Patent Publication Number: US-9413999-B2

Title: CMOS image sensor implementing correlated double sampling with compression

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This present application is a Continuation of application Ser. No. 14/014,145, filed Aug. 29, 2013, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to image sensor systems and, in particular, to an active pixel CMOS image sensor implementing correlated double sampling with compression of the pixel reset values. 
     DESCRIPTION OF THE RELATED ART 
     Digital image capturing devices use image sensors to convert incident light energy into electrical signals. An area image sensor includes a two-dimensional array of light sensing elements called pixels. Each pixel in the array works with the lens system to respond to incident light within a local area of the scene, and produces an electrical signal describing the local characteristics of the scene. Outputs from the light sensing elements are converted to digital form and stored digitally to form the raw data representing the scene. The raw data can be processed by an image processor to produce rendered digital images. 
     Correlated double sampling (CDS) is a method that uses a reset value and a reset plus light dependent value for each pixel to eliminate noise and non-uniformity in the pixel responses of an image sensor. In the following description, the term “light dependent pixel value” will be used to refer to “a reset plus light dependent value” of a pixel. The use of CDS enables the image sensor to achieve better signal-to-noise ratio performance. To perform CDS, a pixel is reset and either the reset voltage value or a corresponding digital value is measured, resulting in a first sample value. The first sample value is stored in a memory buffer. Then the pixel is exposed to light for a pre-determined amount of time and the pixel integrates photons from the incident light to generate a voltage value dependent on the incident light level. Either the light-dependent voltage value (including the reset voltage value) or a corresponding digital value is measured, resulting in a second sample value. This is repeated for a selected image region of pixels in the sensor where the first sample values for the pixels are stored in the memory buffer. The selected image region can be the entire image, or a cropped region thereof. 
     In some cases, the first sample value of a pixel location is subtracted from the second sample value of the same pixel location to generate the CDS corrected output pixel value for the pixel location. In other cases, the reset value and the light dependent pixel value for the same pixel location can be thought of as two points on a line that represents the voltage of the light sensing element as a function of time, where a CDS corrected output pixel value can be calculated based on interpolation or extrapolation along the line. In each case the goal is to calculate an output pixel value for each pixel within the selected image region where pixel value variations represented by the reset value are cancelled. CDS can be implemented using analog or digital methods, and furthermore it can be designed to work on a sequential (e.g., row by row) basis, a partial frame or a full frame basis. To implement full frame digital CDS, memory is required for storing at least the array of first sample values for the entire image sensor. 
     Image sensor designs include Charged Coupled Devices (CCD), Complementary Metal Oxide Silicon (CMOS) image sensors, and Digital Pixel System (DPS) sensors. Image sensors can be designed to support either rolling shutter or global shutter exposure method. The difference between rolling shutter and global shutter resides in the timing that the pixels in the array are reset and exposed to light. Specifically, rolling shutter refers to an exposure method wherein the pixels in the sensor are reset and exposed to light one line at a time, thus resulting in a delay in reset and exposed time between every pair of consecutive rows. Global shutter refers to an exposure method wherein all the pixels in the array are reset and exposed to light at substantially the same time. An example of a CMOS image sensor with global shutter and full frame CDS support has been disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 13/435,071, entitled “CMOS image sensors implementing full frame correlated double sampling with global shutter,” filed Mar. 20, 2012 and having at least one common inventor hereof, which patent application is incorporated herein by reference in its entirety. 
     One disadvantage of image sensors implementing full frame or partial frame CDS is that the image sensor requires a memory buffer to store the reset values of all pixels within the selected region. The required memory buffer size is proportional to the number of pixels times the number of bits needed to represent the CDS value. The size of the memory buffer often increases the system cost especially when the pixel count in practical image sensors keeps increasing. 
     SUMMARY OF THE INVENTION 
     An image sensing device and image sensing method are described herein. By way of example, the imaging sensing device includes a two-dimensional array of light sensing elements, each light sensing element being configured to generate an output signal indicative of an intensity level of light impinging on the light sensing element, one or more analog-to-digital converters configured to digitize output signals read out from the array of light sensing elements to generate digital output pixel values, a control circuit configured to generate digital pixel reset values, and a compression module configured to receive the digital pixel reset values and to generate a compressed digital pixel reset value corresponding to each digital pixel reset value. 
     Further, by way of example, the imaging sensing method includes providing a two-dimensional array of light sensing elements, each light sensing element is capable of generating an output signal indicative of an intensity level of light impinging on the light sensing element, reading out digital pixel reset values associated with the light sensing elements, and compressing the digital pixel reset values to generate a compressed digital pixel reset value corresponding to each digital pixel reset value. 
     The present invention is better understood upon consideration of the detailed description below and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an image sensing device that supports full frame or partial frame digital CDS. 
         FIG. 2  is schematic diagram of an image sensing device that supports compression in full frame or partial frame digital CDS according to one embodiment of the present invention. 
         FIG. 3  is a schematic diagram of an encoding module which can be used to implement the CDS compression module in the image sensing device of  FIG. 2  according to one embodiment of the present invention. 
         FIG. 4  illustrates the neighborhood of image pixels used in a predictor according to one embodiment of the present invention. 
         FIG. 5  illustrates the structure of the codeword generated to encode the prediction error of the pixel reset value according to one embodiment of the present invention. 
         FIG. 6  is a schematic diagram of a decoding module which can be used to implement the CDS decompression module in the image sensing device of  FIG. 2  according to one embodiment of the present invention. 
         FIG. 7  is a schematic diagram of a CDS compression module for encoding multiple digital pixel reset values according to one embodiment of the present invention. 
         FIG. 8  illustrates the line buffer which can be incorporated in the CDS compression module of  FIG. 7  according to one embodiment of the present invention. 
         FIG. 9  is a schematic diagram of a CDS decompression module for decoding multiple digital pixel reset values according to one embodiment of the present invention. 
