The Camera - Essentials of Interactive Computer Graphics - page 364

Figure 14.2.

Rotating the camera to change the up direction.

direction of the camera by rotating the camera. Figure 14.3 shows the resulting

snapshots taken from the different camera up directions. In computer graphics,
we specify the camera up direction as a vector, the up vector. For example, the
three different up vectors in Figure 14.2 are

V a =( 0 , 1 , 0 ) ,

V b =( 1 , 1 , 0 ) ,

V c =( 1 , 0 , 0 ) .

The up vector dictates the y -axis direction of the image plane, and eventually the
photograph. For example, in Figure 14.1, the up direction of the image plane V n
is derived based on the camera up vector V a . As another example, the V c up
vector ( x -direction) in Figure 14.2 says that the x -direction of WC should be the
up direction of the image generated. Notice that in Figure 14.2, the V e V a V b edge
is aligned along the x -axis direction. Now if we look at the corresponding image
in Figure 14.3 (lowest of the three), we see the edges that correspond to V e V a V b
are aligned along the up/down (or y ) direction of the image. As programmers, we
can specify any direction as the up vector to orientate our camera. In general, the
up vector:

Figure 14.3. The top
photograph is taken from
the camera with 0 ◦ twist
angle (or up vector = V a );
the middle photograph is
taken from the camera with
θ = 45 ◦ ( V b ); the bottom
photograph is taken from
the camera with β = 90 ◦
( V c ) .