Image Feature Detection

Question

I am working on an application to recognize line-like features inside circle/elliptical shapes. The shapes look like the following (two shown here):

• The shapes themselves can vary slightly between circle and oval.

• There are at most 5 lines inside the shapes and they are found in roughly the same area on every shape.

• These lines can vary slightly in length, thickness, rotation, and curvature.

• The lines can sometimes touch/intersect slightly.

• Usually, there are exactly 5, but occasionally a line could be missing entirely.

• I don't care about color, black and white thresholding is fine.

Each object (of 100+) will be captured individually by video; the capture is a manual/physical process (i.e. I am holding the camera each time). I have full control over the camera so I can position it consistently for each capture.

Right now I'm trying to use OpenCV to do the recognition. I was able to modify the sample 'face recognition' app to use another Haar identifier XML file, but this seems to only handle detection of the outside circle/ovals.

I am interested in generating an object per sample, to describe the 5 interior lines for further processing:

{

    1: { length: 20, avg_thick: 2.3 },

    2: { length: 4, avg_thick: 2.0 },

    3: { length: 9.1, avg_thick: 2.1 },

    4: { length: 2, avg_thick: 1.9 },

    5: { length: 17, avg_thick: 2.1 }

}

This is my first project involving image recognition. What algorithms or procedures should I use/research to achieve this? Thanks!

UPDATE:

Since the images will be photographed by hand, they are not pure black/white. Trying to apply thresholding makes the (feint) lines inside the shapes sometimes disappear. How could I improve the thresholding results?

Answer 1

You can use the Hough transform if the lines are approximately straight, to find all lines, and the circle version of the Hough transform to find all circles/ellipses.

If the lines are not straight, according to you: "narrow elongated regions" not lines, right? You'd have to skeletonize the threshold first. You can refer to the following link: http://opencvpython.blogspot.com/2012/05/skeletonization-using-opencv-python.html