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SMIL and SVG - Towards Accessible Multimedia

Charles McCathieNevile, Marja-Riitta Koivunen and Ian Jacobs
Web Accessibility Initiative
World Wide Web Consortium
545 Technology square, Cambridge MA 02139
Emails: charles@w3.org, marja@w3.org, ijacobs@w3.org 
Website: http://www.w3.org/

The development and use of accessible, interoperable specifications for multimedia will improve the accessibility of these popular features on the Web, and therefore improve the accessibility of the Web as a whole. This paper examines two of these new specifications, Synchronized Multimedia Integration Language (SMIL) and Scalable Vector Graphics (SVG), and the accessibility features they offer.

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As the Web grows to rely more and more heavily on multimedia content for consumption by the general public, it is necessary to ensure the accessibility of that formats for publishing that content and the tools for viewing it. W3C promotes accessibility in its specifications in a variety of ways. Adopting XML and related specifications as the basis of markup languages promotes accessibility through increased structure, separation of structure from presentation, interoperability, and reuse. In this paper, we discuss some ways in which the Scalable Vector Graphics (SVG) and Synchronized Multimedia Integration Language (SMIL) specifications will ensure that all users can access rich multimedia on the Web. Some of these benefits arise as a result of using XML, while others are features included so that authors and software can make images, video, and synchronized multimedia accessible to people with disabilities.

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The Benefits of XML

Both SMIL and SVG are XML applications. This means they are plain text, and can be created by hand, making it possible to read and author them at some level on any platform, with almost any device. In addition, as the Web moves towards XML this will ensure broader interoperability, and therefore easier integration with assistive technologies.

The use of XML Namespaces and RDF allows a lot of supplementary information to be added to SVG and SMIL. As RDF and XML become more widespread this offers very interesting possibilities for the future.

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Re-usable Objects

A feature of XML is the ability to group and name something. With the development of Xlink, it is possible to re-use objects such as parts of an image, or a section of a presentation. These can either be kept as a local file or available somewhere on the web. This allows people to use a known object such as an image or sound, even if they cannot hear or see it themselves. It is also possible to combine an image or audio track from one place on the web with a description or captions from another.

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Structured Documents

Because they are XML, SVG and SMIL naturally provide the ability to write multimedia objects as logically structured documents. This in turn allows the reader to search through the structure of a presentation or image for the part that they are most interested in, or to focus on a particular component or type of component such as audio or text descriptions.

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Accessibility Challenges of Multimedia

Multimedia presents a number of challenges to people with disabilities and to authors of accessible content:

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What are SMIL and SVG?

The Synchronized Multimedia Integration Language [SMIL] allows an author to specify how to combine various time-based presentation components. Scalable Vector Graphics [SVG] is a new vector-based graphics format for the Web currently being developed by the W3C.

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Equivalent Alternatives

One of the biggest challenges in multimedia accessibility is to provide alternative forms of content. Both SMIL and SVG introduce new features to do this. In SVG there are the title and desc elements, which can be used with almost any element to provide a human-readable title and description for the image or image component. SMIL provides alt, longdesc, and abstract attributes for all media object elements, and title for almost all elements.

Synchronized Alternatives

SMIL allows an author to synchronize various media-specific presentation tracks. Any number of equivalent alternatives (such as text captions, audio descriptions, subtitles, parallel video tracks of signed interpreters, simplified versions of presentation elements) can be synchronized, and various test attributes allow the user to select which tracks they want.

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User Control

An important factor in accessibility is user control - the ability to slow a presentation down, zoom in on an image, change a font, etc. The SMIL and SVG specifications include some requirements for players (User Agents) to allow the necessary level of control. For example the system-captions attribute in the current generation of SMIL players allows the user to specify that captions should be rendered. More extensive information on accessibility requirements for User Agents are provided by the WAI User Agent Accessibility Guidelines [WAI-USERAGENT].

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Layout and Style Sheets

The accessibility benefits of style sheets are well established [CSS-access]. SMIL and SVG both use Cascading Style Sheets to control many aspects of presentation and layout, which allows the user to change the presentation according to their needs. CSS implementation in SMIL players is, like in HTML browsers, slow to become widespread. In SVG rendering CSS is critically important, and is implemented in all seven of the SVG players known to the authors in October 1999.

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One of the features of vector graphics, as opposed to traditional "raster-based" graphics, is that magnifying and reducing the image does not result in any loss of quality. The following small image was produced as a demonstration for the CSIRO SVG viewer. It is shown as a small image, then enlarged as a PNG image, and then enlarged to the same size as an SVG. Enlarging the PNG (or a GIF or JPG image) results in a loss of resolution, while enlarging the SVG image simply makes the details clearer.

