While PDF pages often include many sorts of images and other graphic objects, not every line, fill, background or bitmap is vital to accessibility. It’s more common, in fact, for graphics to be strictly cosmetic in nature, playing no role in conveying information to readers.
For this reason, the first question when thinking about the accessibility of graphics is simply: what is this object for? Does it represent information that’s needed to understand the document, or is it entirely incidental?
In PDF, graphics that aren’t important to the document’s content are marked as “artifacts” to distinguish them from so-called “real content” – the text and graphics that make up the substance of the document.
Marking content as an artifact causes assistive technologies to skip over it, which is precisely the desired effect.
For images, charts and other non-text graphics, we assign a <Figure> tag. These require either alternative text or actual text in order to make the object accessible.
Alternative text is often called “Alt text” or even just “Alt.” The term refers to any text provided as an alternative description to a graphic or image.
Since alt text can be subjective, it’s best to have the document author provide it. This may not always be possible, so here are some suggestions for writing alt text.
If you’re struggling with ALT text for a document created by someone else, consider reaching out to them to provide it for you.
- Look at more than the main subject of the image. Would it benefit to describe the environment around the main subject?
- If it’s a chart, what’s the trend? Are things increasing or decreasing? By how much? Is it between a span of years?
- Don’t forget color. Think what sort of implication green leaves give in contrast to brown leaves, for example.
- Is it an image that is actually read as in the case of a company logo?
- If a logo contains text, for example a motto or slogan, be sure to include that text in the ALT text for the logo.
- Don’t repeat captions. If the caption itself is adequate alt text, the Figure tag’s alt text can be simply: “Described in the caption.”
- Be sure to include relevant punctuation in the ALT text. Adding punctuation at the end, for example, forces the screen reader to stop before continuing on to the next item to be read.
Adding Alternative Text
To place an image or other graphics into a Figure tag, select the image and click Add Figure.
There will be occurrences where a single graphic (for example, a chart) appears as multiple items in the logical view. To address this situation, set your selection tool to images only, then highlight the image in the physical view. Each component will be highlighted in the logical view. Next, click Add Image. This will place all of the pieces into a new Figure tag. Remove the empty tags using Cleanup page.
Some graphics, such as ligatures, are used to represent characters. These graphics require a special solution that differs from Alt text; a solution in which the character represented by the graphic is simply pronounced as part of a word.
To illustrate, a paragraph begins with “The” and the letter “T” is made from paths, as in the illustration. Placing this image inside of a figure tag and typing the letter “T” in the alt text property would cause assistive technology to announce “T” followed by a pause before “he”, which would obviously be misleading and incorrect.
In such cases use the <Figure> tag’s Actual Text property, which causes the “T” to be read inline as the word “The.”
Charts and graphs often consist of collections of images. To address this situation, combine the graphics making up the chart under one Figure tag and assign ALT text. To do this:
- Set the Selection tool to Images Only.
- Highlight the chart in the physical view and click Add Figure.
- Choose “single parent” from the dialog that appears.
- Add alternative text in the Alt Text property of the Figure tag.
The description for a chart should indicate the time frame and number changes or scale. A possible example would be “chart showing a decline in numbers between 2001 and 2008”. Any values present on the chart should also be represented in the alt text.
Make sure to artifact any text runs in charts. Otherwise, they will be read even through ALT text is present.
You should also look for text runs that form part of the chart. It’s important to select these text runs and either include them in the <Figure> tag, or mark them as artifacts. This can be done easily by setting the selection tool to “text runs only”, highlighting the chart in the physical view and clicking Add Artifact.