Abstract:
An image sensor may include an array of image pixels arranged according to a predetermined pattern. A 3-by-3 pixel sub-sampling method is provided that supports a high-speed sub-resolution video mode. The 3-by-3 sub-sampling method may involve organizing the image pixel array into groups, each of which contains a 3-by-3 array of nine pixels. Image pixels at the four corners of each group may be sampled and combined to form a final output. Final outputs produced from each group may form a sub-sampled array that is used by the sub-resolution video mode.

Description:
This application claims the benefit of provisional patent application No. 61/843,816, filed Jul. 8, 2013, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to imaging systems, and more particularly, to imaging systems that support pixel sub-sampling video modes. 
     Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. In a typical arrangement, an electronic device is provided with an array of image pixels arranged in pixel rows and pixel columns. Circuitry is commonly coupled to each pixel column for reading out image signals from the image pixels. 
     Image sensors for which all image pixels in the array are read out are referred to as operating in full resolution mode. To support high-resolution video processing, image sensors are often operated in a reduced resolution or sub-resolution mode in which the amount of data that is read out is only a fraction of the full resolution image size. The sub-resolution mode is implemented via a pixel array sub-sampling technique in which only fraction of the pixels are sampled for readout. 
     A key mode for high resolution video processing is the 3×3 sub-sampling mode in which the output image data is reduced by a factor of 3 in both the X (horizontal) dimension and the Y (vertical) dimension. A conventional way for implementing 3×3 sub-sampling involves accessing data from every third row (i.e., by ignoring data entirely from the two intervening rows). Skipping two-thirds of the rows in this way may be referred to as Y-Skip3. 
     In each row that is actually being accessed, data from each pair of adjacent pixels is read out and combined to produce intermediate pixel values. Adjacent pairs of intermediate pixel values may then be combined to produce final pixel values. Each group of six image pixels in a row generates two final pixel values using this approach. Generating two image pixel values for every group of six image pixels in this way may be referred to as X-Bin2 with 1.5 scaling. Implementing 3×3 sub-sampling via the combination of Y-Skip3 and X-Bin2/Scaling-1.5 may cause inaccuracies since two-thirds of the data is thrown out in the Y dimension and image aliasing may result from the X-Bin2 and scaling operation. 
     It may therefore be desirable to provide improved ways of implementing 3×3 sub-sampling. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an illustrative electronic device in accordance with an embodiment of the present invention. 
         FIG. 2  is a diagram of an illustrative image sensor having an image pixel array and associated row and column control circuitry in accordance with an embodiment of the present invention. 
         FIG. 3  is a diagram illustrating a 3×3 sub-sampling technique that can be used to preserve the Bayer pattern and spacing uniformity in accordance with an embodiment of the present invention. 
         FIG. 4  is a diagram showing how read data can be rearranged before being processed by column control circuitry in accordance with an embodiment of the present invention. 
         FIG. 5  is a timing diagram showing illustrative waveforms associated with performing 3×3 sub-sampling in accordance with an embodiment of the present invention. 
         FIGS. 6A-6F  are diagrams showing column control circuitry configured in various arrangements in different phases of the sub-sampling operation of  FIG. 5  in accordance with an embodiment of the present invention. 
         FIG. 7  is a flow chart of illustrative steps for performing 3×3 sub-sampling in accordance with an embodiment of the present invention. 
         FIG. 8  is block diagram of a processor system that may employ some of the embodiments of  FIGS. 2-7  in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices such as digital cameras, computers, cellular telephones, and other electronic devices include image sensors that gather incoming light to capture an image. The image sensors may include arrays of imaging pixels. The pixels in the image sensors may include photosensitive elements such as photodiodes that convert the incoming light into image signals. Image sensors may have any number of pixels (e.g., hundreds or thousands of pixels or more). A typical image sensor may, for example, have hundreds of thousands or millions of pixels (e.g., megapixels). Image sensors may include control circuitry such as circuitry for operating the imaging pixels and readout circuitry for reading out image signals corresponding to the electric charge generated by the photosensitive elements. 
       FIG. 1  is a diagram of an illustrative electronic device that uses an image sensor to capture images. Electronic device  10  of  FIG. 1  may be a portable electronic device such as a camera, a cellular telephone, a video camera, or other imaging device that captures digital image data. Camera module  12  may be used to convert incoming light into digital image data. Camera module  12  may include one or more lenses  14  and one or more corresponding image sensors  16 . Image sensor  16  may be an image sensor system-on-chip (SOC) having additional processing and control circuitry such as analog control circuitry  31  and digital control circuitry  32  on a common image sensor integrated circuit die with image pixel array  20 . 
