Clone and unzip the repository. polyUnion.oct file has been made already.

To make oct file on your own :
Run the following command in the respective dirtectory :
mkoctfile clipper.cpp polyUnion.cc -o polyUnion

Input is taken in the form of cells.
First cell represents the set of x coordinates of each of the subject polygons.
Second cell represents the set of y coordinates of each of the subject polygons.

Take a test input in octave, for e.g. x = {[0 100 100 0],[50 150 150 50]} , y = {[0 0 100 100],[0 0 100 100]}
Call function : polyUnion(x,y)

Function will return the vertices of Union of the two subject polygons.