text alternative
A text alternative is a sentence written into the document to stand in for a picture, read out to somebody who cannot see it.
In more detail
It is not a caption. A caption is printed under the picture and everybody reads it; a text alternative is never shown, and only software reading the page aloud says it.
The useful one says what the picture shows, not what it is. A photograph of a shop front, a graph of sales falling by a fifth over the year, a signature in ink — never simply a photograph, a graph, an image.
What it does on a real document
Each picture in the structure tree carries its own sentence. A picture that carries no meaning — a rule, a corner ornament, a background — carries no sentence either and is marked as decoration instead, so that nothing reads it out.
No program can write it. What a picture means is known to whoever put it there, which is why the accessibility standard asks for it and no engine can supply it.
Where it comes from, and why
The idea is older than the PDF: it came from the web, where a picture has carried a written stand-in since the early nineteen nineties.
The accessibility standard for PDF carried it over, and the European rules on what public bodies publish are what turned it from good practice into an obligation.
See it on a finished document
A list whose pictures speak too The rules behind this word, and the texts they are read from
Glossary
- standard
- A rule argued out in committee, published under a number anybody may buy and read, and identical for every firm claiming it. A claim to follow one can therefore be checked against the text. A way of working that merely spread because it worked is a habit of the trade: useful, widespread, and answerable to no text at all.
- CMYK
- The four inks a printing press lays down: cyan, magenta, yellow and black. A screen mixes light instead, so a document going to a press states its colours in inks, not in screen colours.
- structure tree
- The list, held inside a tagged PDF, that says what each part of the page is and what it belongs to: this block is a section, this line its heading, this run of cells one row of that table. It is the shape of the document, kept apart from the ink.
- signature
- A seal inside the document that proves two things at once: who approved it, and that not one byte has changed since. A reader shows it as a banner at the top of the page.