2008.01.27 19:18 "[Tiff] OMR bubble detection", by Leonardo Serni

I am looking into creating n application that will read OMR images stored in TIFF files extracting the bubbles that are marked. I was wondering if anyone had some algorithms or examples of doing this. A search of the Internet has not resulted in any success. Thank you in advance for your help.

The most practical approach would be, I think, to employ grayscale images and calculate the minimum white and maximum black threshold... then count the number of "black" pixels in each bubble. Empty and full balloons will sport different blackness averages.

Things get a bit more complicated if images aren't aligned (but still lay parallel to axes, i.e., you have an offset on X and Y) and may get really hairy if you need to cope with rotations (even if for very small rotation values you might get off by employing shearing).

A much more complicated approach would require correlation between a full balloon and an empty one, but this assumes people mark consistently their balloons, which, let's face it, is not a realistic constraint.

Leonardo