Trying the SimpleBlobDetection to identify circumferences in low-resolution images, i'm in doubt about the meaning of the x and y properties of the returned keypoints. In particular, since i need to map from the pixel domain to a physical domain (in my problem every pixel has a side length expressed in meters), i believed that keypoint.x = 0 and keypoint.y = 0 indicated to the upper left corner of the upper left pixel, instead it seams that keypoint.x = 0 and keypoint.y = 0 refer to the center of the upper left pixel. Is my impression right?