How annotation layers define “segments of interest” for new kinds of applications

Here are some analogies we use when talking about software:

Construction: Programs are houses built on foundations called platforms.

Ecology: Programs are organisms that depend on ecosystem services provided by platforms.

Community: Programs work together in accordance with rules defined by platforms.

Architecture: Programs are planned, designed, and built according to architectural plans.

Economics: Programs are producers and consumers of services.

Computer hardware: Programs are components that attach to a shared bus.

All are valid and may be useful in one way or another. In this essay I focus on the last because it points to an important way of understanding what web annotation can enable. My claim here is that the web’s emerging annotation layer forms a shared bus for a new wave of content-oriented applications.

A computer’s bus connects devices: disk drive, keyboard, network adapter. If we think of the web in this way, we’d say that devices (your computer, mine) and also people (you, me) attachment to the bus. And that the protocol for attachment has something to do with URLs.

You can, for example, follow this link to display and interact with the set of Hypothesis annotations related to this web page. You can also paste the link’s URL into a message or a document to share the view with someone else.

That same URL can behave like an API (application programming interface) that accesses the resource named and located by the URL. A page like this one, part of the DigiPo fact-checking project, uses the link that way. It derives the Hypothes search URL from its own URL, and injects the resulting Hypothesis view into the page.

Every time we create a new wiki page at, we mint a new URL that summons the set of Hypothesis annotations specific to that page. In principle there’s no limit to the number of such pages — and associated sets of annotations — we can add. And that’s just one of an unlimited number of sites. The web of URL-addressable resources is infinitely large.

Even so, URLs address only a small part of a larger infinity of resources: words and phrases in texts, regions within images, segments of audio and video. Web annotation enables us to address that larger infinity. The DigiPo project illustrates some of the ways in which annotation expands the notion of content as a bus shared by people and computers. But first some background on how annotation works.

The proposed standard for web annotation defines an extensible set of selectors:

Many Annotations refer to part of a resource, rather than all of it, as the Target. We call that part of the resource a Segment (of Interest). A Selector is used to describe how to determine the Segment from within the Source resource.

When the segment of interest is a selection in a textual resource, one kind of selector captures the selection and its surrounding text. Another captures the position of the selection (“starts at the 347th character, ends at the 364th”). Still another captures its location in a web page (“contained in the 2nd list item in the first list in the seventh paragraph”). For reasons of both speed and reliability, Hypothesis uses all three selectors when it attaches (“anchors”) an annotation to a selection.

When a segment of interest is a clip within a podcast or a video, a selector would capture the start and stop (“starts at 1 minute, 32 seconds, ends at 3 minutes, 12 seconds”). When it’s a region in a bitmapped image, a selector would capture the coordinates (“starts at x=12,y=53, ends at x=355,y=124”). When it’s a piece of a vector image, a selector would capture the Scalable Vector Graphics (SVG) markup defining that piece of the image.

The W3C’s model of web annotation lays a foundation for other kinds of selectors in other domains: locations in maps, nodes in Jupyter notebooks, bars and trend lines and data points in charts. But let’s stick with textual annotation for now, consider how it expands the universe of addressable resources, and explore what we can do in that universe.

Here’s a picture of what’s happening in and around the above-mentioned DigiPo page:

The author has cited a Hypothesis link that refers to a piece of evidence in another web page. The link encapsulates both the URL of that page and a set of selectors that mark the selected passage within it. When you follow the link Hypothesis takes you to the page, scrolls to the passage, and highlights it. That’s a powerful interactive experience!

Now suppose you want to review all the evidence that supports this investigation. You can do it interactively but that will require a lot of context-disrupting clicks. So another program embedded in the wiki page summarizes the cited quotes for you. It uses a variant of the Hypothesis direct link that delivers the interactive experience. The variant is a Hypothesis API call that delivers the annotation in a machine-friendly format. The summarization script collects all the Hypothesis direct links on the page, gathers the annotations, extracts the URLs and quotes, injects them into the Footnotes section of the page, and rewrites the links to point to corresponding footnotes.

To enable this magic, an app that people can use to annotate regions in web pages is necessary but not sufficient. You also need an API-accessible service that enables computers to create and retrieve annotations. Even more fundamentally, you need an open web standard that defines how apps and services work not only with atomic resources named and located by URLs, but also segments of interest within them.

What else is possible on a shared content bus where segments of interest are directly addressable both by people and computers? Here’s one idea being pondered by some folks in the world of open educational resources (OER). Suppose you’re creating an open textbook that attaches quizzes to segments within the text. The quizzes live in a database. How do you connect a quiz to a segment in your book?

Because a quiz is an URL-addressable resource, you can transclude one directly into your book near the segment to which it applies. Doing that normally means encoding the segment’s location in the book’s markup so the software that attaches the quiz can put it in the right place. That works, but it entangles two editorial tasks: writing the book, and curating the quizzes. That entanglement makes it harder to provide tools that support the tasks individually. If you can annotate segments of interest, though, you can disentangle the tasks, tool them separately, build the book more efficiently, and ensure others can more cleanly repurpose your work.

Analogies are necessary but imperfect. The notion of a shared bus, formed by an annotation layer and used by applications oriented to segments of content, may or may not resonate. I’m looking for a better analogy; suggestions welcome. But however you want to think about it, the method I’m describing here works powerfully well, I’ll continue to apply it, and I’d love to discuss ways you can too.


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