Front and Back

• The plane equation allows us to calculate “front” and “back” for any point easily

• S = Ax+By+Cz+D

  • If S >0 then (x,y,z) is in front of the plane
  • If S <0 then (x,y,z) is behind the plane
  • If S =0 then (x,y,z) is on the plane

• S is the perpendicular distance from the plane

• Example: Where is point (2,3,1) in relation to the xy plane

  • S = 0*2+0*3+1*1+0 = 1
  • The point is in front of the plane