Working with the Camera - Essentials of Interactive Computer Graphics - page 408

Of course, as humans, we usually combine and apply all of the above manipula-
tion operations simultaneously when observing objects. On a computer, we have
to be contented with simulating each as distinct operations.

15.3.1 Tumble: Rotating (Orbiting) the Camera

Let's take a moment to do a little experiment. Find a small object on your desk,
for example, the eraser on a pencil. Use this object as your fixed look-at position.
You want to examine this object without touching it. Notice how you move your
head horizontally left to right and vertically up and down when examining this
object. Because the object is small and your look at position does not change
much, notice that when you move your head, unconsciously, you actually try to
maintain the view distance. In effect, you are actually rotating your eye position
orbiting the object. As Figure 15.4 shows, we usually orbit the camera position
when examining a stationary object.
As depicted in Figure 15.6, when you thought you were moving your head
horizontally left to right, you are actually rotating your eye position around the
object with respect to the up orientation. Figure 15.5 shows that a vertical up/-
down movement corresponds to rotating the eye position with respect to the V w
vector of the EC orthonormal basis. Recall that V w is the vector that is perpen-
dicular to both the view vector V v and the up vector V up . In this way, the vertical
up/down rotation actually rotates the eye position on the plane defined by the view
and up vector.

Vertical Orbit
(Up/Down)

Horizontal Orbit
(Left/Right)

Look At
position is fixed

Current
Camera Position

Figure 15.4. Tumble: orbiting of camera position.