I’ve been fiddling with OpenCV on-and-off in my spare time for a few weeks now, and I’m starting to get a feel for the challenges involved in number plate recognition. There’s plenty of papers and background material out there to keep me reading for months.
The simple approach as used by a number of researchers is to concentrate on thresholding the image (sometimes many levels of thresholding), and then doing a bit of connected component labelling to find candidate regions of the image to extract and pass to a second-stage character recognition algorithm.
Here’s my first attempt. Source image, then thresholded and labelled image.