A perspective projection is a pictorial drawing represented on a vertical plane called the picture plane when viewed from a fixed position. The position of the picture plane relative to the object determines the size of the perspective view. When the object is in the picture plane, its perspective will be in true size. When the object is placed in front of the picture plane, its perspective will be of enlarged size. When the object is placed behind the picture plane, the perspective will be reduced in size. The picture plane in this case is placed between the eye and the object.
All the projections converge to a point known as vanishing point. This effect will be seen vividly by looking at a long straight railway track. Such an effect will be noticed in photographs too. Photograph therefore is a perspective view. Perspective is chiefly employed in architectural drawings. The key terminology is as follows:
Ground Plane (GP)- Ground plane is a horizontal plane representing the ground on which the object is generally placed.
Picture Plane (PP)- It is a vertical and transparent plane o which the perspective view of the object is projected. Normally it is placed in between the observer and the object.
Station Point (S)- Station point is the position of the observer’s eye. Its position is fixed at any convenient distance either from the picture plane or the object.
Horizon Plane (HP)- It is an imaginary horizontal plane at the level of the observer’s eye or station point. It is above and parallel to the ground plane and perpendicular to the picture plane.
Central Plane (CP)- It is an imaginary vertical plane which passes through the station point. It is perpendicular to both the picture plane and the ground plane.
Ground Line (GL)- It is the line of intersection of the ground plane with the picture plane.
Horizon Line (HL)- Horizon line is the line of intersection of the horizon plane with the picture plane.
Axis of Vision (AV)- It is a line drawn perpendicular to the picture plane from the station point. It is also termed as “Perpendicular Axis”.
Centre of Vision (Cv)- It is the point at which the axis of vision pierces the picture plane. It also lies on the horizon line.