Example: enlargements of PNG and SVG
The small image:

Example of a small image

Enlarged PNG:

Example of an enlarged PNG

Enlarged SVG:

Example of an enlarged SVG


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SMIL and SVG bring the important accessibility features we have presented to the mainstream of Web multimedia, and show the way forward for accessible XML. As they are further developed and deployed more user feedback can be incorporated into the development cycle. We hope that this feedback, and the Web Accessibility Initiative's work in the W3C and with W3C working groups will lead to greater accessibility of Web content for all users.

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Further Reading

Notes about the accessibility of CSS [CSS-access], SMIL [SMIL-access], and SVG [SVG-access] are published by the W3C Web Accessibility Initiative, as well as guidelines for the accessibility of Web Content [WAI-WEBCONTENT], Authoring Tools [WAI-AUTOOLS] and User Agents [WAI-USERAGENT]. W3C Specifications (including SMIL, SVG, CSS, XML and RDF) are available from

Further information about the W3C's Web Accessibility Initiative is available.

SMIL 1.0 was released as a W3C Recommendation [SMIL] in 1998, and the first public draft of the new SMIL "Boston draft" was released in August 1999. The new version of SMIL is expected to become Recommendation around the middle of the year 2000. SMIL is implemented in a number of players and authoring tools.

At the time of writing (October 1999) SVG was a "last call" Working Draft [SVG], but it is expected to be a W3C recommendation by the end of 1999. There are already (October 1999) seven editing and/or rendering tools for SVG.

Appendix A Sample Accessible SMIL Presentation

The following sample has been provided by the CPB/WGBH National Center for Accessible Media (refer to [WGBH-NCAM]). The presentation contains captions, subtitles and auditory descriptions in English and German. To play the presentation, you will need a SMIL player such as the G2 player from [REAL]. Information about this and other SMIL players is available at the W3C Synchronized Multimedia Home Page ([SYMM]).

Once you have installed the player, follow the instructions below:

  1. Download the sample presentation, which is stored in a zip file.
  2. Unzip the zip file, which contains "car.smi" and other files.
  3. Start the player. Before playing the movie, you'll need to set preferences to display captions (or subtitles) and descriptions in either English or German:

    For the G2 player running in Windows

    1. Open the Options menu.
    2. Choose Preferences.
    3. Choose the Content tab.
    4. Choose English or German from the Language listbox.
    5. Check the Enable Captions box at the bottom of the window.
  4. Open "car.smi" and play it. You should see captions or subtitles below the video window and hear audio descriptions interspersed with the program narration.

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"Accessibility Features of CSS" I Jacobs, J. Brewer eds. Available at http://www.w3.org/TR/CSS-access
"Namespaces in XML" T. Bray, D. Hollander, A. Layman, editors, 14 January 1999. Available at http://www.w3.org/TR/REC-xml-names/
"Resource Description Framework (RDF) Model and Syntax" O. Lassila, R. Swick eds. Available at http://www.w3.org/TR/REC-rdf-syntax/
The Real Networks Home Page is http://www.real.com/.
"Synchronized Multimedia Integration Language" P. Hoschka ed. http://www.w3.org/TR/smil
20 August 1999 working Draft of "Synchronized Multimedia Integration Language Boston Specification" J. Ayars et. al. eds. available at http://www.w3.org/TR/WD-smil-boston-19990820
"Accessibility features in SMIL" M-R. Koivunen, I Jacobs eds. Available at http://www.w3.org/TR/SMIL-access
"Scalable Vector Graphics 1.0 Specification", J. Ferraiolo ed. Available at http://www.w3.org/TR/SVG
"Accessibility of Scalable Vector Graphics", C. McCathieNevile, M-R Koivunen ed. Available at http://www.w3.org/TR/SVG
The W3C Synchronized Multimedia Home Page is http://www.w3.org/AudioVideo.
"Authoring Tool Accessibility Guidelines (working Draft)", J. Treviranus, J. Richards, I. Jacobs, C. McCathieNevile eds. Available at http://www.w3.org/TR/WAI-AUTOOLS
"User Agent Accessibility Guidelines (Working Draft)", J. Gunderson, I. Jacobs eds. Available at http://www.w3.org/TR/WAI-USERAGENT
"Web Content Accessibility Guidelines 1.0" G. Vanderheiden, W. Chisholm, I. Jacobs eds. Available at http://www.w3.org/TR/WAI-WEBCONTENT
The National Center for Accessible Media at WGBH home page is http://www.wgbh.org/ncam.

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