Page 145 - Computer Graphics Handout

perspectives. This seeming d screpancy arises in classical graphics as a result of the desire to show a specific relationship among an
object, the viewer, and the projection plane, as opposed to the computer-graphics approach of complete independence of all
specifications.

4.1.1 Classical Viewing
When an architect draws an image of a building, she knows which side she wishes to display and thus where she should place the
viewer in relationship to the building. Each classical view is determined by a specific relationship between the objects and
the viewer. In classical viewing, there is the underlying notion of a principal face. The types of objects viewed in real-world
applications, such as architecture, tend to be composed of a number of planar faces, each of which can be thought of as a principal
face. For a rectangular object, such as a building, there are natural notions of the front, back, top, bottom, right, and left faces. In
addition, many real-world objects have faces that meet at right angles; thus, such objects often have three orthogonal directions
associated with them. Figure 4.3 shows some of the main types of views. We start with the most restrictive view for each of the
parallel and perspective types, and then move to the less restrictive conditions.

4.1.2 Orthographic Projections
Our first classical view is the orthographic projection shown in Figure 4.4. In all orthographic (or orthogonal) views, the projectors
are perpendicular to the projection plane. In a 145ormal145ew orthographic projection, we make multiple projections, in each
case with the projection plane parallel to one of the principal faces of the object. Usually, we use three views—such as the front,
top, and right—to display the object. The reason that we produce multiple views should be clear from Figure 4.5. For a box-like

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