Information Technology Reference
In-Depth Information
Figure 6.13.
A polyhedral object and an impossible variant
6.3.2
Impossible objects
This idea of images having only local interpretations is at the root of so-called
impossi-
ble objects
. Consider figure 6.13. The image on the left is somewhat unusual but can be
interpreted as an object with two 90
◦
turns. The one on the right (due to Roger Pen-
rose), however, cannot be interpreted as a polyhedral object at all. By covering parts
of the figure it is possible to interpret the rest of the figure in one way or another. In
doing visual interpretation, however, it is necessary to look for local cues and to try
to assemble a global interpretation that accounts for everything that is seen.
6.4 Object recognition
Given an image of some sort, the task of object recognition is to determine whether
the image contains a depiction of a particular object, say, a door that is an exit, or a
banquet table with food on it, or a parcel to pick up for delivery. This is the sort of
perceptual task that we do all the time, and it is absolutely crucial when we arrive at
a new location. It is also somewhat different from the visual interpretation discussed
in the previous sections. Rather than interpreting all the image elements, the goal is
to pick out elements in the image corresponding to the kind of object being sought.
Here is a simple case. The image given is like the one in figure 6.14 and the job is
to see if there is a
cuboid
(a rectangular-shaped block) in the image. As shown in the
figure, a list of vertices in the image is given (of the four types mentioned in the pre-
vious section) together with the vertices to which they connect. The predicates from
