The first thing you notice about the cube shown in figure 5-48 is the fact that it doesnt look like a cube because the depth dimension appears to be longer than the height and width dimensions. The reason for this is the fact that a cavalier projection corrects a human optical illusionthe one that causes an object to appear to become smaller as its distance from the eye increases. This illusion. in turn. causes receding parallel lines to appear to the eye to be shorter-than they really are, and also to be converging toward a point in the distance. But receding parallel lines on a cavalier projection appear in their true lengths, and they remain constantly parallel. Also, the far edges of the cube shown in figure 5-48 are equal in length to the near edges.

The distortion in figure 5-48 is only apparent. It is sometimes desirable to reduce this appearance of distortion. This can be done by reducing the length of the receding axis (OC in fig. 5-39). This axis can be reduced by any desired amount, but it is customary to reduce it by one half.

Figure 5-49.-Cabinet projection of the cube in figure 5-48. (Note receding axis OC reduced by one half its length.)

When the receding axis is reduced by one half, the projection is called a CABINET PROJECTION. Figure 5-49 shows a cabinet projection of a cube. The length of the receding axis OC has been reduced by one half. As you can see, this representation looks more like a cube.

Cavalier and cabinet projections are compared in figures 5-50 and 5-51.

In an oblique projection drawing of a rectangular object, one face (usually the most prominent or most important) is parallel to the plane of projection. All features appearing on this plane, such as circles or oblique lines, are in their true dimension. However, in the side or top views, these same features are somewhat distorted because of the receding axis angle. When drawing these features, you can use various techniques to aid you in their construction.

For convenience, the angles chosen for the receding axis are either 30 degrees, 45 degrees, or

Figure 5-50.-Cavalier projection. Distances along front axis and along receding axis are all true.

Figure 5-51.-Cabinet projection. Distances along front axis are true; distances along receding axis are reduced by one half.

60 degrees because they are easily constructed with triangles (fig. 5-52).