**Theory**

The observer is standing on the horizontal reference plane **π _{1}**;

**h**measures the eye

**O**height above this plane.

The reference plane

**π**intersects the picture plane in the line

_{1}**t**, which is named

*trace*of

**π**.

_{1}Imagine a plane

**π**

_{1}**°**, belonging to the point

**O**and parallel to the plan

**π**,

_{1}This plane intersects the picture plane in the horizon

**i’**, which is named

*vanishing line*of

**π**.

_{1}Since the planes

**π**and

_{1}**π**

_{1}**°**are parallel, the lines

**t**and

**i’**are parallel too.

Since the picture plane is vertical, the horizon

**i’**passes through the principal point

**O°**.

The line **i’** is image of the line at infinity **i**,** **that the plane **π _{1}** has in common with all the planes parallel to it.

This is why we say that the vanishing line is the image of the

*laying*of a plane and all the planes parallels to it.

If you want to explore this topic you can read: Perspective and infinity.

**Practice**

Measure the height of the eye (**h**).

Draw two parallel lines (**i’** and **t**).

Note that, if the picture plane is vertical, **i’** belongs to **O°**.

**Note
**The two lines

**i’**and

**t**`represent’ the plane

**π**, according to geometry, but do not describe it effectively. So it is always best to add a landscape allusion (e.g. the ground, the sky …).

_{1}