Quaternion and Slerp

http://www.euclideanspace.com/maths/algebra/realNormedAlgebra/quaternions/transforms/

http://www.euclideanspace.com/maths/algebra/realNormedAlgebra/quaternions/functions/index.htm

http://ggt.sourceforge.net/html/group__Interp.html#a1

https://en.wikipedia.org/wiki/Slerp

http://number-none.com/product/Understanding%20Slerp,%20Then%20Not%20Using%20It/