I am conducting a research on the topic of mobile location recognition and recently I have reached the issue of extracting discriminative features. I've read that for upright images is strongly suggested using upright keypoints since they are more discriminative. My problem is that I don't understand clearly what is exactly an upright SIFT descriptor, the reference I found coining the term is:
Concretely in the reference they say the following:
... for images in which gravity direction information exist we project the gravity direction onto the facade and align the keypoints with this direction (upright SIFT) ...
My only clue is an implementation of this upright SIFT descriptor as part of an academic project from a course of Digital Image Processing at Stanford
Thank you very much, I would appreciate any insight.