Page 138 - Introduction to Autonomous Mobile Robots

Perception

focal plane image plane 123

(x, y, z)

(x , y )
l
l
f
d e δ
Figure 4.19
Depiction of the camera optics and its impact on the image. In order to get a sharp image, the image
plane must coincide with the focal plane. Otherwise the image of the point (x,y,z) will be blurred in
the image, as can be seen in the drawing above.

mobile robot applications. Without such assumptions, a single picture does not provide
enough information to recover spatial information.
The general solution is to recover depth by looking at several images of the scene to gain
more information, hopefully enough to at least partially recover depth. The images used
must be different, so that taken together they provide additional information. They could
differ in viewpoint, yielding stereo or motion algorithms. An alternative is to create differ-
ent images, not by changing the viewpoint, but by changing the camera geometry, such as
the focus position or lens iris. This is the fundamental idea behind depth from focus and
depth from defocus techniques.
In the next section, we outline the general approach to the depth from focus techniques
because it presents a straightforward and efficient way to create a vision-based range sen-
sor. Subsequently, we present details for the correspondence-based techniques of depth
from stereo and motion.

Depth from focus. The depth from focus class of techniques relies on the fact that image
properties not only change as a function of the scene but also as a function of the camera
parameters. The relationship between camera parameters and image properties is depicted
in figure 4.19.
The basic formula governing image formation relates the distance of the object from the
lens, in the above figure, to the distance from the lens to the focal point, based on the
d
e
f
focal length of the lens: