The early Renaissance painter Leon Battista Alberti stated that his painting "....is considered to be an open window through which I see what I want to paint" [1]. That thought inspires much of Renaissance perspectival painting --- a constant goal is the illusion of three-dimensionality in a two-dimensional surface. In order to appreciate that illusion best, we should view paintings in perspective at an ideal location. For example, consider the image of the interior of a hallway as seen in the applet.
If you move nothing in the applet, it appears that you are viewing the hallway from a point centered between the two walls. You could not be standing much to the left of A, or you would see much more of the right wall and less of the left wall. However, if you move the point A to the left, you produce an image where you do appear to be standing on the left side of the hallway. The point A should always show you the location you should stand to make the image seem most three-dimensional.
Now assume you know the length of the hallway (which you can change if you wish by moving the point B). The further back you stand from the image, the shorter the hallway will look. The closer you stand, the longer the hallway will appear. There should be an ideal distance to stand in order to make the hallway look its proper length.
When we find the ideal viewing location, we have found the artist's viewpoint --- where the artist stood to create this painting. To find this viewpoint accurately, we need to determine two things:
To find the principal vanishing point and viewing distance, there are well established procedures. (You can find a guide to them by looking at Marc Frantz's site [2] --- much of the material in the next section is based on his exposition.) We will review these methods as well as provide an introduction to three-dimensional perspective.