Change direction getter dictionary keys from floats[3] to int

[gabknight]

Description

Following discussion in PR #1342.

Dipy currently uses the 3 float defining the direction as the dictionary key to get the neighboring directions (cdf = self._adj_matrix[tuple(*direction)]). This call is slow and is done at each tracking steps. The solution would be to replace the 3 floats by an int. The 3 floats are already associated to an int as each direction is one of the vertices of the sphere.

A potential solution would be to keep track of both the floats and index on the sphere during the tracking, instead of solely the direction. This is fine for all direction getters that used a discrete sphere. However, not all direction getters use a discrete sphere (e.g. EuDX / PeaksAndMetricsDirectionGetter). We will need to be careful and well encapsulate this behavior in the direction getter.

File

dipy/direction/probabilistic_direction_getter.py

Functions

ProbabilisticDirectionGetter.get_direction(.)
DeterministicMaximumDirectionGetter.get_direction(.)

[gabknight]

We remove the adj_matrix when optimizing the tracking in PR #2805.