What does size and response exactly represent in a SURF keypoint?

Question

I\'m using OpenCV 2.3 for keypoints detection and matching. But I am a bit confused with the size and response parameters given by the detection al

Answer 1

1. "size" is the size of the area covered by the descriptor in the original image (it is obtained by downsampling the original image in the scale space, hence it varies from key point to key point based on their scale).

2. "reponse" is indeed an indicator of "how good" (roughly speaking, in terms of corner-ness) a point is.

3. Good points are stable for static scene retrieval (this is the main purpose of SIFT/SURF descriptors). In the case of tracking, you can have good points appearing because the tracked object is on a well formed background, of half in the shadow... then disappearing because this condition has changed (change of light, occlusion...). So there is no guarantee for tracking tasks that a good point will always be there.