Why Use Long Text Alternatives?
Some types of complex images such as charts, graphs, and pictures can convey a significant amount of information visually or provide a specific sensory experience to a sighted person. These images can expand on statements made in a document; enhance understanding; or act as the central medium of information. Providing long text alternatives helps convey this information.
Types of Images That May Need Long Text Alternatives
2 types of images that may need a long text alternative are complex data images and pictures.
1. Complex Data Images
Complex data images include: charts, graphs, and diagrams. They allow sighted users to understand data and data relationships. These visual representations can convey a vast amount of non-text content. In addition to a short text equivalent, for many data images it may be necessary to provide more detailed information or supplemental content in the form of a long text alternative. The National Center for Accessible Media's Effective Practices for Description of Science Content within Digital Talking Booksprovides useful information on writing long descriptions for complex data.
2. Pictures
Images of pictures include visual representations of objects, artwork, people, scenes, abstractions, etc. This non-text content can convey a significant amount of information visually or provide a specific sensory experience to a sighted person. Descriptions of what a picture looks like can be useful to those who are interested in the subject matter or the visual qualities of an image and who would otherwise be deprived of the content. 3 Tips for Writing Long Descriptions of Pictures provides guidance.
When to Use Long Text Alternatives
Long text alternatives can provide for rich, expressive
documentation of a visual image and should be used when alt is insufficient to embody the visual qualities of
an image.
If the short text alternative does not provide a text equivalent for the image, a long text alternative is also needed to supply supplemental content via a secondary mechanism.
To help decide whether a long text alternative is necessary turn off images in your browser and ask yourself:
Long Description: Long text alternative decision tree
What Long Text Alternatives Provide in Contrast to
the alt Attribute
User Choice
Screen readers read out the value of an alt attribute automatically. The old saying goes "a
picture is worth a thousand words". While this is true, that
doesn't mean everyone wants to hear all one thousand words at once
or at this particular moment. Stuffing a long description into an alt attribute would force a screen reader user to
listen to it.
In contrast a long description provides users the ability to pause and obtain a description of a complex image. The difference is akin to
- glancing at a sophisticated chart or painting (
altfunctionality) versus - studying it (long description functionality).
Sighted users, have that choice (to glance or study). Long text alternatives offer that choice to users with disabilities.
Ability to Utilize Structured Markup
It is impossible to use links or structural markup with an alt attribute as its value only allows string text.
In contrast a long description can utilize headings, tables, lists, etcetera which can facilitate far better communication and provide critical user functionality as discussed in the structure eClass.