2.4 L3 SC3: Images have structure that users can access.

This technique relates to the following sections of the guidelines:


Using markup to give images structure that users can access

The structure of complex images may be identified by using the W3C's Scalable Vector Graphics (SVG) format. SVG provides the ability to identify the structural components of graphics, and provides additional accessibility benefits as well:

" [SVG's g]rouping constructs, when used in conjunction with the "desc" and "title" elements, provide information about document structure and semantics. Documents that are rich in structure may be rendered graphically, as speech, or as braille, and thus promote accessibility. "

The SVG specification includes built-in support for magnifying and panning across images, and many other features that make SVG an appropriate technology for presenting maps, geospatial data, and other complex images that are accessible to users with disabilities.

It is important that the zoom functions can be operated from the keyboard or a keyboard interface as well as a pointing device, as required by Guideline 2.1 Level 1 SC1. When third-party applications such as SVG viewers are required in order to view content for which user agents do not provide native support, the third party application must satisfy Guideline 4.2.

User agent issues

As of September 2004:

  • No user agents provide full native support for SVG. Mozilla and Firefox offer partial support for native rendering of SVG, but SVG is not included in the standard builds at this time. See the Mozilla SVG Project page at http://www.mozilla.org/projects/svg/ for current information and updates.

  • No screen reader support for any SVG viewer, including Mozilla and Firefox (no screen reader support for Mozilla and Firefox generally).

  • No SVG viewer satisfies Guideline 2.1 L1 SC1 or Guideline 4.2 L1 SC1.

Editorial Note: Does lack of screen reader and keyboard support for SVG mean that we will have to provide interim techniques (the dread "until user agents" clause rearing its ugly head again)?

  • Vaingo and Pulsifer (2002) provided an extensive rationale for using SVG to deliver geospatial data interactively. This technical report included a comprehensive survey of OpenSource applications for creating and viewing such data (see "Internet Mapping for the Visually Impaired with Scalable Vector Graphics." Accessed August 27, 2004.).

  • Campin and McCurdy (2003) described other techniques for making maps accessible to blind users by combining SVG and Javascript to yield tactile and audio feedback as well as visual information (available at SVG Maps for People with Visual Impairment accessed August 27, 2004).

  • Rotard and Ertl (2003) described a transformation schema which transforms SVG into a tactile representation while also providing Braille and speech output (Rotard and Ertl, Tactile access to scalable vector graphics. . Accessed August 27, 2004.)

  • . The Canadian government's Maps for the Visually Impaired project has also produced example maps and research data (Accessed August 27, 2004).