Editing Hough transform (section)

=== Circle detection process ===
{{main|Circle Hough Transform}}
Altering the algorithm to detect circular shapes instead of lines is relatively straightforward.

* First, we create the accumulator space, which is made up of a cell for each pixel. Initially each cell is set to 0.
* For each edge point (i, j) in the image, increment all cells which according to the equation of a circle <math>(i - a)^2 + (j - b)^2 = r^2</math> could be the center of a circle. These cells are represented by the letter <math>a</math> in the equation.
* For each possible value of <math>a</math> found in the previous step, find all possible values of <math>b</math> which satisfy the equation.
* Search for local maxima in the accumulator space. These cells represent circles that were detected by the algorithm.

If we do not know the radius of the circle we are trying to locate beforehand, we can use a three-dimensional accumulator space to search for circles with an arbitrary radius. Naturally, this is more computationally expensive.

This method can also detect circles that are partially outside of the accumulator space, as long as enough of the circle's area is still present within it.