Graphics Reference
In-Depth Information
Fig. 1.4
The mouse pad was paper, sold in packs of 25 sheets. The pattern was a hexagonal array
of light dots in a dark field, shown here at approximately actual size. Effective mouse pads could
be made by a copier
that sense. Further, it was specifically neuromorphic and biomimetic in the way it
incorporated lateral inhibition in its imager.
Xerox's optical mouse development was celebrated on the cover of their internal
Xerox World magazine in 1982, as shown in Fig.
1.2
. The Xerox optical mouse
was sold with a range of office workstations, such as the 6085 shown in Fig.
1.3
,as
well as with Xerox Lisp machines, Tektronics SmallTalk machines, and high-end
copier/duplicator products—none of which were high-volume products. Xerox was
not successful in their attempts to sell licenses to their optical mouse patents, even
after the market for mice exploded on the introduction of the Apple Macintosh with
its low-cost ball mouse in 1984.
This chapter reviews the ideas that went into the optical mouse's very application-
specific embedded vision system.
1.2 Image Sensing with Lateral Inhibition
The elementary light detector in an nMOS process is a PN photodiode, with the P
region being the substrate and the N region being a diffusion region, as shown in
Fig.
1.5
(a “green” area as we taught it in the red/green/blue/black scheme at the time).
As shown in Fig.
1.6
, a reset transistor back-biases the photodiode to an initial high
voltage, and the voltage decays as the diode collects photoelectrons. The photodiode
voltage can be used as an input to digital logic (shown in the figure as an inverter),
provided the logic is designed to tolerate the intermediate analog values that the
voltage will necessarily go through slowly.
The imaging strategy in the optical mouse relies heavily on an engineering inter-
pretation of lateral inhibition as a nonlinear scheme for arriving at a stable image,
independent of light level. In the simplest example, as shown in Fig.
1.7
, a two-pixel
imager uses mutual inhibition in a form that forces the system to decide which one
of the two pixels receives more light than the other.
In systems of more than two pixels, each pixel can inhibit, and be inhibited by, pix-
els within some radius. For example, in the four-pixel imager of Fig.
1.8
,pixelsatdis-
tance 1 and 2mutually inhibit, but the end pixels, at a distance of 3, do not inhibit each
other. This radius-of-inhibition idea has an obvious extension into two dimensions.
