Information Technology Reference
In-Depth Information
image is spatially sampled, those samples contain enough information to represent the brightness and colour all
over the screen.
Figure 3.31:
Address generation is the fundamental process behind transforms.
When the address generator produces an address of 2.5, it actually means that what is wanted is the value of the
signal interpolated half-way between pixel 2 and pixel 3. The output of the address generator will thus be split into
two parts. The integer part will become the memory address, and the fractional part is the phase of the necessary
interpolation. In order to interpolate pixel values a digital filter is necessary.
In order to follow the operation of warping, some knowledge of perspective is necessary. Stated briefly, the
phenomenon of perspective is due to the angle subtended to the eye by objects being a function not only of their
size but also of their distance.
Figure 3.32
shows that the size of an image on the rear wall of a pinhole camera can
be increased either by making the object larger or bringing it closer. In the absence of stereoscopic vision, it is not
possible to tell which has happened. The pinhole camera is very useful for study of perspective, and has indeed
been used by artists for that purpose. The clinically precise perspective of Canaletto paintings was achieved
through the use of the camera obscura ('darkened room' in Italian)
[
13
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.
Figure 3.32:
The image on the rear of the pinhole camera is identical for the two solid objects shown because the
size of the object is proportional to distance, and the subtended angle remains the same. The image can be made
larger (dotted) by making the object larger or moving it closer.
Warping processors work by producing the correct subtended angles which the brain perceives as a three-
dimensional effect.
Figure 3.33
shows that to a single eye, there is no difference between a three-dimensional
scene and a two-dimensional image formed where rays traced from features to the eye intersect an imaginary
plane.
