Homogeneous coordinates
- replace 2d points with 3d points, last coordinate 1
- for a 3d point (x,y,w) the corrsponding 2d point is (x/w,y/w) if w is not zero
- 3d points (x,y,0) correspond to directions (x,y), if x or y are not zero
- (0,0,0) does not correspond to a 2d point
- Geometric construction: 3d points are mapped to 2d points by projection to the plane z =1