@ahojnnes : The elements in `keypoints` list are tuples containing location of keypoints. This is the data-structure that IMO should be for storing keypoints extracted from an image. OpenCV has a separate class for this http://opencv.willowgarage.com/documentation/cpp/features2d_common_interfaces_of_feature_detectors.html#keypoint. This class acts as the output of FeatureDetector instance and input to the FeatureDescriptor instance. Analogous to that, I feel a list of tuples should be ideal for storing keypoints. If you have better ideas, please let me know.