Recognize numbers in images

Question

I've been searching for resources for number recognition in images on the web. I found many links providing lots of resources on that topic. But unfortunately it's more confusing than helping, I don't know where to start.

I've got an image with 5 numbers in it, non-disturbed (no captcha or something like this). The numbers are black on a white background, written in a standard font.

My first step was to separate the numbers. The algorithm I currently use is quite simple, it just checks if a column is entirely white and thus a space. Then it trims each character, so that there is no white border around it. This works quite well.

But now I'm stuck with the actual recognition of the number. I don't know what's the best way of guessing the correct one. I don't think directly comparing to the font is a good idea, because if the numbers only differ a little, it will no more work.

Could anyone give me a hint on how this is done?

It doesn't matter to the question, but I'll be implementing this in C# or Java. I found some libraries which would do the job, but I'd like to implement it myself, to learn something.

Answer 1

Why not look at using an open source OCR engine such as Tesseract?

http://code.google.com/p/tesseract-ocr/

C# Wrapper for Tesseract

http://www.pixel-technology.com/freeware/tessnet2/

Java Wrapper for Tesseract

http://sourceforge.net/projects/tessocrinjava/

While you might not consider using a third-party library as implementing it yourself, there's a tremendous amount of work that goes into just integrating the third-party tool. Keep in mind also that something that may seem simple (recognizing the number 5 versus the number 6) is often very complex; we're talking thousands and thousands of lines of code complex. In the least, look at the source code for tesseract and it'll give you a good reason to want to leverage a third-party library.

Here's another SO question that'll give you some ideas about the algorithms involved: https://stackoverflow.com/questions/850717/what-are-some-popular-ocr-algorithms