structure tree
The structure tree is the list, held inside a tagged PDF, that says what each part of the page is and what it belongs to.
In more detail
It is called a tree because the parts nest: a document holds sections, a section holds a heading and paragraphs, a table holds rows and a row holds cells. Every branch says what it is, and the leaves point at the marks on the page that belong to it.
Nothing of it shows. Two documents can look identical on screen and on paper while one carries a tree and the other carries none, and the difference only appears when software tries to make sense of the page.
What it does on a real document
Software reading a page aloud walks the tree rather than the ink, which is what gives it headings to jump between and a table it can read column heading by column heading. A program pulling figures back out does the same, and so does a search engine deciding what the document is about.
Most of the tree can be built as the page is laid out — a table knows its own rows, a paragraph knows where it ends. What no program can work out from the ink is the outline: which block is a section, what a picture means, and in what order it all reads. That part is declared.
Where it comes from, and why
It arrived with the tagged PDF in 2001, for two needs at once: reading a page aloud, and reflowing a page onto a small screen.
It is what the level of the archiving rules written for documents that must be read aloud asks for, and what the whole accessibility standard is built on. Both refuse a document that claims them and carries no tree.
See it on a finished document
A document that can be read aloud 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.
- tagged PDF
- A PDF that carries the structure of its own text alongside the drawing: this is a heading, this is a table, this cell belongs to that column. It is what lets a reading aid, or a program pulling data back out, make sense of the page.