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GOT IT AT A GLANCE

What is
an AprilTag?

A sticker that lets machines understand real objects
for people who have never heard of it
No. 20Inside Computers & the InternetPart 8 of 20
1

In one line

A palm-sized black-and-white square, stuck on something.
One glance from a camera far away, and it knows
which number it is, how far, and which way it faces.

It isn't meant for people. It is drawn for machines. To a human it's just a pile of black and white squares.

2

How is it different
from a QR code?

QR CODE
Tells you "what this is"
It must face you, be close, and be clear. A bit crooked, dim or far and it won't scan. What comes out is a string of text.
THIS STICKER
Also tells you "where it is, which way"
Seen at an angle, in poor light, from far away, it's still recognized. And you can work out how far it is from the lens and how much it is rotated.

Put simply: a QR code is for a phone to scan up close; this is for a machine to see from afar. And it doesn't just identify, it locates.

3

How does
it work?

① Find the squareFirst look in the image for a box with a black border and white inside, and find its four corners
② Read the cellsThe pattern of black and white cells is its number, like a license plate
③ Use the shape to find positionStraight on it's a square; from the side it becomes a trapezoid. How skewed and how small it looks tells you how far and which way
④ Not sure? Don't guessThe number has a built-in check, so it knows when it misread. It would rather say "couldn't see clearly" than blurt out a wrong number

Step four is its key trait: missing one is fine, getting one wrong is not. Get it wrong once and the machine heads to the wrong place.

4

What is it
used for?

Robot navigationStick a row on the wall and it knows where it is at a glance
Drone landingThe sticker on the ground is its parking spot
Warehouse slotsOne on every shelf slot, one scan tells you which
Camera calibrationPhotograph a few stickers of known size to work out the lens distortion
Film effectsStickers act as anchors so virtual objects can be lined up later
Student contestsNearly every robotics team uses a set: free and it works

What they share: they all want a machine to know where something is right now, without buying piles of expensive equipment.

5

Two things
it doesn't do

It doesn't know people or things It only knows its own few hundred numbers. Anything without a sticker doesn't exist to it, people included. It can't tell who just walked up, nor whether a box holds meat or vegetables. It only knows "sticker number 7 is two meters ahead, to the left."

The other: if the sticker is dirty, torn or half-covered, it can't be recognized. It relies on the paper being intact, not on being clever.

6

Who made it,
and does it cost money?

A lab at the University of Michigan in the US made it around 2011, and it has been free for everyone to use ever since, including for business.

The commonly used set has 587 numbers, enough for most situations. The sticker itself is just printed paper: the cost is a sheet of paper and a bit of ink.

To make a machine understand reality,
it doesn't have to get smarter.
Sometimes the easier way is the reverse:
put a mark on things that it is sure to recognize.
The price of one sheet of paper buys a machine that stops guessing.
UP NEXT

What Is the MIT License?

A sticky note on a recipe: copy it, change it, even sell it, just keep the author's name and expect no refunds.

No. 21 · 3 minKeep going →
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