     During image capture operations, light from a scene may be focused onto an image pixel array (e.g., array  20  of image pixels  22 ) by lens  14 . Image sensor  16  provides corresponding digital image data to analog circuitry  31 . Analog circuitry  31  may provide processed image data to digital circuitry  32  for further processing. Circuitry  31  and/or  32  may also be used in controlling the operation of image sensor  15 . Image sensor  16  may, for example, be a backside illumination image sensor. If desired, camera module  12  may be provided with an array of lenses  14  and an array of corresponding image sensors  16 . 
     Device  10  may include additional control circuitry such as storage and processing circuitry  18 . Circuitry  18  may include one or more integrated circuits (e.g., image processing circuits, microprocessors, storage devices such as random-access memory and non-volatile memory, etc.) and may be implemented using components that are separate from camera module  12  and/or that form part of camera module  12  (e.g., circuits that form part of an integrated circuit that includes image sensors  16  or an integrated circuit within module  12  that is associated with image sensors  16 ). Image data that has been captured by camera module  12  may be further processed and/or stored using processing circuitry  18 . Processed image data may, if desired, be provided to external equipment (e.g., a computer or other device) using wired and/or wireless communications paths coupled to processing circuitry  18 . Processing circuitry  18  may be used in controlling the operation of image sensors  16 . 
     Image sensors  16  may include one or more arrays  20  of image pixels  22 . Image pixels  22  may be formed in a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology or any other suitable photosensitive devices. 
     The analog circuitry on image sensor  16  may include control circuitry such as amplifier circuitry and analog-to-digital converter (ADC) circuitry for converting the analog signals generated by pixels  22  into digital signals. As shown in  FIG. 2 , image sensor  16  may include image sensor pixel array  20  (e.g., an array of image sensor pixels  22  arranged in rows and columns) that is coupled to row decoder circuitry  224  and column control circuitry  222 . Row decoder circuitry  224  may receive row addresses and supply corresponding row control signals row_ctr such as pixel reset signals (RST), pixel row-select signals, charge storage gate transfer signals (TX), and other row control signals to image pixels  22  over control paths  228 . In the example of  FIG. 2 , a first row control signal row_ctr&lt;0&gt; may be fed to a first row of image pixels via row line  228 - 1 , a second row control signal row_ctr&lt;0&gt; may be fed to a second row of image pixels via row line  228 - 2 , etc. 
     Image pixels  22  in each column of image pixel array  20  may be coupled to a corresponding column line  140 . For example, image pixels  22  in a first pixel array column may be connected to column readout line  140 - 1 , image pixels  22  in a second pixel array column may be connected to column line  140 - 2 , etc. Column lines  140  may be used for reading out image signals from image pixels  22  and for supplying bias signals (e.g., bias currents or bias voltages) to image pixels  22 . During image pixel readout operations, a pixel row in array  20  may be selected using row decoder circuitry  224  and image data associated with image pixels  22  in that pixel row can be read out along column lines  140 . Each column line  140  may be coupled to corresponding column control circuitry including column amplifiers, memory circuits, and data converting circuitry, just to name a few. 
     As shown in  FIG. 2 , image pixel array  20  may be formed in a Bayer mosaic pattern exhibiting repeating unit cells of two-by-two image pixels. Each unit cell may, for example, include two green pixels  22 G (e.g., a green (Gr) pixel formed in a common row with a red pixel and a green (Gb) pixel formed in a common row with a blue pixel) diagonally opposite one another and adjacent to a red (R) pixel  22 R diagonally opposite to a blue (B) pixel  22 B. However, this is merely illustrative. If desired, other pixel color patterns may be used. 
     The ability of pixels  22 G,  22 R, and  22 B to receive different colored light may be provided via use of different color filter elements positioned above the image pixels. In particular, a green color filter element that only passes through green light may be positioned over pixels  22 G; a red color filter element that only passes though red light may be positioned over pixels  22 R; and a blue color filter element that only passes through blue light may be positioned over pixels  22 B. In general, other ways of providing color selectivity may be used. 
     In accordance with an embodiment of the present invention, a novel 3×3 sub-sampling method for processing data from image pixel array  20  is provided.  FIG. 3  is a diagram illustrating the 3×3 sub-sampling method that preserves the Bayer pattern and spacing uniformity across the entire array  20 . As shown in  FIG. 3 , array  20  may include colored pixels arranged in the Bayer pattern described in connection with  FIG. 2 . The array may also be divided up into groups of three-by-three image pixels (i.e., array  20  may be organized into groups of nine individual pixels). 
     There may be at least three different types of groups. A group having green pixels at its corners may be categorized as being the first type. A group having red pixels at its corners may be categorized as being the second type. A group having blue pixels at its corners may be categorized as being the third type. In general, other types of groups having the same color at its four corners may be identified. When processing data from the first group type, the four green pixels at the corners of that group may be read out and combined to form a corresponding combined green output signal. When processing data from the second group type, the four red pixels at the corners of that group may be read out and combined to form a corresponding combined red output signal. When processing data from the third group type, the four blue pixels at the corners of that group may be read out and combined to form a corresponding combined blue output signal. 
