Lines and directed lines in space

### 27.4.1 Lines and directed lines in space

See Section 26.6.1 for lines in the plane.

The line command returns and draws a directed line. It can take its arguments in three different ways.

1. Two points.
   - line takes two arguments: P,Q, two points (which can also be given as a list).
   - line(P,Q) returns and draws the line whose direction is from the P to Q.
2. A point and a direction vector.
   - line takes two arguments:
     - P, a point.
     - [u1,u2,u3], a direction vector.
   - line(P,[u1,u2,u3]) returns and draws the line through the given point with the direction given by the direction vector.
3. Two planes.
   - line takes two arguments: eqn1,eqn2, the equations of two planes.
   - line(eqn1,eqn2) returns and draws the line which is the intersection of the planes.
   - The direction of this line is given by the cross-product of the normals for the planes. For example, the intersection of the planes x=y (normal (1,−1,0)) and y=z (normal (0,1,−1)) will be (1,−1,0)× (0,1,−1)=(1,1,1).

#### Examples

line([0,3,0],point(3,0,3))

line([0,3,0],[3,0,3])

line(x=y,y=z)