DSP应用:CMS图像处理-关于CMOS DSP, CMS, CMOS, 图像处理, 应用

BlueSeaHaiChao

A CMOS image sensor uses photodiodes or photogate transistors as the light
detecting element. In contrast to CCD sensors, each photo element is individually
addressable in a row and column matrix, similar to a DRAM structure, which
allows partial image readout (windowing, area of interest). Windowing is especially
helpful in motion detection and object tracking algorithms. It reduces intermediate
storage memory requirements and allows higher frame rates.

Image quality of modern CMOS sensors is almost comparable to CCD devices,
with slightly higher noise levels, mainly caused by the integration of analog and
digital circuitry on the same chip. CMOS however does not suffer from blooming
and smearing effects that are caused by charge leakage in CCD devices.
CMOS sensors can be built with logarithmic characteristics, achieving a dynamic
range up to 100 dB and more, which is essential for many 'outdoor' applications
like object detection and tracking in a vehicle.

Color sensors include a Bayer color filter on top of the inherently monochrome
sensor array. The analog pre-processor is complemented with gamma correction
circuits and with a RGB to YCrCb matrix. An additional ADC converts the CrCb
color difference signal.

BlueSeaHaiChao

Image data is transmitted via a parallel bus interface, qualified by a pixel clock
and horizontal and vertical synchronization pulses.
A CMOS sensor is typically controlled via a SCCB (I²C) bus, a serial two-wire
interface. Depending on the sensor the host processor can control exposure,
gain, brightness, contrast, gamma correction, windowing, frame rate and output
data format.

Besides the described progressive-scan architecture which outputs image data
sequentially line by line, random-access sensors exist which are connected to the
host via row and column address lines, similar to a DRAM device. These sensors
provide fully random access to any pixel within the array and are particularly
useful for very fast object tracking systems.

CMOS sensors exist in various resolutions. Typical are CIF (352x288 pixels),
VGA (640x480), SVGA (800x600) and XGA (1024x768) devices, but also a number
of special formats are available. Low resolution sensors (128x128) allow
hundreds of frames per second and are used for very fast motion detection,
whereas material inspection applications call for highest resolution up to several
Megapixels.

The OV6120 monochrome sensor was chosen for this application for no particular
reason except the availability of an evaluation board, readily populated with a
lens holder and some passive components. Most CMOS image sensors do have
an identical architecture and should work with minor adjustments, mainly concerning
the software setup and sensor configuration.