Graphics Reference
In-Depth Information
Fig. 1.5
The photodiode is an
n
-type region in an nMOS process. These and other diagrams are
scanned from my 1981 PARC optical mouse report [
24
]
Fig. 1.6
The photodiode was incorporated as a dynamic node, going directly into a logic gate, just
as other switched dynamic nodes were used in the methodology we had been teaching [
9
,
10
]
The stable images from the four-pixel imager, that is, images where nomore pixels
can change from “dark” to “light,” are just three: 1001, 0100, 0010. These patterns
are ideally suited to imaging and tracking light lines spaced at about 3 pixels, in a
dark background, as shown in Fig.
1.9
. Figure
1.8
also shows logic for comparing
successive images, by local binary cross-correlation, and keeping track of position
by a counter, under control of the timing logic of Fig.
1.10
.
When I studied the literature on lateral inhibition in the 1970s, based mostly on
the compound eye of the horseshoe crab
Limulus
, it seemed to be based mostly on
a linear systems view, with subtraction from neighboring sensor elements resulting
in a bandpass or highpass filtering effect. In his 1967 topic
Sensory Inhibition
,von
Békésy [
36
] explored the effects of lateral inhibition in various sensory systems,
including vision, hearing, touch, and taste, in essentially linear systems terms, for
“sharpening” the response to stimuli. Papers argued that the effect could be very
accurately modeled as linear. For example, while Knight et al. [
19
] noted that crab
eyewas quite compressive, with a “graded response to awide range of light intensities
(to a factor of about 10
7
in intensities for
Limulus
),” they also said that “the response to
an intensity decrement is the close mirror image of the response to the corresponding
increment. This suggests that we are dealing with a so-called 'time-invariant linear'
system.” In most subsequent analysis, the locally short-time approximately linear
filtering behavior was modeled as if it were accurate, and the extremely nonlinear
large-scale long-time adaptive behavior was ignored, so the important property of
lateral inhibition as a strategy for normalizing the response across a wide dynamic
range was missed. Barlow finally forcefully explained why lateral inhibition is a key
