Introduction - If you have any usage issues, please Google them yourself
Basic Principle:
First ,initial point Pzero,assumed that Pzero belong to Gzero,moves the distance deltaX along one direction,assuming X axes,and reaches the point Pone belonging to Gplus or Gminus,assumed that it is Gminus.Then,it moves the distance deltaY along another direction ,that is Y axes,and reaches the point Ptwo.It will change the directon of moving if Ptwo belongs to Gplus or it will keep moving to Gplus.