         FIG. 10  illustrates the line buffer which can be incorporated in the CDS decompression module of  FIG. 9  according to one embodiment of the present invention. 
         FIG. 11  is a plot of the ideal data bit fill rate of the memory buffer together with upper and lower bounds to illustrate the operation of the rate control algorithm according to one embodiment of the present invention. 
         FIG. 12  is a schematic diagram of a predictor which can be used in the encoder of  FIG. 3  and the decoder of  FIG. 6  according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the principles of the present invention, an image sensing device implements full frame or partial frame digital correlated double sampling (CDS) with compression of the pixel reset values. In this manner, the memory buffer used to store a selected region in a frame of pixel reset values can be made with reduced memory size. In one embodiment, the compression of the pixel reset values is implemented using a predictor applying a predict function to a set of neighboring decoded pixel reset values. Context information, which is the result of classification calculations, can be used in encoders to improve the compression performance. In an embodiment, a prediction function is designed so that both lossless and near lossless compression can be realized with comparable performance and without the use of context calculations, which reduces the complexity of implementing the compression of the pixel reset values. 
     In the present description, data compression refers to a method to reduce the data size of information by encoding the information using fewer data bits than the original representation. Compression can be either lossy or lossless. Lossless compression reduces data bits by identifying and eliminating statistical redundancy. No information is lost in lossless compression. Lossy compression reduces data bits by identifying marginally important information and removing it. 
     Since the purpose of the pixel reset values for CDS is to use as a reference in the CDS subtraction process to eliminate or reduce noise in the output image data, lossless compression methods are often desired. For a given output image data size, the size of the compressed pixel reset data with lossless compression is dependent on the statistical characteristics of the data. In some cases, the compressed pixel reset data, compressed using lossless compression methods, may still be too large for the desired memory buffer size. In some embodiments, lossy compression methods are applied to further reduce the size of the compressed pixel reset data. In one embodiment, a rate control method is used to dynamically adjust the compression steps so that the resulting compressed pixel reset data will fit into a memory buffer of the desired size. 
     In some embodiments of the present invention, the compression method in the image sensing device is configured to process multiple pixel reset values in each clock cycle. In image sensor design, for speed and timing considerations, the image sensor may process image data for multiple pixels in each clock cycle at the digital image processing module. Accordingly, in some embodiments, the compression of the pixel reset values is configured to compress or decompress (i.e. encode and decode) multiple pixels in each clock cycle so as to maintain a streamline flow of data through the image sensing device. Furthermore, in embodiments of the present invention, processing pixel reset values for multiple pixels per clock cycle does not impact the compression performance. 
     The image sensing device of the present invention with compression of CDS data realizes many advantageous features. First, the size of the memory buffer needed to store a selected region of a frame of pixel reset values is reduced, thereby reducing the overall image sensor cost. Second, the image sensing device implements compression rate control to control the fill rate of the memory buffer so that all the data bits of the compressed pixel reset values can fit into the memory buffer. Third, the compression of the CDS pixel reset values can be configured to process multiple pixels concurrently without impacting the compression performance. 
       FIG. 1  is a schematic diagram of an embodiment of an image sensing device that supports full frame and partial frame digital CDS. Referring to  FIG. 1 , an image sensing device  1  includes an image sensor  2  including a two-dimensional array of light sensing elements, also referred to as “pixels.” In the present illustration, the pixels are arranged in a rectangular array of M (height) by N (width) pixels. Each pixel in image sensor  2  includes a photodiode and multiple control transistors connected in a configuration to control the reset, charge transfer and readout operation of the pixel. 
     To implement CDS, the pixels in the image sensor  2  in  FIG. 1 , under the control of a control circuit  3 , are reset. The pixel reset values of all the pixels in the image sensor  2  are measured and digitized by the ADC circuits  4 . The switch  5  is connected to the lower switch position directing the digital pixel reset values to a memory buffer  6  to be stored. When the pixels are exposed to light, the light-dependent pixel values of all the pixels in the image sensor  2  are measured and digitized by the ADC circuits  4 . The switch  5  is connected to the upper switch position directing the light-dependent pixel values read out from the image sensor to the adder  8 . Meanwhile, the digital pixel reset values are read from the memory buffer  6  and are subtracted from the light-dependent pixel values to generate CDS-corrected digital pixel output values. In the image sensing device  1 , the memory buffer  6  must be large enough to store a selected region of a frame of digital pixel reset values. The size of the memory buffer  6  increases the cost of the image sensing device  1 . 
       FIG. 2  is schematic diagram of an image sensing device that supports compression in full frame and partial frame digital CDS according to one embodiment of the present invention. Referring to  FIG. 2 , an image sensing device  20  includes an image sensor  2  including a two-dimensional array of light sensing elements or pixels. In the present illustration, the pixels are arranged in a rectangular array of M (height) by N (width) pixels. Each pixel in image sensor  2  includes a photodiode and multiple control transistors connected in a configuration to control the reset, charge transfer and readout operation of the pixel. The image sensing device  20  further includes analog-to-digital circuits  4  for digitizing the pixel data generated by the image sensor  2 . A control circuit  3  controls the operation and timing of the control transistors in the pixels to operate the image sensor  2  in a reset mode, a light integration mode and a read-out mode. 
     In embodiments of the present invention, the pixels in image sensor  2  can be configured using any active pixel architecture, presently known or to be developed, having reset, and row/column select controls. Furthermore, the image sensor can be formed using various pixel cell designs, including the 4-transistor, 5-transistor or more complex active pixel cell architectures. Exemplary active pixel architectures are described in Bigas et al., “Review of CMOS image sensors,” Microelectronics Journal, 2006. 
     In embodiments of the present invention, the image sensor  2  can either be a monochrome or a color sensor with a color filter array. To implement a color image sensor, an array of selectively transmissive filters is superimposed and in registration with each of the pixel elements. The array of selectively transmission filters includes at least a first group of filters associated with a first group of photodiodes for capturing a first color spectrum of visible light and a second group of filters associated with a second group of photodiodes for capturing a second color spectrum of visible light. Construction of color image sensors is known in the art. In some embodiments, the pixel elements are coated with individual RGB color filters arranged in a Bayer pattern. A demosaicing algorithm is used in the image processing pipeline to produce color images based on pixel data obtained from the color image sensor. In other embodiments, the pixel elements are coated with CMY color filters or other color filter patterns, in a Bayer pattern or other color filter configurations. 
