A question that comes up from time to time from people learning how to make PDFs accessible concerns the tooltips that you sometimes see displaying the alt text for images when you mouse over them. People are often confused as to why sometimes you see such tooltips and sometimes you don’t.
Am I doing something wrong?
Often learners assume they are doing something wrong when no tooltip is displayed. Sometimes they resort to somewhat tortuous (and even torturous) workarounds to “fix” the perceived problem.
Vector graphics vs. JPEG/PNG
The answer is quite simple: in a PDF the alt text for PNGs and JPEGs will be displayed in a tooltip when you mouse over the image in question. However, with vector graphics no tooltip will be displayed.
It is absolutely not a problem that no tooltip is displayed with vector graphics. In fact, it is actually the correct behaviour. Alt text should only be displayed if, for whatever reason, the image itself cannot be displayed. PDFs have historically got it wrong when it comes to PNG, JPEG and tooltips.
For document authors, for several reasons including the above, the best way to check the alt text in your PDF is not to mouse over the images one by one. The most effective way to check your alt text is with a screen reader. In either JAWS or NVDA, just press “G” to go from one graphic to the next. Not only can you easily assess the appropriateness of your alt text in this way, it’s also a great proof reading method. If there’s a typo anywhere in there, you will almost certainly hear it. It’s a bit like someone sticking a screw driver in your ear: you are going to notice!