Eigen
3.3.7

Represents a generic uniform scaling transformation.
This is defined in the Geometry module.
_Scalar  the scalar type, i.e., the type of the coefficients. 
This class represent a uniform scaling transformation. It is the return type of Scaling(Scalar), and most of the time this is the only way it is used. In particular, this class is not aimed to be used to store a scaling transformation, but rather to make easier the constructions and updates of Transform objects.
To represent an axis aligned scaling, use the DiagonalMatrix class.