LINEAR PERSPECTIVE

Linear perspective is an illusion.

Linear perspective assumes:

1. A horizon line.  The horizon line is the same as the horizon (the edge of the land against the sky), and is easy to see on a large flat plane like the ocean. Most of the time geographic features (hills) and other objects (trees and buildings) hide the true horizon line.

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

Here, the horizon line is the red horizontal line that cuts through the center of this drawing.

The horizon line should be selected based on how much you want drawn above or below the horizon line.  If you have a lot of objects to draw from above, then you can place the HL higher on the page.  If you have a lot of objects to draw from below (for example, things floating in the sky above you), then you can place the HL lower on the page.

See how the location of the horizon line changes the drawing in each of these examples.

2. A fixed viewing point (Station Point).  The station point represents the eye of the observer. It is like the camera in relation to a photograph.  For example, in many renaissance paintings, the station point is very obvious.

If you stand off to the side and view this piece, the illusion is ruined.  The station point is directly in front of the painting at the center (as if the person taking this "photo" was on a ladder directly in front of this action, and their eye/camera is at the same height as the horizon line).

3. Vanishing point(s) (where parallel lines converge to, or meet; they are always located ON the horizon line, unless you are working with 3-point perspective, and we'll get to that later).

Here, the vanishing points are where the sidewalks disappear on the left and the right.  All lines (accept the vertical lines) converge, or meet, at these two points.

Here, there is a single vanishing point, and it is located at the very center of the image, and labeled "VP."

4. Orthagonals.  Lines drawn from the corners of your objects back to the vanishing point(s).

Here, the orthagonals are labeled in red.

NOTE:

Linear perspective is limited to the 'cone of vision' (a space between the vanishing points, and the viewer).

Here, you can see how the human cone of vision limits the illusion of perspective. 

Here, you can see how the cone of vision interacts with the picture plane and the ground plane/line. 

The picture plane is an imaginary flat surface located between the observer (artist) and the object being drawn.  The drawing surface (paper) is the same as the picture plane. 

The ground plane is the surface on which  objects rest.  The ground line represents where the picture plane and the ground plane intersect (it is often the bottom edge of your drawing).