SIFT orientation normalization

Question

I want to normalize the sift descriptors by rotating them so that the horizontal direction is aligned with the dominant gradient orientation of the patch.

I am using vl_feat library. Is there any way in vl_feat to normalize the sift descriptos?

or

what is the effective way of doing this using matlab?

Answer 1

I believe the ones in VLfeat are already oriented in the dominant gradient direction.

It shows them rotated if you look here: http://www.vlfeat.org/overview/sift.html

 [f,d] = vl_sift(I) ;

f is a Nx4 matrix of keypoints. N being the keypoint indexand the other 4 being, x position, y position, scale, and orientation. d is a Nx128 matrix, where N is the keypoint index, and the 128 dimensions belong to the SIFT descriptor.

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If all of your images are rotated upright, it is actually beneficial to not use rotational invariance. See this paper which assume a gravity vector: https://dspace.cvut.cz/bitstream/handle/10467/9548/2009-Efficient-representation-of-local-geometry-for-large-scale-object-retrieval.pdf?sequence=1