Perspective -First part

As a premise, let me tell you that i will not be able to write about all the different perspective techniques in detail at this point.All i can do is give you an idea of the basics, that you will then be able to expand using other resources, such as books and website.
We are going to learn the basics for working with perspective and we’re going to learn them for two reasons:
Because Perspective helps us in representing volume
Because Perspective helps us in putting different objects of a scene in relation by giving them a different depth position.
First of all, the concept of “Perspective” is a lot simpler than what we might expect.
There are different techniques for creating drawings with a perspective view and here I am going to list all the ones that are useful for someone who wants to create an illustration without knowing any complex technical details:

One-point perspective or central perspective (with one vanishing point)
Two-point perspective(with two vanishing points)
Tree-point perspective(with three vanishing points)

A perspective view is based on some fundamental elements:

Horizon Line (Abb. HL)
Ground Plane (Abb. GP)
Projection Plane (Abb. PP)
Point of View (Abb. PV)
Vanishing Point (Abb. VP)

_The HORIZON LINE is an horizontal line drawn on a sheet and it represents the Horizon line for the observer. The line of Horizon determines from how high from the ground the observer is, watching the scene.
If an object is above the Horizon line, the observer is watching it from below; If an object is under the Horizon line, the observer is watching it from the top… this is something important for drawing superman while he’s flying through New York’s skyscrapers or for drawing a Chihuahua that is peeing on your shoes.
_The GROUND PLANE is the plane where the observer stands. This plane intersects the plane of projection and this intersection creates a line called Ground Line that is parallel to the Horizon Line.
_The PROJECTION PLANE is a convectional rule for drawing and it represent s an invisible plane on which we observe the projection of the image.
The POINT OF VIEW is the observer’s point of view from which starts the visual cone that frames the image.
This cone has an angle of 60 degrees
_The VANISHING POINT(s) is one or more points located on the line of Horizon in which all the parallel lines meet. In a Central Perspective, parallel lines that reach the vanishing point on the Horizon line are perpendicular to the point of view. For convention the intersection of two lines starting from a vanishing point and finishing in to a point of view has an angle of 45 degrees.
Having an angle of 45 degrees helps creating a natural perspective view similar to the one we have in real world.
If we decreases the angle putting the vanishing points closer to each other, we will enlarge the visual cone gaining an effect called “wide angle” in photography.

Here you can find some examples WA

WIDE ANGLE

This kind of perspective view allows us to draw on the sheet (and in our visual cone ) more objects than we can see in the environment, but these object will be distorted the more we lower the angle.
On the opposite, if we increases the angle putting the vanishing points far from each one, we will enlarge the angle more than 45 degrees and we will increase the convergence of the parallel lines.
In this way we will obtain a flattening effect that is like the one obtained using zoom in photography.
This is the reason why in photography for picturing a person it is better to be a little bit far from the subject and to use the zoom for decreasing the perspective distortion of the face!
It is good to know that in 3D Graphics, the visual cone is settable editing the FOV (field of view) parameter of the camera: this parameter is also used for cataloging real lenses for cameras used for filming!
Perspective technique obtained projecting objects points on one or more points on the line of Horizon allows us to create what we perceive as a volume giving a third dimension to the object.
Perspective and Volume are inseparable buddies! Let'em be your friends!

Some link on perspective

WIKIPEDIA

ArtyFactory

Translation by Christian Russo and Ryan Lovelock Thanx!