Detecting movement of edges of dot pattern determines relative movement of image pattern across image sensor
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Introduction In a mechanical mouse, the motion of a ball
is transformed into cursor control signals via an encoding
wheel. However, this method of generating cursor control signals
is inapplicable for an optical mouse. An optical mouse uses
an image sensor to detect an image pattern. The image sensor
includes a plurality of pixels, each of which can generate
an output signal related to the amount of light that reaches
that pixel. A set of pixel signals corresponds to the amount
of the captured light image. A method for determining the
relative movement of an image pattern across an image sensor
in order to generate cursor control signals is needed.
Description To determine the relative movement of an image
pattern, detecting the movement of the edges of the dot pattern
is proposed. An image pattern on the image sensor array surface
consists of irregularly spaced dots or speckles. On average,
a dot size is selected so that the image pattern of one dot
covers approximately five pixels of the image sensor array.
With the help of a threshold circuit, the signal from each
pixel is compared with a predetermined threshold circuit,
and a decision is then made whether this pixel is "white"
or "black". The "black" and "white"
pixels correspondingly represent a low and high level of light
reflected from the working surface and received by the image
sensor array. A differential sensor then detects the so-called
edges of the dot pattern. The edges of the image pattern are
defined as the border between adjacent "white" and
"black" pixels. Data representing values for edges
on the X-axis, edges on the Y-axis, and the color of the pixel
are stored as first memory data. In the short time interval
of approximately 1 millisecond, subsequent edge data are received,
which are stored as second memory data. Comparator logic compares
the states of the first and second memory and determines the
temporal intensity change of the pixels. The temporal intensity
change of the pixel (the transfer of the pixel from the "white"
state into the "black" state or vice versa) represents
the motion of the edges of the dot pattern along the image
sensor array surface. Correspondingly, the motion of the edges
of all dot patterns represents the movement of the image pattern
across the image sensor. Thus, detecting the edges of a dot
pattern determines the relative movement of an image pattern
across the image sensor.
Additional information With the help of a light source and appropriate
optics, the pattern of the working surface is obtained on
the image sensor array surface. The working surface can be
any surface with an irregular pattern of spots. In particular,
it can be the surface of a ball with a human fingerprint pattern.
