantage of this approach is that it is independent of image formats and operating systems and can be used with existing technology. The downside of it is that its scope does not include the browser. It needs a program on the server that processes the image before delivering it. Since most web frameworks use some kind of thumbnailer, this should not be a big issue.

Although the metadata standard has been developed with the web as its primary use case, there are other areas where it could be used as well. I am thinking of desktop image viewers, digital billboards or embedded devices. Everywhere you need to show a full screen image but don’t know the shape of the screen it is viewed on.

Photoshop Plugin

I have created two pieces of software that use the metadata standard. The first is a plugin for Adobe Photoshop and an extension for the file info dialog that lets users add responsive information to any image. The second is a filter for a web service that interprets the standard and crops an image according to the rules if there is a need for it.

The results of such a workflow can be seen on the demo page.

I chose to create a filter for the Thumbor image server because I know Python better than PHP. Otherwise, I would have built a plugin for WordPress. The ultimate goal is that in the future most programming languages and web frameworks have a library that can read and process that metadata.

I would have preferred to have a plugin for Adobe Lightroom instead of Photoshop but this turned out to be rather difficult. I think it is not possible to access the image canvas from within the context of a plugin. Another option would have been a web interface but there is the issue of not being able to save a file to disk.

The processing of the responsive metadata turned out to be a fairly complex process. I drew an activity diagram to help figure out the decisions that have to be made when applying the art direction rules to the image.

Naming Things

When talking about responsive or universal images, size is the most important property. Unfortunately the most common unit web developers use for the size of an image, pixel, is ambiguous. It has two meanings:

  1. A physical pixel on a screen or a single image point in a bitmap.
  2. A unit of measure.

Usually the context of where the word “pixel” is used defines which one it is.

But how do you correctly measure the width of an image for a responsive website? There are two factors involved: The physical size of the image and the distance from the device to the viewer. Obviously an image appears larger if it is closer to the viewer. Therefore, the unit of measure for universal images should not be based on an absolute width like Millimeter but on a viewing angle.

Luckily that’s what the “Pixel” unit is based on in the CSS 2.1 specification. So a CSS pixel is resolution independent. It is 0.213° or 12.8′. The definition is: “The visual angle of one pixel on a device with a pixel density of 96dpi and a distance from the reader of an arm’s length.” Since “an arm’s length” is not an exact value, the unit is not scientifically exact, but good enough for what it is used for.

This is the reason why the viewport width of an iPhone 4 in portrait mode is still 320 pixels even though the display has twice the number of physical pixels.

In Android the unit is called “Density independent pixel” or dp, which is maybe a better way to explicitly separate the unit from the pixel unit used by all image editing programs which is based on the actual pixels in an image.

It is therefore necessary to always think about which version of the pixel unit is used in the current context. If it is not clear, it is better to explicitly specify it.

What this group is about

Art direction in the context of universal images means cropping and reframing an image in one or two dimensions depending on the context it is in.

Art directed responsive images start with the photographer, not just in the browser. That’s why we should come up with ways to help content producers create responsive images.

Before we start discussing technical issues, let’s recap the use cases for responsive images that require art direction:

  • Small devices: Smartphones, especially when held in portrait mode, simply cannot show an image the size the photographer has envisioned it. Sometimes details cannot be recognized in a small image due to their size. To keep up with  editorial integrity, one has to find a balance between zooming in to show what is going on and the overall image composition.
  • Different layouts: Websites could use the same image in different places. For each layout the image would be displayed in the size and aspect ratio required by the layout.
  • Fluid images: Websites often use a fluid grid where the column with is relative to the browser width. If images have a fixed height and a dynamic width then parts of the image can get cropped in the browser.
  • Adaptive images: Images can be displayed full screen on mobile devices no matter if the user is holding the device in portrait or landscape mode.

The following graphics describes the usual workflow for creating an image for a website.

Responsive Image Flowchart

The roles used is based on the roles defined in MWG, Guidelines for Handling Image Metadata.

Apart from the technical challenges there are also legal ones. In general, copyright prohibits to alter a work of art in any way, even cropping it (although the interpretation of this differs between countries). However, most image agencies allow it in their license agreements. Nevertheless, this is a factor that has to be taken into consideration when creating a universal images: It could just have a metadata entry that says “Don’t crop me”.

A big part of the discussions are going to be about metadata. There are quite a few different standards (Exif, IPTC, XMP) but none fulfills the needs of universal images completely.

The goal of this group is to find a way for content producers to make an image responsive in a way that it can be distributed together with the information needed so that all responsive derivates can be created automatically.

What this group is not about

The issue of delivering and displaying images to the end user (browser). That’s what the Responsive Issues Community Group is for.

Call for Participation in Universal Images Community Group

The Universal Images Community Group has been launched:


Our goal is to provide tools and specifications for creatives and agencies to create and distribute multi-format images. These are
images which contain metadata that lets them adjust to different sizes, depending of the format of the output device or the layout of the website it is used in. These images can be used to have a web-server automatically create the alternative versions needed for a responsive website.


In order to join the group, you will need a W3C account. Please note, however, that W3C Membership is not required to join a Community Group.

This is a community initiative. This group was originally proposed on 2015-12-14 by Simon Bächler. The following people supported its creation: Simon Bächler, Christopher Regan, Christophe Avon, Hui Jing Chen, Mads Ulsø Østergaard. W3C’s hosting of this group does not imply endorsement of the activities.

The group must now choose a chair. Read more about how to get started in a new group and good practice for running a group.

We invite you to share news of this new group in social media and other channels.

If you believe that there is an issue with this group that requires the attention of the W3C staff, please email us at site-comments@w3.org

Thank you,
W3C Community Development Team