Incorrect center detection.

[blackball]

In the first release of this pattern detector, I used the ellipse center as the circle center. In fact, under perspective projection, a circle will be a ellipse, and the center of the circle will not be the center of the ellipse. You could not estimate the the circle center by a single ellipse. Using all the ellipse we could get the circle centers using some optimization algorithm. Here is another simple way to do. Luckily, I have a black rectangle border outside the circles, and for every circle, I have a black background rectangle. I could first rectify the rectangle, and then detect the center of the circle, then project the points back using the homography matrix from the rectangle rectification.

I will improve the codes in near future. I believe this will bring a improvement. And I will add a benchmark framework for the pattern detection.

http://math.stackexchange.com/questions/214259/how-to-define-a-perspective-circle-in-xy