Circular Hough Transform

                   First we find all the edge in the image .This step has nothing to do with Hough transform and any edge detection technique can be used .It could be canny or sobel or prewitt. Here we used sobel edge detection technique. At each edge point we draw a circle with centre in the point with the desired radius .This circle is drawn in the parameter space, such that our x!axis is the a! value and y!axis in the b!value and z! axis is the radii. At the coordinates which belongs to the parameter of the drawn circle .We increment the value in our accumulator matrix which essentially has same size as parameter space .In this way we sweep over energy edge point in the input image drawing circle with the desired circle with desired radii and incrementing the value in our accumulator

When every edge point and every desired radius is used ,we can turn our attention to accumulator will now contain numbers corresponding to the number of circles passing through the individual coordinate .

The parameter space for CHT

One important difference between the Hough Transform and other approaches is resistance of the former to noise in the image and its tolerance towards holes in the boundary line.