## Perspective Grid

2. His parents at ground level might have this view. The horizontal and vertical lines appear to converge and all surfaces are foreshortened. (The vertical lines give a sense of vertically but they are not actually parallel to the edge of the page.) Note that this viewpoint is more revealing than the first.

4. If Junior walked around the bench and looked at the end head-on, the true geometry of the end rectangle would appear. The bench top, though, would strongly converge and foreshorten. Notice that the horizontal as well as the vertical lines of the end maintain their real directions.

Chapter 3: HOW WE SEE FOR PERSPECTIVE DRAWING

Cone Of Vision... Central Visual Ray... Picture Plane

A perspective drawing will look correct only if the artist's viewpoint and his direction of viewing the subject are relatively fixed. This means drawing with a limited "field of vision." This field is usually called the CONE OF VISION because of the infinite number of sight lines which radiate in a cone-like pattern from the eye. (In reality these lines are light rays coming from the subject to the eye. ) The angle of this cone is between 45 and 60 degrees. If a greater angle is used in a drawing, it implies a moving cone of vision — and the picture will be distorted. You can test your cone of vision by looking straight ahead and swinging your outstretched arms in and out of sight.

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When we look about, essentially what we do is focus upon a succession of spots or "centers of interest," each of which is fixed by a sight line at the exact center of the cone of vision. This line is sometimes called the "center line of sight" or the "central direction of seeing." We shall call it the CENTRAL VISUAL RAY. When you look through a telescope or hold a pencil so that it appears as a point the telescope or the pencil is exactly on your central visual ray.

To understand perspective drawing a PICTURE PLANE must be imagined between the observer and subject. THIS PLANE HAS A CONSTANT RIGHT ANGLE RELATIONSHIP WITH THE CENTRAL VISUAL RAY. So when drawing a subject, whether it is above, below or straight ahead, imagine viewing it through an omnipresent picture plane which is perpendicular to your central visual ray surrounded by a cone of vision.

Basis Of Perspective - Lines Of Sight Through A Picture Plane [19]

Basis Of Perspective - Lines Of Sight Through A Picture Plane [19]

The concept of the picture plane may be better understood by looking through a window or other transparent plane from a fixed viewpoint. Your lines of sight, the multitude of straight lines leading from your eye to the subject, will all intersect this plane. Therefore, if you were to reach out with a grease pencil and draw the image of the subject on this plane you would be "tracing out" the infinite number of points of intersection of sight rays and plane. The result would be that you would have "transferred" a real three-dimensional object to a two-dimensional plane.

The concept of the picture plane may be better understood by looking through a window or other transparent plane from a fixed viewpoint. Your lines of sight, the multitude of straight lines leading from your eye to the subject, will all intersect this plane. Therefore, if you were to reach out with a grease pencil and draw the image of the subject on this plane you would be "tracing out" the infinite number of points of intersection of sight rays and plane. The result would be that you would have "transferred" a real three-dimensional object to a two-dimensional plane.

You can refine this experiment by looking through a window that has vertical and horizontal pane divisions, or through any transparent sheet that has been marked off in a similar grid pattern with crayon or by scoring.

Here the vertical and horizontal lines of each small rectangle clarify the direction of oblique or converging lines beyond. Working with such a "reference grid" one can easily transfer the scene to a sketch pad. This is surely not a recommended drawing technique but it does dramatically show the basic theory of perspective. In fact, the word perspective comes from the Latin word "perspec-ta" which means "to look through."

YOUR CANVAS, SKETCH PAD, OR DRAWING BOARD, THEREFORE, IS THE PICTURE PLANE. ON IT IS DRAWN WHAT WOULD BE SEEN IF IT WERE TRANSPARENT AND HELD PERPENDICULAR TO YOUR CENTRAL VISUAL RAY.

Chapter 4: WHY APPEARANCE DIFFERS

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