Editing Scale-invariant feature transform (section)

=== Orientation assignment ===
In this step, each keypoint is assigned one or more orientations based on local image gradient directions. This is the key step in achieving [[rotational invariance|invariance to rotation]] as the keypoint descriptor can be represented relative to this orientation and therefore achieve invariance to image rotation.

First, the Gaussian-smoothed image <math>L \left( x, y, \sigma \right)</math> at the keypoint's scale <math>\sigma</math> is taken so that all computations are performed in a scale-invariant manner. For an image sample <math>L \left( x, y \right)</math> at scale <math>\sigma</math>, the gradient magnitude, <math>m \left( x, y \right)</math>, and orientation, <math>\theta \left( x, y \right)</math>, are precomputed using pixel
differences:

:<math>m \left( x, y \right) = \sqrt{\left( L \left( x+1, y \right) - L \left( x-1, y \right) \right)^2 + \left( L \left( x, y+1 \right) - L \left( x, y-1 \right) \right)^2}</math>

:<math>\theta \left( x, y \right) = \mathrm{atan2}\left(L \left( x, y+1 \right) - L \left( x, y-1 \right), L \left( x+1, y \right) - L \left( x-1, y \right) \right)</math>

The magnitude and direction calculations for the gradient are done for every pixel in a neighboring region around the keypoint in the Gaussian-blurred image L. An orientation histogram with 36 bins is formed, with each bin covering 10 degrees. Each sample in the neighboring window added to a histogram bin is weighted by its gradient magnitude and by a Gaussian-weighted circular window with a <math>\sigma</math> that is 1.5 times that of the scale of the keypoint. The peaks in this histogram correspond to dominant orientations. Once the histogram is filled, the orientations corresponding to the highest peak and local peaks that are within 80% of the highest peaks are assigned to the keypoint. In the case of multiple orientations being assigned, an additional keypoint is created having the same location and scale as the original keypoint for each additional orientation.