     In some embodiments, the image sensor  2  can support either rolling shutter or global shutter. An example of a CMOS image sensor with global shutter and full frame CDS support has been disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 13/435,071, entitled “CMOS image sensors implementing full frame correlated double sampling with global shutter,” filed Mar. 20, 2012. 
     In embodiments of the present invention, the image sensing device  20  includes a memory  24  for storing digital pixel reset values. The memory buffer may be formed on the same integrated circuit as the pixel array. Alternately, the memory buffer or memory may be formed on an integrated circuit separated from the integrated circuit on which the pixel array is formed. The level of integration of the memory with the array of pixel elements of the CMOS sensor is not critical to the practice of the present invention. The image sensing device  20  may further include a CDS control and cancellation circuit to perform CDS cancellation on the image sensor and to generate the CDS corrected digital pixel output values. In the present embodiment, the CDS control and cancellation circuit is modeled as a switch  25  and an adder  28 . The switch  25  and the adder  28  are used to illustrate the functional operation of the CDS control and cancellation circuit symbolically and the actual implementation of the CDS control and cancellation circuit may be different and may not include an actual switch or adder. 
     To implement CDS with compression, the pixels in the image sensor  2  of  FIG. 2 , under the control of a control circuit  3 , are reset. The pixel reset values of all the pixels in the image sensor  2  are measured and digitized by the ADC circuits  4 . The switch  25  is connected to the lower switch position directing the digital pixel reset values to the CDS compression device  22 , which compresses the digital pixel reset values and stores the compressed values in the memory buffer  24 . Then during the light integration phase, the pixels are exposed to light, and the light-dependent pixel values of all the pixels in the image sensor  2  are measured and digitized by the ADC circuits  4 . The switch  25  is connected to the upper switch position directing the light-dependent pixel values read out from the image sensor to the adder  28 . Meanwhile, the compressed digital pixel reset values are read from the memory buffer  24  and are routed through the CDS decompression device  26 . The decompressed pixel reset values are subtracted from the light-dependent pixel values to generate CDS-corrected digital pixel output values. 
     In the present description, the term “light dependent pixel value” will be used to refer to a pixel value including the reset value and the light dependent value for a pixel. That is, the “light dependent pixel value” refers to “a reset plus light dependent pixel value” of a pixel for simplicity. In the present description, the terms “exposure” and “light integration” refer to the action of the light sensing element to integrate photons from incident light. The “exposure period” or “exposure time” does not necessarily refer to the time period when the light sensing element is exposed to light. In some cases, such as when electronic shutter is used, the pixel array may be exposed to light but not yet integrating photons. In the present description, a pixel element or a pixel array is quantized prediction error to be “exposed to light” or “integrating incident light” when the light sensing element of the pixel element is integrating photons from the incident light. 
     In the image sensing device  20 , the digital pixel reset values are compressed to reduce the quantity of data that will need to be stored in the memory buffer  24 . To that end, the image sensing device  20  includes a CDS compression module  22  coupled to receive the digital pixel reset values through the switch  25 . The CDS compression module  22  generates compressed digital pixel reset values which are then stored in the memory buffer  24 . Accordingly, the size of the memory buffer  24  can be reduced to reduce the cost of image sensing device  20 . The image sensing device  20  further includes a CDS decompression module  26  in communication with the memory buffer  24  to read out compressed digital pixel reset values and to generate decompressed digital pixel reset values which are provided to the adder  28  for CDS cancellation. 
     The primary use of the pixel reset values is as a reference in the CDS cancellation process to eliminate or reduce noise in the output image data. In some embodiments, the CDS compression module  22  implements lossless compression methods. In this case, the CDS Compression input values  32  and the CDS Decompression output values  34  are identical. In some cases, the lossless compression methods may generate a total size of compressed data that is still too large for a memory buffer of a desirable size. Accordingly, in some embodiments, the CDS compression module  22  also implements lossy compression methods to realize further reduction in the data bits of the compressed pixel reset values is desired. In this case, the CDS Compression input values  32  and the CDS Decompression output values  34  are different in a way controlled by the CDS Compression  22  and CDS Decompression  26  blocks. 
     U.S. Pat. Nos. 5,680,129 and 5,764,374, both to Seroussi et al., disclose a compression method for lossless compression of image data. The method described in Seroussi&#39;s patents encodes the image one pixel at a time in an array scan order. Specifically, for each pixel, the method includes the use of a predictor, a variable length coder, and context calculations. The purpose of context calculations is to classify the pixel into a pre-defined number of classes, so that encoding parameters corresponding to the chosen class is used in the variable length coder to encode the pixel. In other words, the compression method of the Seroussi patents uses a pre-defined number of independent variable length encoders, and the optimum encoder is chosen for each pixel according to the context. In the case where an adaptive variable length encoder is used, such as an adaptive Huffman coder or an adaptive Golomb-Rice coder, the compression method will need to maintain multiple sets of state information, one for each class, and update the state variables of the respective class according to the context. 
     It is well known that using a context in the encoder and decoder can provide good compression performance. However, this significantly complicates the encoder and decoder implementation because it will be necessary to maintain and update multiple sets of state variables, one for each context. In image sensor designs where the compression module will need to encode multiple pixels at each clock cycle, the context based compression method can be exceedingly complex. According to embodiments of the present invention, the CDS compression module  22  in image sensing device  20  implements a compression method using an adaptive predictive coding method without using contexts or context calculations. The CDS decompression module  26  is implemented using a corresponding adaptive predictive decoding method. In embodiments of the present invention, the CDS compression module and the CDS decompression module are implemented using at least the following elements:
         Predictor;   Variable Length Coder (or Entropy Coder)   State Calculation
 
