# Optimization, Maintainability and Major Milestones

The recent silence on the blog was a result of the debates of optimizations vs maintainability that were going on behind the scenes. Nevertheless, a major chunk of this project is now complete and is now ready to be merged into scipy.

# Enhancements

## Rotation Application

We can finally do what rotations were meant to do: rotate vectors. If an initial frame rotates to a final frame by rotation R, then the application of this rotation onto a set of vectors can be seen in two possible ways:

1. As a projection of vector components of the original frame expressed in the basis of the final frame. In this respect, a rotation is simply a change of basis.
2. As a physical rotation of a vector being glued to the original frame as it rotates. In this visualization, we stay in the initial reference frame and simply change coordinates.

There were a number of ways to approach this implementation, but we quickly settled on the simple approach of a matrix vector product.

The set of input output semantics to support, however, was infinitely more interesting and took some discussion. We now support applying N rotations on P points, where N and P follow numpy broadcasting rules: either one of them equals unity (in which case we broadcast the single object across the larger one), or they both equal each other (in which case we apply corresponding rotations on corresponding points). The output would be shaped (N  {or P}, 3) unless we had a single rotation being applied onto a single vectors.

Devising a simple implementation that would handle all these cases was challenging, but I ultimately found what I think is an elegant solution based on `numpy.einsum`.

## Rotation Compositions

Compositions of a rotation were also added. There were quite a few implementations of this one. After some benchmarking and some discussion, we finally chose to compose quaternions using the Hamilton product. While not the fastest approach, it is easier to maintain in the long run and quickly catches up for large numbers of rotations.

Again, the number of rotations contained in two composable rotations R1 and R2 must follow the same rules as for the application case. Implementing this broadcasting was a much simpler task as numpy functions handled this implicitly for us.

## Rotation Inversions and Indexing

We now support inverting rotations and extracting particular rotations from an object containing multiple rotations. This was one of the more straightforward and least controversial additions done since the last post.

# Major Milestone

I’m happy to announce that the process of merging the Rotation class into the main scipy repository has been initiated and my work will soon be available to thousands of users across the globe.

This represents the completion of the first phase of this project. Now we need to focus on interpolations and estimations, all of which are interesting, mathematically and otherwise, in their own right. Stay tuned!