A the Hough, now there’s an amazingly simple method for finding circles, lines, and other shapes. Lest this sounds too narrow an application, consider the wild possibility of finding a pool ball. The hough can find these in almost linear time. Amazing!

The idea is this: build an “accumulator”, which is really a voting array for parameters describing the object you wish to find. In the case of circles, you could have one accumulator for each approximate radius. Your task is then to go through the source image and find features which “might” be parts of a pool ball; in this case, edges might work. For each point on an edge, we “vote” in a radius around the point. If we do this for all points around a ball, the locus of the ball will have a high number of votes.

Here’s an example image with some pictures of the accumulator for circles of radius 32 and 48.

Source Image:

Paramater Space at radius 48

Paramater Space at radius 32

Shown below, is the final result of my Hough Transform for radius 32. Note that it missed one, but had no false positives. Not bad, especially considering I use a general approach that isn’t “hand tuned” to this image.