The Camera - Essentials of Interactive Computer Graphics - page 363

Figure 14.1. Parameters controlling a computer graphics camera.

Look-at position. This is where the pinhole camera is aiming. It is also referred
to as the camera focus position. Figure 14.1 shows that the camera is aiming at the
center of the large piece of paper, or at the origin of our coordinate system: (0 cm,
0 cm, 0 cm). The viewing direction is the direction from the camera (pinhole) po-
sition toward the look-at position. The vector describing this direction is referred
to as the view vector. As illustrated in Figure 14.1, implicitly defined is the image
plane , a plane perpendicular to the viewing direction. We can consider the image
plane to be the plane upon which the final image is captured. This plane is like
the film in the traditional camera, or the charge-coupled device (CCD) sensor ar-
ray of the contemporary digital camera. In either case, it is a 2D rectangular area
that is perpendicular to the viewing direction and upon which the photograph is
captured. In the real world, the image plane is a physical part of the camera. For
this reason, in computer graphics the term “camera” is sometimes used to refer to
“the image taken by the camera.” For example, in the following discussion, the
“camera up vector” is really referring to the up direction of the image plane. In
most cases, the distinction is subtle and unimportant. However, as in the case of
the up vector, understanding the distinction helps understand the concept.

Up direction. This is the up direction of the camera and dictates the up direction
of the image plane. For example, Figure 14.2 shows that we can change the up