The used CMOS sensor board is EVT202. EVT202 specification:
z Number of Active Pixels: 640 x 480
z Number of Physical Pixels: 650 x 490
z Main Clock Frequency: 3 to 24 MHZ
z Frame rate: from 1 to 30 fps
z Output data format: 8/16-bit YCbCr, 24-bit RGB, 16-bit RGB and 8-bit raw data
z Input / output interface: SIF
EVT202 is a CMOS sensor boards with an ICM205B vision chip from IC media Corp [18]. Figure 2.3 Illustrates the EVT202 CMOS sensor board. The ICM205B sensor chip is a single-chip digital color imaging device. It incorporates a 640 x 480 sensor array operating at 1 - 30 frames per second in progressive manner. Each pixel is covered by a color filter which formed a so-called Bayer pattern.
There are several module built in the sensor that can be used for enhanced the image quality, such as the brightness of the scene by adjusting the digital gain for all pixels, color correctness by using automatic white balance circuit, image sharpening by correcting the value of the sharpening function and gamma correction to boost darker signal by selecting the appropriate gamma value. The modules that available inside the chip are described as below:
Figure 2.3 EVT202 and its specification
z Real time color interpolation
The functionality of this module is to reconstruct each pixel that covered by a color filter that form so-called Bayer pattern into complete set of RGB values.
z Programmable / Automatic exposure control
The Automatic Exposure (Auto Exposure) module is to control the Exposure time, Digital gain and Anti-flickering function automatically.
z Programmable / Automatic white balancing and color correction
The Automatic white balance is to perform white balancing when the lighting is suddenly changed in the environment by changing the R (Red), G (Green) and B (Blue) to produce a new value of white color. When this function is disabled, the value of the red gain, green gain and blue gain is set manually.
z Programmable sharpening control
The functionality of this module is to sharpening the edge of the image. The edge will sharpen according to the weighting value that given to the CMOS sensor.
z Programmable gamma correction
The gamma correction module is to boost the image from darker. The gamma correction function of the CMOS sensor is γ
1 i
o V
V = , where Viis normalized (ranged from 0 to 1) R, G, or B signal coming from the white balancing module and Vo is normalized final output. So by adjusting the γ value we can adjust gamma correction.
z Programmable brightness correction
The brightness correction is to increase / decrease the brightness level of the CMOS sensor.
z Programmable anti-flickering (50 Hz, 60 Hz or off, like outdoor)
The functionality of this module is to prevent flicker that can occur from light source by making the flicking stable at 50 Hz / 60 Hz.
All of these modules can be changed or activated via serial bus control which known as I2C by changing the value of the register in each modules. Although these feature are available inside the chip, but not of these entire feature are activated or changed. It is because some of available function can affect the image that want to be process, such as sharpening function, auto exposure and auto white balance function. The sharpening function can sharpen the edge of the image by changing its value but because the inconsistency of the sharpening edge between image at time t and time t+1, so it can make false motion detection for our optical flow algorithm so we disabled the sharpening module.
Another function that we disabled is the auto exposure time function, by disabling this function the frame rate of the image will be fix although there is any changes of the environment such as lighting changes that coming from the light source so by fixing the frame rate, we can acquire the image from CMOS sensor with a fix frame rate for all the time.
The last function that we disabled is auto white balance function. Activated the function of auto white balance function can also make false motion detection for our optical flow algorithm. It is because when there are any suddenly changing in environment from dark to
white the auto white balance function will try to adapt by changing the R, G and B value to make a new white color for several image frames. In this time, there will be false motion detection even there is no any motion in the image. Generally, the need of motion detection algorithm is that the image must be consistently same for all the time without any suddenly changes so it will be clearly detect motion occur in the image instead of the changing of the environment. Instead of the above module, we also change the gamma correction from the gamma=1 to gamma=1.5 for boosting the darker signal so the image will be brighter.
Meanwhile the brightness correction will not be change; it is no necessary to change the value of brightness correction after changing the gamma correction value. The value of the module that has been changed or disabled can be seen in the table 2.1.
Another configuration of the CMOS sensor that should be considered is data output format and data output mode. The ICM205B CMOS sensor offer a various data output format such as: 8/16 bit YCbCr, 24-bit RGB, 16-bit RGB and 8-bit raw data and also output modes such as VGA and its sub-sampling QVGA/QQVGA.
Table 2.1 list of CMOS sensor built in function module that has been changed
No Module name Default value New value
1 Auto exposure Automatic Disable
2 Auto white balance Automatic Disable
3 Sharpening Sharpen weight = 2 Disable sharpening 4 Gamma correction Gamma = 1 (no gamma) Gamma = 1.5
5 Brightness correction 0 No changes
In our embedded image processing platform, we configure the output data as 16-bit YCbCr with the 8-bit low output data is Y and the next 8-bit high output data is sequential output of Cb/Cr which we are just using the 8-bit low output data (Y), so we doesn’t use the 8-bit high output data (Cb/Cr). And for the output data mode, we are using VGA data output mode. The YCbCr data format itself is a similar with YUV color space format which is used in European TVs.