Visual Tracking - Rectification

To perform rectification, the yardlines, sidelines, and hashmark lines are found using the Canny edge detection algorithm and line-finding software from the INRIA research lab in France.

Intersection points between lines are identified and matched with the field model at each frame. The intersection points can be recovered regardless of the state of the players on the field. Even when player is directly occluding an intersection point, as indicated by the arrow, the intersection point will still be recovered with sub-pixel accuracy.

The field model is initialized to the first frame of the video sequence (by manually marking four corresponding points), after which the intersections are tracked automatically.

Once corresponding intersection points are recovered, the rectification is performed using a simple homographic transform (perspective four-point transform).

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