     The combined output signals generated by each row of groups are represented as signals  302  in  FIG. 3 . Signals  302  generated from each row of groups may serve as the final sub-sampled output signals  304 . As illustrated in  FIG. 3 , the number of output signals  304  has been reduced from the original array size by a factor of three. Sub-sampling array  20  in this way may be referred to as performing X-Bin3 and Y-Bin3 operations. The X-Bin3 operation may refer to “binning” or combining data signals from two out of every three pixels that are arranged along a row. Note that only data signals from non-adjacent columns should be binned together to form the final output signal. The Y-Bin3 operation may refer to combining data signals from two out of every three pixels that are arranged along a column. 
       FIG. 4  is a diagram showing two adjacent groups of 3×3 pixels. A first group of the first type may have green image pixels Gr formed at the intersections of rows 0 and 2 and columns 0 and 2, whereas a second group of the second type may have red image pixels R formed at the intersections of rows 0 and 2 and columns 3 and 5. Image pixel signals may be read out from array  20  sequentially on a row-by-row basis. When performing 3×3 sub-sampling using the Y-Bin3 approach, only rows 0 and 2 (and rows 3 and 5, etc.) need to be read (i.e., data from rows 1, 4, 7, 10, need not be read). The image pixels marked with a circle in  FIG. 4  are the ones that need to be taken into account using the desired 3×3 sub-sampling method. 
     The “OUTPUT” in  FIG. 4  shows data that has been read from row 0. In particular, data signals Gr-R-Gr-R-Gr-R has been read out from row 0, columns 0-5, respectively. The bolded signals above correspond to the pixels that are relevant and that need to be sampled for the X-Bin3 operation. 
     Routing circuitry within column control circuitry  222  may subsequently be used to “swap” the positions of at least some of the read data, as indicated by arrows  350 . In particular, the swapping operation may effectively shift the relevant signals so that they are all arranged in adjacent columns (e.g., Gr-R-Gr-R may be arranged in columns 0-3, respectively, while the Gr-R signals in columns 4 and 5 are not relevant in computing the final output). 
     Column amplifiers  352  may receive the rearranged signals and produce corresponding amplified versions of these signals. The amplified signals may then be provided to A/D converting circuits  354  for conversion into digital signals. There may be one column amplifier and one A/D converter in each column. Rearranging the image signals as described above allows the column control circuitry  222  to only enable the column amplifiers corresponding to the first four columns in each group of six columns while deactivating the two remaining column amplifiers corresponding to the last two columns in each group of six columns (e.g., column amplifiers in columns 0-3 may be switched into use while column amplifiers in columns 4 and 5 may be switched out of use). Note that in the six columns shown in  FIG. 4 , three of the column amplifiers (in columns 0, 2, and 4) may be used for amplifying green pixel signals, whereas the other three of the column amplifiers (in columns 1, 3, and 5) may be used for amplifier green pixel signals. As described above, the swapping operation allows one of the three column amplifiers processing the green pixel signals to be idle and one of the three column amplifiers processing the red pixel signals to be idle. 
     The discussion of  FIG. 4  relates only to the first and second types of groups. This is merely illustrative. In general, the principles described above in connection with  FIG. 4  may apply to the third type of group in which blue pixel signals are being processed. Referring back to  FIG. 3 , blue and green alternating pixels may be processed based on pixels in rows 3-5. Processing groups of 3-by-3 image pixels in this may be extended across the entire array  20 . 
       FIG. 5  is a timing diagram showing illustrative waveforms relevant to the 3×3 sub-sampling operation. During a first time period, data may be read from a first image pixel in a first row (e.g., row 0). At time t 1 , a reset signal RST may be pulsed high to reset a sense node in the first image pixel. The sense node (sometimes referred to as the floating diffusion node) may serve as a temporary storage node for the image pixel and may exhibit a potential level that directly affects the output value that is read out from the image pixel. 
     At time t 2 , a sample-and-hold reset signal SHR representing a reset signal value of the floating diffusion (FD) node of the first image pixel may be read out and held at a first storage element. At time t 3 , a transfer signal TX in row 0 may be pulsed high to transfer any charge that has accumulated in the photodiode of the first image pixel since the last charge transfer operation to the floating diffusion node. Doing so may change the potential level of the floating diffusion node from the reset level by an amount that is proportional to the amount of charge that is currently being transferred. 
     At time t 4 , a sample-and-hold image signal SHS representing an image signal value of the floating diffusion (FD) node of the first image pixel may be read out and held at a second storage element. At this point, even though SHR and SHS of the first pixel have been sampled and stored, data from a second pixel in another row can be read out prior to performing A/D conversion on the reset and image signal values obtained from the first image pixel. Doing so allows the pixel signals from multiple rows to be combined (or “binned” together) prior to conversion. 
