Why is this licence plate not recognized by OpenALPR?


#1

Hello all,

I’m testing the OpenALPR Cloud API to recognize licence plates in order to blur them for privacy purpose.
The whole thing works great, but some licence plates are not recognized by the software, like the following one:


  • The quality of the picture is pretty good
  • The licence plate is fully visible
  • The picture is correctly oriented
  • The licence plate is from France, and OpenALPR is set for European plates

Thank you for your answer,

Édouard


#2

The reason is due to how we resize the images prior to recognition. In order to be efficient, we first resize the image to a smaller size to try and find the license plate. The default is 1280x720. After we found the plate we use the original resolution for more detailed analysis.

This setting is adjustable for the On-Premises SDK. For example, you can change the setting to something large (in /etc/openalpr/openalpr.conf):

max_detection_input_width = 5000
max_detection_input_height = 5000

After doing so, we are able to find the plate.

$ alpr /tmp/plate.jpg  -c eu`
plate0: 1 results
State ID: eu-fr (98% confidence)
    - EM186VS	 confidence: 94.9984	 pattern_match: 0

In the Cloud API, you cannot do that. However, I think the reason that this is not found is because the size of the image is so tall. When the height is resized to 720 pixels, the plate is barely recognizable. Perhaps, for this aspect ratio, you could send the image in 3 sections (top third, middle third, bottom third) with some overlap so that you don’t miss anything. Then you can map the X/Y offsets back to the original.

-Matt