Why we read number plates on the phone, not in the cloud

The obvious way to build a plate-reading stocktake app is to send each photograph to a cloud recognition service and get the registration back. Several are excellent. We used one during development and it read plates very well. We replaced it, and the reasons are worth explaining because they are reasons a dealer should care about.

Where the photographs go

A stocktake photograph is a picture of a car on your premises. Sometimes it has a customer in the background, or a customer’s car, or your staff. Sending every one to a third party in another jurisdiction to be read means that third party processes personal data on your behalf, needs to be in your privacy notice, and needs a processing agreement. It is manageable. It is also entirely avoidable.

Reading the plate on the phone means the photo is stored privately in your account as evidence and goes nowhere else. Nothing to declare, nothing to trust.

What a per-scan price does to behaviour

When each photo costs money, people are told to be careful with photos. Photograph only stock, do not photograph the customer cars, do not retake. That is the opposite of what a good stocktake needs, which is: photograph everything with a plate and let the software sort it out. Reading on the device costs nothing per scan, so the instruction becomes "snap everything", and the count gets better.

Signal

A cloud reader needs a connection at the moment of the photo, or a queue and a delay. Compounds, underground car parks and steel-framed workshops are where signal dies. An on-device reader gives the answer immediately and queues only the upload of the evidence.

How it works, briefly

The phone finds plate-shaped white or yellow regions in the photo, straightens them, reads the characters with an open-source recogniser running in the browser, and fits what it read to the real UK plate formats, correcting the usual confusions (0 and O, 1 and I, 5 and S, 8 and B) only where the format demands it. Square two-line plates are handled, and so are windscreen VINs; the last eight characters are enough. The candidates are then matched against your stock list, which is the only place a plate is ever confirmed. Nothing is counted from a photo alone.

The first time the scan screen opens it fetches about 6 MB of reader files from our site, once. After that it works with no signal at all.