Perspective. The terminology.

In the Composition and Perspective post I explained what perspective is.  In this article I will introduce the basic terms used in perspective. Almost all the books or articles about perspective have their own terminology. To avoid confusion I will report the terminology that will be used on this blog,  Hoping  it will be helpful to people moving their first steps into this fascinating world.

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Point of View (PV)

It’s the point from which the viewer’s eyes see the subject. The visual cone has an angle of 60°. To test this, close one eye and stand or sit with your back straight, look at a spot located in front of you, at eye level. Lift up and stretch your arms in front of you, then open them until you are not able to see your hands with precision. The two arms form an angle of 60 degrees.

Projection plane (PP)

It is an imaginary vertical plane through which the observer looks at the subject. What is visible depends on several factors:

• Size and position of the plane;
• Height of the point of view;
• Distance between the point of view and the projection plane.

To simplify the discussion you can imagine the projection plane as the classic pass par tout used by artists to frame a scene.

Horizon Line (HL)

This is a horizontal line on the projection plane at the same level of the point of view. Everything in perspective is related to this line. In landscapes this line coincides with the horizon and, depending on its location, lets you know if the viewer is watching the scene from above, below or from middle position. For example, consider the three images below. In the first case, the scene is viewed from below, from middle position in the second and from above in the third case.

Photo from 2D Design Notes

Typically artists avoid drawing the horizon line in the middle of the paper, to avoid excessive symmetry always harmful to the artistic result.

Vanishing Points (VP)

These are points placed on the horizon line. A first vanishing point is called “central vanishing point” (CVP), located just opposite the point of view, it is the intersection of the horizon line and its perpendicular starting from the point of view. We will call this perpendicular  projection ray (red line in the first figure). Then there are two lateral vanishing points, which are on the horizon line on the right and left of the central vanishing point at the same distance. We will call these two points: the right vanishing point (RVP) and left vanishing point (LVP). The lines that join these vanishing points and the point of view form an angle of 45 degrees. Hence, the distance from these vanishing points and the central vanishing point is equal to the distance of the observer from the projection plane.The figure below shows an example where the vanishing lines converge towards the central vanishing point.

Photo from 2D Design Notes

In this other figure, however, the central box has a side parallel to the projection plane, then, as we shall see in next articles, the perspective lines converge to the central vanishing point. The light pole and the box to the right, however, are oblique to the projection plane, and then the perspective lines converge towards the lateral vanishing points.

Photo from 2D Design Notes

Reference Plane (PR)

It’s the plane on which the observer is located. It comes from the observer’s feet and goes perpendicular to the projection plane.