Complementary Metal Oxide Semiconductor Imager Median Computation Circuit
Original Publication Date: 1997-Nov-01
Included in the Prior Art Database: 2005-Apr-01
Gowda, S: AUTHOR [+5]
Disclosed is a method to compute the median of pixel charge values across an image pixel array. This method can be used in image acquisition applications where there is need for the contrast of the image to be maximized. The method does not require the storage of pixel values, and hence, results in a compact implementation.
Complementary Metal Oxide Semiconductor Imager Median
a method to compute the median of pixel charge
values across an image pixel array. This method can be used in image
acquisition applications where there is need for the contrast of the
image to be maximized. The method does not require the storage of
pixel values, and hence, results in a compact implementation.
Fig. 1 shows
the block diagram of a typical CMOS imager
chip. It consists of a pixel array having M rows and N columns of
pixels. The pixels generate analog signals that are proportional to
the light intensity that they are exposed to. The pixels are
addressed one row at a time by the row decoder circuits, and the
analog signal values are placed on the column lines as shown in Fig.
2. The column lines of the array drive the A/D converter block. As
shown in Fig. 3, the A/D converter block can contain a single A/D,
one A/D per column, or any number of A/Ds between these limits.
Depending on the number of A/Ds, appropriate muxing of the column
signals is done by the column control circuits. The digital signal
from the A/D is passed through image processing logic and then driven
conversion of digital signals to analog signals in the
A/D converter, there is loss of information due to the use of a
limited number of discrete digital values to describe data in the
continuously varying analog domain. For example, if the output is 8
bits wide, then 256 digital values are available to describe the
analog gray scale values between black and white. However, this loss
of information can be minimized by computing certain characteristics
of the data in the digital domain and feeding this information back
to the A/D converter by varying certain parameters of the A/D
converter. For example, the high and low limits of the digital data
can be used to reset the reference of the A/D and use all 256 digital
intervals for the smaller set of analog values that actually exist in
the image, rather than the entire range from black to white. Another
useful metric of the image data is the median of the pixel value
distribution. The median value enables the ADC to perform the
conversion of a...