Introduction - An Introduction to Computer Graphics for Artists - page 3

1.1.1 Linear Perspective and Projection

To reproduce linear perspective on paper, artists draw a straight horizontal line to
represent the horizon, then a point in the center of the line to represent the vanish-
ing point (this corresponds to the center of one's pupil). Another horizontal line is
then drawn either above or below the horizon line and subdivided equally, with
evenly spaced lines drawn directly into the vanishing point. These lines represent
parallel lines and may be used to measure height, width, or depth. These lines
simulate how parallel lines as seen by the human eye converge in towards the pupil
(Fig. 1.2 , right).
The original form of linear perspective did not take into account lens curvature,
so it was only an approximation of what a human eye sees. Nevertheless, it worked
well enough that for 500 years it was the basis for almost every great work of art
made during that period. In more recent times, axonometric projection is one
method invented to solve the problem of curve distortion in visual perception
(Reggini 1975 ).
With the invention of the computer, it was inevitable that the rules behind
linear perspective would be turned into software, and they were. When this
happened, the modern era of 3D computer graphics, perhaps the most signifi cant
advance in the visual arts since the discovery of perspective, was made (Kozbelt
2006 ). The work of artists has also been used as the basis for other innovations
in computer graphics.

1.2 Aerial Perspective

Leonardo da Vinci's observation that colors become less distinct over distance
became known as Aerial Perspective. He used this to more accurately describe
distant details in his paintings, most famously in the background of the painting
Mona Lisa. Other artists used the technique as it became better known, and descrip-
tions of it became commonplace in topics about art (Farago 1994 ). The phenome-
non Leonardo saw, described, and went to some trouble to understand, was caused
by many tiny light-occluding particles such as dust and fog suspended in the
atmosphere. At near distances, they do not noticeably affect our vision because
there are not as many of these particles between our eye and the object we are
observing as when we refocus our eyes on a distant object. In computer graphics,
aerial perspective is known as environmental fog. The effect is used effectively to
simulate great distances in computer renderings (Shannon 1995 ). An example of
this is provided with the painting by Stephan Martiniere reproduced in Fig. 1.3 .
Color in the background is muted because it is fl ooded with the primary light color.
At extreme distances, individual color differences are normally indistinguishable
due to atmospheric perspective.