What you need is called OCR, "Optical Character Recognition". Please see:
http://en.wikipedia.org/wiki/Optical_character_recognition[
^].
Unfortunately, I must warn you: I never found any Open-Source OCR solutions which would be perfect, but it could be good enough for just the license plate number. There are few CodeProject solutions I like, but, to best of knowledge, they are all incomplete, would need serious amount of work to make a working application out of them, and they are for .NET, not Java.
Please see these CodeProject articles:
Neural Network OCR[
^],
Creating Optical Character Recognition (OCR) applications using Neural Networks[
^],
Neural Network Classifier[
^],
A C# Project in Optical Character Recognition (OCR) Using Chain Code[
^],
Unicode Optical Character Recognition[
^],
Hebb neural network[
^],
Backpropagation Artificial Neural Network in C++[
^],
OCR Line Detection[
^].
Maybe there are newer projects I'm not familiar with; so it would not hurt if you do your own search in CodeProject and elsewhere. Please, if you find something good, please reply in the comment to this answer.
[EDIT: in response to a follow-up question to Solution 1:]
Right, you might also need some general image processing to be used before OCR. I recommend the open-source library AForge.NET written by
Andrew Kirillov, a CodeProject member:
http://en.wikipedia.org/wiki/AForge.NET[
^],
http://www.aforgenet.com/[
^].
Please see Andrews articles at CodeProject:
http://www.codeproject.com/Articles/Andrew-Kirillov#articles[
^].
—SA
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