Revision history [back]

1. Calibrate your camera and undistort the image - your chessboard looks slightly rounded and so your final image is rounded too.
2. You do't need to threshold the warped image - just leave as it is and the result will look better.

I am not sure if the Canny detector can successfully get the chessboard corners. It would be much easier if the edge of your chessboard was solid white.