To further improve compression performance and to ensure that the compressed data will fit into a desired memory buffer size, a DC value calculator and a rate control algorithm can be optionally used.
       

       FIG. 3  is a schematic diagram of an encoding module which can be used to implement the CDS compression module in the image sensing device of  FIG. 2  according to an embodiment of the present invention. Referring to  FIG. 3 , an encoding module  50  is configured to process digital pixel reset values from the image sensor  2  one pixel at a time using an adaptive predictive coding method. In embodiments of the present invention, the encoding module  50  processes the digital pixel reset values in an array scan order. In the present description, an “array scan order” refers to the order where pixel data are read out of the image sensor array row by row. The pixel data are read out of the N×M pixel array  80  in an array scan order where pixels are read out from rows i=1 to i=M and within each row, from column j=1 to j=N. In the present embodiment, the array scan order starts from the upper left corner of the pixel array  80 . This is illustrative only and in other embodiments, the array scan order can start from other corners of the pixel array. 
     The present invention also works with image sensors that include sequencer circuits where the sequencer circuits control the readout ordering of the pixels. For example, the pixels in an image sensor array can be read out starting from the r th  row where r is a number between 1 and M. The readout proceeds from the r th  row towards the bottom of the array, i.e. towards the M th  row. After the pixel values of the M th  row are read out, the readout proceeds from the top of the array and the remaining rows (first row to r−1 th  row) are then read out. The readout ordering that has just been described is exemplary. Sequencers in image sensors can be designed to readout the pixels in other row orderings. 
     In one embodiment, the encoding module  50  includes a predictor  70  generating predicted pixel reset values for each pixel, an adder  53  receiving the incoming digital pixel reset values and generating a prediction error for each pixel, a quantizer  55  generating quantized prediction errors (qpe) from the prediction error (pe)  54 , a mapping module  56  to map the quantized prediction errors (qpe) to absolute-value prediction errors (ape), and an adaptive encoder  58  generating encoded or compressed digital pixel reset values. In  FIG. 3 , an adaptive Golomb-Rice coder is illustrated. Other entropy coder can also be used in the embodiment. The encoding module  50  further includes a state update module  62  generating the encoding parameter k for the adaptive Golomb-Rice encoder  58  and the state value C for the adders  53  and  67 . The encoding module  50  further includes a rate control module  64  to modulate the distortion parameter of the quantizer  55  to control the output bit rate for the compressed digital pixel reset values. The encoding module further includes an inverse quantizer  65  to calculate a decoded prediction error dpe, which is added to the predictor output  72  and the state value C at the adder  67  to produce decoded pixel reset values. The decoded pixel values of the neighboring pixel locations (coordinates) are used by a predictor to produce a predicted pixel reset value for the current pixel. 
     In an embodiment of the present invention, the predictor  70  operates on a group of neighboring decoded pixel reset values associated with pixels in the neighborhood of the current pixel being processed as illustrated in  FIG. 4 . The predictor  70  applies a predict function on the neighboring decoded pixel reset values to generate the predicted pixel reset value. The difference between the digital pixel reset value, the corresponding predicted pixel reset value and the state value C is the prediction error pe (node  54 ). 
     Referring to  FIG. 4 , in an embodiment of the present invention, for a digital pixel reset value x(i, j) associated with a given pixel at an array location (i, j), the predictor  70  uses five neighboring values x(i, j−s), x(i−s, j−s), x(i−s, j), x(i−s, j+s) and x(i−s, j+2s) to compute the predicted pixel reset value y(i, j). In  FIG. 4 , a digital pixel rest value x(i, j) and five neighboring decoded pixel reset values a, b, c, d and e, with s=2 are shown. In other embodiments, other values of s can be used. However, the value s=2 is particularly useful for a color image sensor where a color filter array with a Bayer pattern or other patterns is used to obtain color pixel values. In an image sensor with a Bayer color filter array, choosing s=2 is equivalent to encoding the pixels corresponding to each color position independently. In the case of a monochrome image sensor, a value of s=1 can be used. Furthermore, in other embodiments, other method can be used to select the neighborhood pixel reset values for use in the compression method of the present invention. 
     In an embodiment of the present invention, the predictor  70  generates the predicted pixel reset value y(i, j) for a given pixel using a prediction device as shown in  FIG. 12 . Let y(i, j) be a predicted pixel reset value for digital pixel reset value x(i, j) and a, b, c, d and e denoting the neighboring decoded pixel reset values as shown in  FIG. 4 . Note that the positions of the pixels a, b, c, d, e are relative to location (i, j) of the pixel x(i, j), i.e., a=x(i, j−2), b=x(i−2, j−2), and so on. The predicted pixel reset value y(i, j) is given by: 
     
       
         
           
             
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                             otherwise 
                           
                         
                       
                       . 
                     
