Detect circles with various radii in grayscale image via Hough Transform

Detect circular shapes in a grayscale image. Resolve their center positions and radii.

Circular Hough transform based on the gradient field of an image.
1. Operates on grayscale images, NOT B/W bitmaps.
2. NO loops in the implementation of Circular Hough transform, which means faster operation but at the same time larger memory consumption.
3. Detection of radii works for concentric circles as well.