I'm doing a scene using openGL (a house). I want to do some collision detection, mainly with the walls in the house. I have tried using the TestIntersionPlane function in Lesson 30 but I don't understand what exactly should the first two parameters be. I've tried the following: Vector planeNor(0,0,1); Vector position(0,0,-10); Plane p(planeNor,position); Vector vel(0,0,-1); double lamda; Vector pNormal; coll= p.TestIntersionPlane(vel,Z,lamda,pNormal); glPushMatrix(); glBegin(GL_QUADS); if(coll) glColor3f(1,0,0); else glColor3f(1,1,1); glVertex3d(0,0,-10); glVertex3d(3,0,-10); glVertex3d(3,3,-10); glVertex3d(0,3,-10); glEnd(); glPopMatrix(); where Z is initially (0,0,0) and everytime I move the camera towards the plane, I reduce its z component by 0.1 (i.e. Z.z-=0.1 ). I know that the problem is with the vel vector, but I can't figure out what the right value should be. Can anyone help me please?