What is a good method to find all occurrences of a specific letter in an image?

Like, finding the positions of all the A's within a .png, regardless of scale, but all rotated properly?

Edit: Those who remarked that this is a more difficult problem than I am making it out to be are correct. I wanted to add some detail; I'm looking for a way to eith开发者_C百科er find the position of a letter from a (black-on-white scanned handwritten image), or given an image of just the letter, find the A-ness of it (WRT a certain font).

I realize this is a large project, so I was wondering if there were any open-source libraries that do something like this, or a simple algorithm. I've tried some corner-matching schemes but had no luck.

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First, your question is in the domain of computer vision (not image processing, though it may seem so for the uninitiated). Then, it makes a big difference whether the png image is a photo of a natural scene, or a scanned document. In the case of natural images you need to check the literature in object detection in general or text detection in particular. For scanned documents your field is document analysis (or document processing).

In any case, I think that your problem is probably solvable, although the solution may be farther (in efforts and time) than you might care to persist.

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Well one very good way to do this is to use a NGC (non-grayscale correlation) with your 'A' as model. The problem is this is not a scale invariant method. Indeed this question could be very complicated. To solve the multiple scales you could generate models for each scale and loop (slow).