Traditional Culture Encyclopedia - Photography major - How to treat the projection of a straight line on a plane?

How to treat the projection of a straight line on a plane?

Case 1, the straight line is parallel to the plane,

Take any two points on a straight line as plane perpendicular lines respectively, and connect two vertical feet on the plane.

A straight line is the projection of a straight line on a plane.

In case 2, a straight line intersects a plane.

Take any point on the straight line out of the plane and make a vertical plane to connect the vertical foot and (the intersection of the straight line and the plane).

The obtained straight line is the projection of the straight line on the plane.