Perspective

Perspective in the graphic arts, such as drawing, is an approximate representation, on a flat surface (such as paper), of an image as it is seen by the eye. The two most characteristic features of perspective are that objects are drawn:

• Smaller as their distance from the observer increases.
• Foreshortened: the size of an object's dimensions along the line of sight are relatively shorter than dimensions across the line of sight.

Linear perspective is a system for drawing objects that use lines and vanishing points to determine how much an object's apparent size changes with space.

BASIC CONCEPTS

The horizon line is a theoretical line that represents the eye level of the observer. The horizon line is the same as the horizon (the edge of the land against the sky) only on a large flat plane like the ocean. Most of the time geographic features (hills) and other objects (trees and buildings) make the horizon above the horizon line.

Indoors the horizon is often not visible but there is still a theoretical horizon line representing the point of view of the observer.

Vanishing points are points (usually) on the horizon line where receding lines (planes) converge. The vanishing point (v.p.) is on the horizon line when an objects has horizontal planes that are parallel to the ground. When the object's planes are inclined the vanishing points can be above or below the horizon line.

There are two basic systems of linear perspective: one-point and two-point named after the number of vanishing points used in each.

All parallel lines follow the same rules. If one goes to a vanishing point then all like lines go to the same vanishing point. In most systems vertical lines are drawn vertical (not in three-point perspective).

The station point represents the eye of the observer. It is the camera in a photograph.

The picture plane is the "window" that is represented by the picture.

The ground line is a line that is parallel to the picture plane at the base of the object being depicted.

Types of perspective

1.      One-point perspective

One vanishing point is typically used for roads, railway tracks, hallways, or buildings viewed so that the front is directly facing the viewer. Any objects that are made up of lines either directly parallel with the viewer's line of sight or directly perpendicular (the railroad slats) can be represented with one-point perspective.

One-point perspective exists when the painting plate (also known as the picture plane) is parallel to two axes of a rectilinear (or Cartesian) scene — a scene which is composed entirely of linear elements that intersect only at right angles. If one axis is parallel with the picture plane, then all elements are either parallel to the painting plate (either horizontally or vertically) or perpendicular to it. All elements that are parallel to the painting plate are drawn as parallel lines. All elements that are perpendicular to the painting plate converge at a single point (a vanishing point) on the horizon.

2.      Two-point perspective

Two-point perspective can be used to draw the same objects as one-point perspective, rotated: looking at the corner of a house, or looking at two forked roads shrink into the distance, for example. One point represents one set of parallel lines; the other point represents the other. Looking at a house from the corner, one wall would recede towards one vanishing point; the other wall would recede towards the opposite vanishing point.

Two-point perspective exists when the painting plate is parallel to a Cartesian scene in one axis (usually the z-axis) but not to the other two axes. If the scene being viewed consists solely of a cylinder sitting on a horizontal plane, no difference exists in the image of the cylinder between a one-point and two-point perspective.

Two-point perspective has one set of lines parallel to the picture plane and two sets oblique to it. Parallel lines oblique to the picture plane converge to a vanishing point, which means that this set-up will require two vanishing points.