From The Living Headlights (our project in Chapter 5, Equipping Your Car with a Rearview Camera and Hazard Detection), we are already familiar with one technique to detect circles. We treated the problem as a special case of blob detection and we used an OpenCV class, SimpleBlobDetector, which allows us to specify many detection criteria such as a blob's size, color, and circularity (or noncircularity, that is linearity).
A blob is a shape filled with a solid (or nearly solid) color. This definition implies that many circular or linear objects are not detectable as blobs. In the following photo we see a sunlit desk with a china teapot, china bowl, and pewter bowl:
The bowls and the lid of the teapot have approximately circular outlines in this top-down view. However, they are unlikely to pass the detection as blobs because the interior of each shape is multicolored, especially in uneven light.
Blob detection starts with a simple threshold filter (marking bright regions...