                   
                 
               
             
           
         
       
     
     In the present embodiment, the predictor  70  uses a predict (f, g, h) function which takes 3 input pixel reset values f, g, and h. The neighboring decoded pixel reset values a, b, c, d and e are partitioned into three overlapping groups, where Group  0  includes the 3 pixel values {a, b, c}, Group  1  includes the 3 pixel values {b, c, d}, and Group  2  includes the 3 pixel values {c, d, e}. A spread value r for each group, which is the maximum value of the group minus the minimum value of the group, is calculated. The group with the lowest spread value, that is the group with minimum of r 0 , r 1  or r 2 , is chosen, and the output of the predictor is the output of the predict( ) function for the chosen group with the smallest spread value. 
     By using a predictor based on an extended region of support (e.g., with 5 pixels in the neighborhood), and choosing a subset of pixels based on the spread value, the encoding module  50  can provide good performance without having to use context or perform context calculations. In this way, the overall complexity of the encoding module is reduced while good compression performance is maintained. 
     In processing the pixels in the image, some pixels may not have the desired number of neighboring pixel reset values. For example, for a pixel that is near the outside boundary of the image sensor, the pixel reset value x(i, j) for such a pixel may not have all five neighboring decoded pixel reset values needed by the predictor. In an embodiment of the present invention, when any particular neighboring decoded pixel rest values a, b, c, d and e are not available, a DC value is used in place of the missing neighboring decoded pixel rest value. In one embodiment, the DC value is calculated by a DC value calculator  74  ( FIG. 3 ). In some embodiments, the first two rows of the pixel reset values are used to calculate the DC value. For instance, the average of the first two rows of the pixel reset values is used as the DC value. In other embodiments, the DC value can be set to 0. In yet other embodiments, the DC value can be calculated as the average of image data from a subset of one or more previous frames. 
     In the encoding module  50  shown in  FIG. 3 , the quantizer  55  is incorporated when lossy encoding is desired to further reduce the number of data bits generated by the encoding module. Quantizer  55  is optional and may be omitted when only lossless encoding is desired. Quantizer  55 , when applied, has a distortion parameter z, resulting in a quantizer bin size of 2z+1. For example, when the distortion parameter z=1 and the quantizer bin size is 3, the quantizer  55  is configured for +/−1 encoding, that is, the decoded value will be within 1 of the original pixel value. Thus, each decoded value of the pe differs from the corresponding original pe value by at most 1. In embodiments of the present invention, a rate control module  64  is coupled to control the distortion parameter z of the quantizer  55  to adjust the rate at which the data bits of the compressed pixel reset values is generated. The operation of the rate control module  64  will be described in more detail below. 
     In embodiments of the present invention, the adaptive encoder is implemented using a Golomb-Rice encoder as the entropy coder for the prediction error. Entropy coders such as adaptive Huffman coder, adaptive run-length coder, or other coders can be used with the present invention. In an embodiment of the present invention, the Golomb-Rice coder is specified by an encoding parameter k. Referring to  FIG. 3 , the predictor error is calculated by
 
 pe=x ( i, j )− y ( i, j )− C  
 
where C is a state value to adjust the prediction error so that the prediction error is close to zero mean. Calculation of C can be performed using known methods such as the method disclosed in the Seroussi&#39;s patents, and will be described in more detail below. The quantized prediction error qpe is mapped to a non-negative value or absolute value by the function:
 
             ape   =     {           2   *   qpe               if   ⁢           ⁢   qpe     ≥   0     ;                   -   2     *   qpe     -   1           otherwise   .                   
Note that qpe and pe may or may not have the same value. When the distortion parameter z equals zero, i.e. when doing lossless encoding, qpe and pe have the same value.
 
     The Golomb-Rice encoder  58  generates a codeword having a variable length l for each pixel determined from the absolute-value prediction error ape.  FIG. 5  illustrates the structure of the codeword generated to encode the prediction error of the pixel reset value according to one embodiment of the present invention. Referring to  FIG. 5 , the codeword for a given pixel reset value x(i, j) is given by a concatenation of q bits of value 1, followed by a single bit of value 0, and then followed by k bits of the least significant bits (LSB) of ape. The value of q is the value of the most-significant bits (MSB) of the ape above k bits, i.e. q=ape/2 k  where the division is integer division with truncation. For example, let ape=13 and the original data be of 12 bit precision, which corresponds to the bit string “000000001101”. Suppose k=2, then q is the value of the ten most-significant bits “0000000011” of the ape above the last two bits. Thus, q=3 and thus the codeword contains 3 bits of “1” as the leading bits. The codeword is then followed by a 0 and then followed by k bits of the LSB of the ape which is “01”. Thus, for an ape value of 13 and k=2, the codeword generated is 111 0 01. Accordingly, the length  1  of the codeword for x(i, j) is q+1+k bits. In this example, l=6. In practice, k is initialized to an arbitrary initial value, e.g. 6. The value k is adaptively updated by the state update module  62  after each pixel reset value has been encoded. 
     In the predictive coding scheme used in the image sensing device of the present invention, both the encoder and the decoder use a predictor to generate a predicted pixel reset value. The encoder encodes the prediction error calculated using the predicted pixel reset value. The decoder decodes the prediction error and calculates the decoded pixel reset value using the decoded prediction error and the predicted pixel reset value. To ensure that the encoder and the decoder are synchronized with each other, both the encoder and decoder should calculate the exact same predicted pixel reset value for every pixel. Accordingly, referring to  FIG. 3 , the encoding module  50  includes an inverse-quantizer  65  which receives the quantized prediction error pe (node  55 ) to generate an inverse-quantized or recovered prediction error. The inverse-quantized prediction error is then added back with the predicted pixel reset value y(i, j) and the value C, such as at an adder  67 , to generate the decoded neighboring pixel reset values for use by the predictor  70 . A line buffer  66  may be used to store the neighboring decoded pixel reset values for use by the predictor  70 . In this manner, the predictor  70  is operated on neighboring decoded pixel reset values that are derived from the quantized and then inverse quantized prediction errors and the encoding and decoding operation can be in sync. 
     As configured, the encoding module  50  generates a variable length code for each prediction error which can be considered the encoded or compressed digital pixel reset value. The compressed digital pixel reset values are bit-packed into an output bit stream on an output node  60 . In some embodiments, the output bit stream of the encoding module  50  may be coupled to the memory buffer  24  where the compressed digital pixel reset values are stored. In other embodiments, the output bit stream may be coupled directly to the decompression module, or to some external storage outside of the image sensor device  20 . Because the output bit stream is formed from a concatenation of variable length codewords, a FIFO (first in first out) buffer may be needed in order to maintain a constant input and output speed for the decompression module. 
     The performance of the Golomb-Rice code depends on the proper adaptation of the values k and C. U.S. Pat. No. 5,764,374 suggests a simple way of updating the value of k and C. To this end, the state of the coder is defined to be a set of parameters A, B, C, L and k. Since the encoding module used in the present invention does not use any context, there is only a single set of A, B, C, L and k. The values of the state variables A, B, C, L and k are calculated by the procedures in Table 1. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Update means 
                 Update C 
                 Calculate k 
               
