Sift descriptor
This descriptor is based on the gradient direction contribution of a neighborhood of the keypoint, SIFT descriptor is computed as follows
flowchart TD
A[select neighborhood of size 16x16]
B[split in regions of size 4x4]
C[compute gradient orientation histogram for each pixel]
A --> B --> C
The descriptor is given by all of the histogram of the regions so the dimension space of a SIFT descriptor is $R^{128}$
The gradient are rotated according to the canonical orientation of the gradient and each pixel is weighted by a Gaussian centered at the keypoint