Types of Accessibility Aids
The following sections describe some of the more common types of accessibility aids—utilities that are added to a computer to make it more accessible to people with certain disabilities. Not all people with disabilities use tools like these, but being familiar with them and how they work can help you to design and build applications that work well with them.
Screen enlargement utilities
A screen enlarger (also called a screen magnifier or large print program) is a utility that allows the user to magnify a portion of his or her screen, effectively turning the computer monitor into a window that shows a portion of an enlarged virtual display. The user can use mouse or keyboard commands to move the window to view different areas of the virtual display. An enlarger needs to track where the user is working, so it can automatically keep the active area in view.
Screen review utilities
People who are blind use the computer with the aid of a screen review utility (also called a blind access utility or screen reader). A screen review utility takes the information displayed visually on the screen and makes it available through alternative, nonvisual media, such as synthesized speech or a refreshable Braille display. Because both of those media present only text, not graphics, the utility needs to render screen elements as text — for example, by assigning a user-friendly name to each graphic object. The utility also needs to track what the user is doing and the location of the focus to be able to describe the important aspects of what is happening on the screen.
In Windows and other graphical environments, a screen review utility works by watching all operations that draw information to the screen. Then it builds up an "off-screen model," a database of objects on the screen, their properties, and their spatial relationships. Some information will be automatically read to the user when it changes on the screen, and other information will be found when the user requests it. A screen review utility often accepts configuration files (also called set files or profiles), which tell it how to work correctly with particular applications.
Voice input utilities
People with severe mobility impairments will often use a voice input utility (also called a speech recognition program) to control the computer with their voice instead of the mouse and keyboard. This kind of utility is also increasingly being used to boost productivity of people who do not have disabilities.
Like a screen review utility, a voice input utility tries to identify objects on the screen that can be manipulated and to determine appropriate names for them so that the user can activate an object with a single phrase. It also needs to be able to manipulate controls programmatically and be able to detect what the user is doing and the changes that result.
On-screen keyboards
Some people with motion impairments cannot use a standard keyboard, but they may be able to use another input method, such as a switch or a pointing device. An on-screen keyboard is a utility that displays a list of commands to the user and allows him or her to choose and execute the commands using a variety of input methods. A common use of this technique is to display a set of keys that the user can point and click on to type into the computer. Variations of this technique include Morse-code input systems and single- or double-switch input systems.
If a single-switch system is used, an on-screen keyboard successively highlights groups of commands until the user selects one group by pressing a switch. Then the utility successively highlights smaller groups of commands within the selected group until the user selects the specific command to run. If a user can point but not click, he or she can activate a command using a head pointer by pausing the pointer over the command for a certain amount of time.
An on-screen keyboard is also commonly used to display buttons with all the commands available at a given time. To display them, the utility needs to identify, name, and activate controls much the way a voice input utility does.
Keyboard filters
Impaired dexterity may make it difficult for a person to use a standard keyboard, but keyboard filters built into Windows 95 compensate somewhat by correcting for erratic motion, tremors, slow response time, and similar conditions. In most cases, however, it is not possible to apply the same corrections to pointing devices, such as the mouse, so users with impaired dexterity are restricted to keyboard input. Other types of keyboard filters include typing aids, such as word prediction and abbreviation expansion utilities, and add-on spelling checkers. These typing aids are also increasingly being used to improve the typing speed and accuracy of users who do not have disabilities.