               
                   
               
             
            
               
                 A += ape/2 
                 if (B &gt;= L) { 
                 for (k=0; k&lt;cds_bpp; 
               
            
           
           
               
               
               
            
               
                 B += pe 
                 C++; B −= L; 
                 k++) { 
               
            
           
           
               
               
               
            
               
                 L += 1 
                 if (B &gt;= L) B = 
                 if ((L&lt;&lt;k) &gt;= A) 
               
            
           
           
               
               
               
            
               
                 if (L &gt; cds_nmax) { 
                 L−1; 
                 break; 
               
            
           
           
               
               
               
               
            
               
                   
                 L &gt;&gt;= 1 
                 } else if (B &lt; 0) { 
                 } 
               
            
           
           
               
               
               
               
            
               
                   
                 A &gt;&gt;= 1 
                 C−−; B += L; 
                   
               
               
                   
                 B &gt;&gt;= 1 
                 if (B &lt; 0) B = 0; 
                   
               
            
           
           
               
               
               
            
               
                 } 
                 } 
               
               
                   
               
            
           
         
       
     
     The quantity L is a running count of the pixels. It can be observed from the left box that A/L is approximately E[ape]/2 and B/L is approximately E[pe] where E is the expected value operator. The purpose of introducing the quantity cds_nmax is to slowly “forget” the samples that are far away from the present time. The quantity C is updated such that (C*L+B)/L is approximately equal to E[pe]. Note that the optimum value of k for the Golomb coder is such that 2 k  is roughly equal to E[ape]/2. The logic in this procedure finds the optimum value of k for the Golomb-Rice coder. 
     In summary, the encoding module  50  operates to encode the pixel reset values one pixel at a time on a sequential basis. For every pixel, a predicted pixel value is calculated, which is used to calculate the prediction error pe. The prediction error pe is quantized. The quantized prediction error is then mapped to an absolute-value prediction error ape. The absolute-value prediction error ape is encoded using the Golomb-Rice encoder with the encoding parameter k and the resulting codeword is provided on the output node. Meanwhile, the encoding module also calculates the inverse quantized prediction error from the quantized prediction error. The inverse quantized prediction error is added to the predicted pixel reset value and the sum is written to a line buffer to be used in the predictor. Then the state variables are updated according to Table 1 and the process continues to the next pixel. 
       FIG. 6  is a schematic diagram of a decoding module which can be used to implement the CDS decompression module in the image sensing device of  FIG. 2  according to an embodiment of the present invention. The decoding module  100  operates in the reverse way in relation to the encoding module  50  of  FIG. 3 . Referring to  FIG. 6 , the decoding module  100  receives the bit stream of compressed digital pixel reset values on an input node  102 . The input bit stream is provided to an adaptive Golomb-Rice decoder  104  to produce a decoded value. The decoded value is the absolute-value prediction error ape, which is a mapped value of the quantized prediction error qpe. The ape is coupled to a mapping module  106  to generate the quantized prediction error qpe. Then, an inverse-quantizer  108  applies inverse quantization to the quantized prediction error qpe to recover the prediction error pe within a distortion given by the distortion parameter used during encoding. The decoded prediction error pe is added to a predicted pixel reset value y(i, j) and the state value C to generate the decoded or decompressed pixel reset value x(i, j) on an output node  110 . The predicted pixel reset value y(i, j) is generated by a predictor  120  which operates on a set of neighboring decoded pixel reset values that may be stored in a line buffer  112 . A DC value  113  may also be used as in the encoder when there is missing neighboring decoded pixel reset values. In operation, the decoding module  100  is synchronized with the encoding module by using a rate control mechanism  116  and state update mechanism that performs the same calculations as in the encoder  50 . 
     As described above, the compression method described above requires s line buffers in both the encoding module and the decoding module, where encoding and decoding of each pixel require pixel values from s prior rows. A value that provides good compression performance in most image sensors is s=2, although other values of s can be used. 
     In some embodiments, the image sensing device incorporates a data path that processes multiple pixels per clock cycle in order to boost the throughput of the image sensor. In that case, the CDS compression module of the image sensing device will need to encode multiple pixel reset values associated with adjacent pixels per clock cycle. Similarly, the CDS decompression module of the image sensing device should decode multiple compressed pixel reset values associated with adjacent pixels per clock cycle. For example, in some embodiments, the image sensing device processes four adjacent pixels on the same row in the data processing data path. 
       FIG. 7  is a schematic diagram of a CDS compression module for encoding multiple digital pixel reset values according to one embodiment of the present invention. Referring to  FIG. 7 , a CDS compression module  150  includes four encoding modules  170  for encoding four digital pixel reset values in parallel in each clock cycle. Each encoding module  170  can be implemented in the same manner as the encoding module of  FIG. 3  for processing a single digital pixel reset value. In another embodiment, one copy of the rate control element  64  can be shared among the encoding modules. The CDS compression module  150  further includes line buffers  160  for storing incoming digital pixel reset values for encoding and compressed digital pixel reset values for read out. In one embodiment of the present invention, the line buffers  160  include 6 lines of buffer memory to manage the flow of data. 
     In the encoding modules, the calculation for updating the state variables according to Table 1 above can be computational intensive and may require multiple clock cycles to complete one update calculation. In one embodiment of the present invention, the state update module in each encoding module is configured to perform the state update calculation for variables in Table 1 in two clock cycles. In one embodiment, when the CDS compression module  150  is configured to process 4 pixels in a row in every clock cycle, and the state update module will perform the state update calculation in two clock cycles, this results in state update for any one of the encoding modules  170  once every 8 pixels in a row. 
     In embodiments of the present invention, the CDS compression module  150  is configured to encode digital pixel reset values stored in the line buffers  160  using a formation different from the array scan order in order to minimize the latency between updating of the state variables in Table 1.  FIG. 8  illustrates the line buffer which can be incorporated in the CDS compression module of  FIG. 7  according to one embodiment of the present invention. Referring to  FIG. 8 , the line buffer  160  includes 6 lines of memories, where each line of memories is disposed to store one row of pixel reset values from the image sensor. That is, each line of the line buffer  160  includes N memory elements.  FIG. 8  illustrates the operation of the encoding module on the pixel reset values stored in the line buffer over several clock cycles. 
     In  FIG. 8 , the encoder is at a state where the top 4 lines (lines  1 - 4 ) of the line buffers  160  are filled with pixel reset values. Specifically, the top 2 lines (lines  1 - 2 ) store decoded pixel reset values while the middle two lines (lines  3 - 4 ) store pixel values that have not yet been encoded. The bottom 2 lines (lines  5 - 6 ) are empty and are ready to receive incoming pixel reset values to be encoded. 
