What Linear Perspective Means
The drawing of three-dimensional form has conventionally been dealt with in a technique called linear perspective.
Linear just means "characterized by an emphasis on line." Linear perspective, therefore, is typically defined as being "a means of producing a three-dimensional image onto a two-dimensional surface by the use of lines." (right)
Notice that depth is primarily achieved through the use of converging lines which seem to extend out into the distance.
Others get more get fancy about it, defining linear perspective as "a mathematical (characterized by the exactness or precision of mathematics) system for creating the illusion of space and distance on a flat surface such as a canvas or wall."
Unfortunately, though, impressive-sounding definitions like that do not explain what lines have to do with anything.
The point is, "linear perspective" and its definitions often lend little or no intuitive understanding for what is actually quite simple.
The Basis of Linear Perspective
Linear perspective assumes that all forms are made of right angles.
This makes a box the ideal shape for demonstrating it. Its form consists of the three basic directions corresponding to height, width and depth. (right)
Direction points are only used for illustrating those forms foreshortening away from us. And so only those parts of a form that are angled away will require a direction point. Ordinarily, such a form allows for up to three, according its three overall directions.
The number of direction points used, then, has nothing to do with where an object is located within our picture but how it is angled to us.
Generally, the more direction points used, the more three-dimensional an object will look since this will give us more viewing angles of it. So to see something as close to one angle as possible, employ one-point perspective. To see it mostly from two angles, use two-point perspective. These, however, are very general rules being that even a form in one-point perspective can clearly display three different angles of it. (left)
Commonly, we will have a combination of one-, two- and three-point perspectives among the differently positioned forms present. These methods, then, should not be confused to mean that all forms in the scene should go to just one, two or three direction points in linear perspective. We may not want all forms pointing the same way since mixing them up can make for a more interesting picture.
Limitations of Linear Perspective
Linear perspective has limited application since it deals mainly with drawing with lines on flat surfaces, excluding it from any art form like stage, window display or designing amusement park attractions. Even though we can still see converging lines in reality, there is no means of looking at a box in a way that would only require one direction point. It will have direction points for every direction pointing away from us. (right) A large form could even extend to direction points that are behind us.
Also, even with a majority of man-made forms being based on right angles and straight, parallel lines, we will find almost nothing of these in nature nor in any life form whether animal or vegetation. We could paint many hundreds of landscapes and seascapes without using one single direction point and set of parallel lines. The fact is, there is only so much you can express through converging lines.
Yet, I have read that "You cannot understand perspective unless you understand the cube." The basis of this limited thinking is merely that one-, two- and three-point linear perspective methods are based on objects which are made up of right angles, as cubes are.
And since everything in the world isn't made of right angles nor can be drawn with converging lines, linear perspective will call for the outlining of all objects, even curved or irregular forms, with rectangles or boxes so its techniques will more-or-less function (which can be workable).