I'm very confused about question 11/12 from the final review.  I know that if I want to find if two lines are parallel I test to see if there cross product is zero and I know that given p0 and p1 that their direction vector is  P0P1 = <x1-x0; y1-y0; z1-z0> which we can use to find the parametric equation, but what vector do I use to test if the two lines are parallel?  I think it's the direction vector (because that makes sense), but i'm not sure.    

 Will you give us the 'hints' section that you give us for question 18 on the exam?

eykang wrote:

1. Two lines are parallel if their DOT product is 1. Or their directions are same.

True, but I'm certain that's true if their cross product is either 0 or 180 as well.   I guess if we normalized the vectors the dot product would be easier, but my question is still if we test using either method should we be using the direction vector c of each line segment ie (C-A)t.

2. On question 16, the points are expressed not in the homogeneous coordinate system. So it means that when we assume they are in the homogeneous coordinate system, w=1.

3. Yes, I will give hint equations.

How do we find the projection of the points/lines on 11 and 12 if we don't know what the near plane (N) is?