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# Visualization and Sketching

This chapter is from the book

## 3.29 Sketching Perspectives

Perspective pictorials most closely approximate the view produced by the human eye. Perspective views are the type of drawing most like a photograph. Examples of a perspective drawing can be seen in Figures 3.69, 3.70a, and 3.72. Although complex perspective views are time consuming to sketch, they are easy to create from 3D CAD models.

Unlike parallel types of projection, perspective projectors converge. The point at which the projectors converge is called the vanishing point. This is clearly seen in Figure 3.70a.

The first rule of perspective is that all parallel lines that are not parallel to the picture plane vanish at a single vanishing point, and if these lines are parallel to the ground, the vanishing point will be on the horizon. Parallel lines that are also parallel to the picture plane remain parallel and do not converge toward a vanishing point (Figures 3.71 and 3.72).

When the vanishing point is placed above the view of the object in the picture plane, the result is a bird’s-eye view, looking down onto the object. When the vanishing point is placed below the view of the object, the result is a worm’s-eye view looking up at the object from below (see Figure 3.74 on page 107).

There are three types of perspective: one-point, two-point, and three-point perspective, depending on the number of vanishing points used.

### The Three Types of Perspective

Perspective drawings are classified according to the number of vanishing points required, which in turn depends on the position of the object with respect to the picture plane.

If the object sits with one face parallel to the plane of projection, only one vanishing point is required. The result is a one-point perspective, or parallel perspective.

If the object sits at an angle with the picture plane but with vertical edges parallel to the picture plane, two vanishing points are required, and the result is a two-point perspective, or an angular perspective. This is the most common type of perspective drawing.

If the object sits so that no system of parallel edges is parallel to the picture plane, three vanishing points are necessary, and the result is a three-point perspective.

#### One-Point Perspective

To sketch a one-point perspective view, orient the object so that a principal face is parallel to the picture plane. If desired, this face can be placed in the picture plane. The other principal face is perpendicular to the picture plane, and its lines will converge toward a single vanishing point.

8:12 Train as Seen by 8:12 1/2 Commuter (Excerpted from Droodles – The Classic Collection by Roger Price ©2000 by Tallfellow Press. Used by permission. All rights reserved.)

#### Two-Point Perspective

Two-point perspective is more true to life than one-point perspective. To sketch a two-point perspective, orient the object so that principal edges are vertical and therefore have no vanishing point; edges in the other two directions have vanishing points. Two-point perspective is especially good for representing buildings and large civil structures, such as dams or bridges.

#### Three-Point Perspective

In three-point perspective, the object is placed so that none of its principal edges is parallel to the picture plane. Each of the three sets of parallel edges has a separate vanishing point. See Figure 3.73.

### Bird’s-Eye View versus Worm’s-Eye View

The appearance of a perspective sketch depends on your view-point in relation to the object. Select some reachable object in the room and move so that you are looking at it from above and really notice its shape. Now gradually move so that you are looking at it from below. Notice how the change of viewpoint changes the appearance of its surfaces—which ones are visible and their relative size.

The horizon line in a perspective sketch is a horizontal line that represents the eye level of the observer. Locating the sketched object below the horizon line produces a view from above (or a bird’s-eye view). Locating the sketched object above the horizon line produces a view from below (or a worm’s-eye view). Figure 3.74 illustrates the horizon line in a drawing and the effect of placing the object above or below the horizon line.

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