     In operation, incoming pixel reset values “I” are stored into the line buffer in array scan ordering. Thus, in the present illustration, four pixel reset values I 1  are received in line  5  of the line buffer  160  during the first clock cycle for the scan line and four adjacent pixel reset values I 2  are received in line  5  the next clock cycle. The character I represents incoming pixels and the numerical index indicates the clock cycle from the beginning of the line. The four pixels in each group, e.g. I 1 , could have different pixel values. The incoming pixel reset values are received in array scan ordering. The decoded pixel reset values in lines  1  and  2  as well as the previous decoded pixel values in lines  3  and  4  (i.e., to the left side of the current pixel value) are used in the prediction calculations. Meanwhile, the encoding modules operate on the pixel reset values stored in the middle two lines, lines  3  and  4 , of the line buffer  160 . More specifically, in order to minimize the latency or the number of pixels between updating of the state variables, the encoding modules  170  process four pixels in a 2×2 configuration within a clock cycle. For example, in the first clock cycle for the scan line, four pixel reset values at the pixel locations E 1 U 1  are encoded using state variables that have been previously calculated. The character E denotes execution of an encoding procedure and the numerical index indicates the clock cycle from the beginning of the line. The predictor may use the decoded pixel reset values stored in lines  1  and  2  as the neighboring decoded pixel reset values to calculate the predicted pixel reset values for the pixel locations E 1 U 1 . Meanwhile, the state update modules are activated to calculate the state variables according to Table 1. The character U denotes execution of a state update procedure and the numerical index indicates the clock cycle from the beginning of the line. When the pixels at the four locations are encoded, the encoded bits are packed and are sent to the output, e.g. to the memory buffer  24  for storage. In addition, the encoder also calculates the decoded pixel values using the inverse quantizer  65  and the adder  67 . The decoded pixel values are written back to the corresponding locations in lines  3  and  4  of the line buffer. 
     In one embodiment, the state update modules uses two clock cycles to complete the state update calculation. Thus, in the next clock, 4 pixels at the pixel locations E 2  are encoded using the same state variables as in the locations E 1 U 1 . Since the previous state update procedure is not yet completed and a new state update procedure is not started, there is no U 2  notation in  FIG. 8 . At this clock cycle, 4 pixels denoted by I 2  enter the line buffers and are stored in row  5  in array scan ordering. The process of encoding 2×2 blocks of pixel reset values continues, with state update invoked in every other clock cycle. At the end of clock  2 , another 2×2 block of pixel reset values at pixel locations E 2  has been encoded, and the encoded bits are sent to the output, e.g. to the memory buffer  24 . The decoded values calculated by the encoder are written back to the corresponding locations of lines  3  and  4 . Meanwhile, the updating of the state variables, which was started at E 1 U 1 , is also completed. Then, at clock  3  for the scan line, another 2×2 block of pixel reset values at pixel locations E 3 U 3  is encoded using the updated state variables. By using the 2×2 block formation, even though the state variables are updated every two clock cycles, the pixel reset values are encoded with updated state variables every 4 pixels instead of every 8 pixels when array scan ordering is being used. 
     When the encoding procedure arrives at the middle of lines  3  and  4 , the buffer at line  5  is filled with new pixel values. The encoding procedure continues from the middle of lines  3  and  4 , where the incoming pixel values are now written to line  6 . When all the pixels in lines  3  and  4  are encoded, the incoming pixels also completely fill up lines  5  and  6 . The data in the line buffers of  FIG. 8  are shifted two lines up, and the procedure repeats. Other methods, such as rotating the line pointer, can also be used to achieve the same effect as in shifting the data in the line buffers. 
       FIG. 9  is a schematic diagram of a CDS decompression module for decoding multiple digital pixel reset values according to one embodiment of the present invention. Referring to  FIG. 9 , a CDS decompression module  200  includes four decoding modules  230  for decoding four digital pixel reset values in parallel in each clock cycle. Each decoding module  230  can be implemented in the same manner as the decoding module of  FIG. 6  for processing a single compressed digital pixel reset value. In another embodiment, one copy of the rate control element  116  can be shared among the decoding modules. The CDS decompression module  200  further includes line  220  for storing decompressed digital pixel reset values for read out. In embodiments of the present invention, the line buffers  220  include 4 lines of buffer memory to manage the flow of data. 
       FIG. 10  illustrates the line buffer which can be incorporated in the CDS decompression module of  FIG. 9  according to one embodiment of the present invention. Referring to  FIGS. 2, 9 and 10 , in embodiments of the present invention, the CDS decompression module  200  receives the bit stream either from the CDS compression module or from the memory buffer  24 . The bit stream includes encoded pixel reset values in a 2×2 block formation. The decoding modules  230  can then operate on the encoded pixel reset values also in a 2×2 block formation. The line buffers  220  include four lines of buffer memories. The top two lines (lines  1 - 2 ) contain decoded or decompressed pixel reset values which can be read out and also be used by the predictor to calculate the predicted pixel reset values. The bottom two lines (lines  3 - 4 ) contain pixel reset values being decoded. The updating steps for the state variables according to Table 1 are started in alternating clock cycles as in the encoding modules. Thus, in clock  1  for the scan line, pixel reset values at pixel locations D 1 U 1  in a 2×2 formation are decoded using the state variables from previous calculation. The character D denotes a decoding operation and the numerical index indicates the clock cycle from the beginning of the line. The notation U 1  has the same meaning as in the encoder. Meanwhile, pixels at locations O 1  in line  1  are sent to the output in pixel scan ordering. The character O denotes sending decoded pixel values to the output and the numerical index indicates the clock cycle from the beginning of the line. In clock  2  for the scan line, pixel reset values at pixel locations D 2  in a 2×2 formation are decoded. There is no U 2  notation in  FIG. 10  because the state update that was started at the previous location has not been completed yet. State update that was started at pixel location D 1 U 1  is now completed. Within clock  2  for the scan line, the next 4 pixels at pixel locations O 2  in line  1  are sent to the output. Then in clock  3  for the scan line, pixel reset values at pixel locations D 3 U 3  in a 2×2 formation are decoded. Meanwhile the state variables have been updated so that pixel reset values at pixel locations D 3 U 3  are decoded using the updated state variables. When the decoding process arrives at the middle of lines  3  and  4 , all the pixels from line  1  has been sent to the output. The process continues with decoding the rest of pixels in lines  3  and  4 , and pixels in line  2  are sent out in pixel scan order. The decoding process continues in this manner. 
     Depending on the statistical characteristics of the pixel reset values and the size of the memory buffer, the size of the output bit stream after compression may not fit the total memory buffer size. In an embodiment of the present invention, the CDS compression module implements a rate control algorithm to control the distortion level of the quantizer to achieve a target memory size. The same rate control calculations are executed at both the encoder and decoder so that the same distortion level is applied to the CDS decompression module to synchronize the encoding and decoding operations. 
     In operation, the rate control module implements a rate control algorithm that calculates at each pixel location p the total number of bits Sp that is used from the first pixel through the p th  pixel. Then, the algorithm makes a decision to either increase the instantaneous distortion, reduce the instantaneous distortion, or maintain the same instantaneous distortion.  FIG. 11  is a plot of the data bit fill rate of the memory buffer to illustrate the operation of the rate control algorithm according to one embodiment of the present invention. Referring to  FIG. 11 , the horizontal axis represents the number of pixels that have been processed (encoded), and the vertical axis represents the total number of output data bits from the first pixel to the current pixel. Let R be the target bit rate on a per pixel basis. Hence for a total of M*N pixels in the image sensor, the total number of bits allowed is R*M*N. In other words, R*M*N is the memory buffer size. The line E in  FIG. 11  represents the ideal performance curve if every pixel would be encoded at a constant bit rate. For a practical image, the actual plot of bit rate versus p will fluctuate and deviate from the line E especially if the encoding is at lossless or near lossless mode. The piecewise linear lines F, G and H set an upper bound to the encoding bit rate, whereas the line I represents a lower bound to the encoding bit rate. In a preferred embodiment, the four lines are given by:
 
