Docs

QR, NFC and labels

A ref is a URL. Anything that can carry a URL can carry a ref: a printed QR code, an NFC tag, a line of text on a spine, a sticker.

QR codes

Each row in My refs has QR SVG and QR PNG. The SVG scales to any size without blur and is what you want for a cutting plotter, a label designer or a book cover. The PNG is 444 pixels wide and fine for a quick print. The code encodes https://shelfref.com/t/{id} with medium error correction, so it survives a scratch or a logo in the middle.

Minimum printed size: about 12 mm for a phone at arm's length, 20 mm if the surface is curved or glossy. Leave a white margin of at least two modules around the code.

Label sheet

Print labels makes one A4 PDF with every ref at three sizes (20, 30 and 45 mm), each with its id and your private label underneath. Print on plain paper and cut, or on sheet labels. The id is printed in letters you can read aloud: the alphabet has no 0/O, 1/l/i or j.

NFC tags

Any blank NFC tag (NTAG213, 215 or 216 are the common ones; a sticker costs cents) can hold a ref. On Android, open My refs in Chrome and press Write NFC, then hold the phone to the tag. The browser writes the URL as an NDEF record. Tapping the tag with any phone afterwards opens the ref: no app needed, on iPhone or Android.

iPhones cannot write tags from the browser. Use any free NFC writer app and paste the URL (the Copy URL button gives it to you). Tags can be rewritten, but you will rarely need to: change the target on shelfref instead.

NFC does not work through metal. Put the tag on or inside wood, cardboard, plastic or paper.

Both at once

A QR and an NFC tag on the same object carry the same URL, so the thing works with a camera and with a tap. This is how the book tokens shelfref was built for are made: QR printed on the back, tag glued inside.