This operator returns the multiplication of a three-dimensional vector with the matrix. The Matrix a has to be a 4 X 4 matrix. So the 3D-point will be "lifted" to a 4D-Point by setting the fourth coordinate to 1. After the well known matrix multiplication the coordinates are divided by the result in the fourth coordinate. In this way, it is for example possible to transform a point by a matrix, which represents a perspectively projection.