 H : ( R+ 0.5)* p  
 
 G : ( R− 1)* p+M*N− 256
 
 F: R*p+N*K  
 
 I: R*p−N*K *(log 2 (2*distortion+1)−log 2 (2*distortion−1))
 
where p is the number of pixels that have been encoded so far, M is the height of the image, N is the width of the image, and K is a predefined constant. The upper and lower bounds are examples that were chosen for ease of calculation. Other bounds can be chosen to work with the present invention.
 
     In operation, at a pixel location p, the total number of bits Sp that have been used up to that point is calculated. The upper threshold UBp is a point on lines F, G and H for any given value of p between 0 and M*N. The lower threshold LBp is a point on line I. The rate control algorithm operates in such a manner that if Sp&gt;UBp, then the distortion parameter for the quantizer is increased by 1. If Sp&lt;LBp, the distortion parameter is decreased by 1. 
     To prevent the distortion parameter from being adjusted upwards too often because of spurious rate fluctuations, the rate control algorithm will stop considering any distortion parameter adjustments for V pixels immediately after each distortion adjustment. Since it is important to operate the rate control method such that the final bit rate, i.e. the total number of bits used to encode the entire image, is within the bit budget R*M*N, the delay value V can be reduced as p approaches M*N. That is, when the encoding process is near the end of the image, the distortion parameter adjustments can happen more frequently. 
     The above detailed descriptions are provided to illustrate specific embodiments of the present invention and are not intended to be limiting. Numerous modifications and variations within the scope of the present invention are possible. For example, different active pixel cell designs may be used in an image sensor to achieve full frame or partial frame digital CDS with global shutter, different pixel value readout methods and timings may be used, etc. Furthermore, the compression methods, rate control approach, and detailed calculations may be changed. The present invention is defined by the appended claims.