Put the rotating QR code from your phone onto the projector from this
computer. A QR code on a phone screen is readable from 1–2 metres;
having students come up to the phone one by one is not a workable
approach in a crowded room.
1. Start attendance on your phone
Open the session in the teacher app and press the
“Hand over to computer (projection)” button under the
QR code.
2. Show the QR code on your phone to this computer
Turn on the camera and hold the phone up to the screen. The key
does not travel over the network while this happens:
between the computer and the phone there is only a camera, and nothing
goes to the server.
Manual entry
Paste here the “The computer has no camera” line from your phone.
This route is riskier than the QR code: the key sits
here as readable text — it gets copied to the clipboard and can be
read over your shoulder. Also, on this route the
clock difference cannot be measured, because the
typing time destroys the measurement.
The system cannot verify this. This page cannot know
which computer it was opened on. Hand the QR code over to your own
computer; not to the classroom computer, not to the shared lectern
machine, not to someone else's machine. The device you hand over to
becomes able to produce valid QR codes for this class for the rest of the
session.
The computer's clock must be correct. The QR code is
generated from time and the freshness window is 5 seconds. If the clock
has drifted, the QR code produced is stale and students are rejected
without understanding why. This page compares against the
phone's clock at hand-off time and warns if there is drift — but there is
one thing it cannot measure: if you change the clock after the
hand-off, the page can only sense it as a jump.
Offline. No internet is needed once the page is open;
the QR code is computed on this computer. For that reason, open the page
before you go into class — the page itself is downloaded over the
network.