Images of text that meets guideline 1.1 should satisfy this criterion. (Refer to Guideline 1.1 Provide text alternatives for all non-text content. )
Editorial Note: The working group is seeking an algorithm that measures contrast in a way that is accurate and testable enough that we could include it in the guidelines. One algorithm, which comes from the Techniques For Accessibility Evaluation And Repair Tools document, is currently under consideration for inclusion in the techniques, but the group has not yet found something that is specific enough to be included at the guidelines level.
A 20 decibel difference in sound level is roughly 4 times quieter (or louder). Background sound that meets this requirement will be approximately four times (4x) quieter than the foreground audio content.
Individuals with low vision can easily read characters in the content even if they don't have the wide field of view or full range of color perception used by fully sighted persons to separate text from background images.
Individuals with hearing impairments that limit their ability to hear all of the frequencies of speech can make out the words from the sounds they can hear because they are not mixed with residual sounds from the music.
Example 1: a background image on a page.
A background image and text are arranged so that there is no image behind the text or the image is so faint that the difference between the darkest part of the image and the text (which is dark) meets the standard foreground/background contrast requirements. The image behind the text also does not contain lines that are about the same width as the characters so they do not interfere with character recognition.
Example 2: speech over background sounds.
Because speech is often naturally mixed with background sounds (movies, live news etc) and cannot be easily removed or separated, captions are provided (under guideline 1.2) to make dialog understandable. However not all people can see or read the captions. Where speech is mixed or recorded so that it is at least 20 db above any background sounds, most people do not need to rely on captions to understand the dialog.
Images that appears behind or to the back of the visual field.
derived programmatically means that the specific value can be determined in a standard, machine or software readable form .
Programmatically located means that the value can be found, though there may be multiple possible values (for example, providing a dictionary definition for a word that has multiple meanings). This term contrasts derived programmatically, where the specific value can be determined.
Editorial Note: This provision is dependent on the definition of a standard way to associate dictionaries and the availability of on-line dictionaries.