How to find rotation angle of image?

Question

I'm feeding in a Bitmap image to my C# program to be able to perform OCR to identify the characters in the image. I can do this fairly well if the image is not rotated. One of the program requirements, however, is that the program automatically determines if the image has been rotated, and that it automatically corrects these rotations.

I've tried implementing a simple method where lines are traced across the image and points which contact a character are recorded, and then performing a simple linear regression on the line points. This works to an extent, although it has not proven very accurate due to curvature of characters, etc.

I was wondering if there was a better method to solve this problem? Many thanks in advance! :)

Answer 1

I use gmseDeskew algorithm to deskew an image in my program. It works very well.

Answer 2

It's an interesting problem to be sure. I'd look for certain letters that are easier to tell rotation for. For example, a capital A or R or K should have both of the lower parts are roughly the same horizontal plane. Another option is to take letters that cannot be identified and rotate them in various ways and re-attempt to identify them. If a letter than could not be identified in the raw scan CAN be identified when you rotate it, that's a pretty big clue. Once you have identified the "correction" rotation that makes a non-recognizable character into a recognizable one, apply the same rotation value to the others.

Answer 3

If it recognizes lines of text, then try to blur the image so that lines are mostly solid and find direction of the lines (either with analysis of Fourier transform or by ridge detection).