     During a second time period, data may be read out from a second image pixel in a second row (e.g., row 2). Note that row 1 has been skipped since the Y-Bin3 operation ignores every other row (see, e.g.,  FIG. 4 ). Referring back to  FIG. 5 , reset signal RST may be pulsed high to reset the floating diffusion node in the second image pixel (at time t 5 ). At time t 6 , signal SHR associated with the second pixel may be read out from the FD node of the second image pixel and temporarily held at a third storage element. 
     At this time, the reset value of the first pixel that is held in the first storage element and the reset value of the second pixel that is held in the third storage element can be combined and converted into a combined digital reset signal (as indicated as SHR conversion at time t 7 ). 
     Sometime during the SHR conversion (e.g., at time t 8 ), a charge transfer signal TX in row 2 may be pulsed high to transfer any charge that has accumulated in the photodiode of the second image pixel since the last charge transfer operation to the floating diffusion node of the second image pixel. Doing so may change the potential level at the FD node of the second image pixel from the reset level by an amount that is proportional to the amount of charge that is currently being transferred. 
     At time t 9 , signal SHS associated with the second image pixel may be read out and temporarily held at the first storage element. The reset value that was previously stored in the first storage element was processed at time t 7  and thus can be overwritten. If desired, the image signal value of the second pixel can also be stored in the third storage element or any available storage element other than the second storage element. 
     At this time, the image signal value of the first pixel that is held in the second storage element and the image signal value of the second pixel that is held in the first storage element can be combined and converted into a combined digital image signal (indicated as SHS conversion at time t 10 ). The combined digital reset signal and the combined digital image signal may then be used to compute a final output signal. For example, the final output signal may be obtained by calculating the difference between the combined digital reset signal and the combined digital image signal. Computing output signals in this way is sometimes referred to as performing correlated double sampling (CDR). 
     In describing  FIG. 5 , operations involving only two image pixels along a given column are discussed. When performing 3-by-3 sub-sampling, however, data from two additional image pixels along a second column needs to be combined with data of the two image pixels from the given column in order to achieve X-Bin3. In other words, the four image pixels at the corners of each group of nine pixels discussed in connection with  FIG. 3  needs to be combined prior to conversion. 
       FIGS. 6A-6F  are diagrams showing how column control circuitry  222  may be configured to support different phases of the 3×3 sub-sampling method in accordance with an embodiment of the present invention. As shown in  FIG. 6A , column amplifier circuitry  352  may include at least a first column amplifier  352 - 1  operable to receive signals from a selected pixel in column 0, a second column amplifier  352 - 2  operable to receive signals from a selected pixel in column 2, and a third column amplifier  352 - 3  operable to receive signals from a selected pixel in column 4. Column amplifiers  352 - 1  and  352 - 2  may be used to amplify signals from relevant green pixels in columns 0 and 2 while amplifier  353 - 3  in column 4 is switched out of use (see, the example of  FIG. 4  in which only the green pixels in columns 0 and 2 are being binned together). 
       FIG. 6A  shows a first operational phase (or time period) during which amplifier  352 - 1  receives a reset signal SHR from pixel p(0,0) (i.e., a pixel in row 0, column 0), amplifier  352 - 2  receives a reset signal from pixel p(0,2) (i.e., a pixel in row 0, column 2), and amplifier  352 - 3  receives a reset signal from pixel p(0,4) (i.e., a pixel in row 0, column 4). Column control circuitry  222  may include switching and routing circuitry operable to couple together any number of column amplifiers  352  and further operable to couple any one of column amplifiers  352  to a selected portion of data conversion circuitry  354  (e.g., A/D converters  354 - 1 ,  354 - 2 , and  354 - 3 ). For example, data converter  354 - 1  may be coupled to column amplifiers  352  via a first switch  360 ; data converter  354 - 2  may be coupled to column amplifiers  352  via a second switch  362 ; and data converter  354 - 3  may be coupled to column amplifiers  352  via a third switch  364 . 
     During the first phase, column amplifiers  352 - 1  and  352 - 2  may be coupled together so that the reset signals from p(0,0) and p(0,2) may be combined and fed to capacitor C 3  at the input of converter  354 - 3  by closing switch  364  (i.e., switches  354 - 1  and  354 - 2  may be turned off). The combined reset signal of these green pixels in row 0 (labeled as signal  0 _R in  FIG. 6A ) may be equal to an amplified version of the average of the reset signals obtained from p(0,0) and p(0,2). The first phase may correspond to the time period immediately following time t 2  in  FIG. 5 . 
       FIG. 6B  shows a second operational phase during which amplifier  352 - 1  receives an image signal SHS from p(0,0), amplifier  352 - 2  receives an image signal from p(0,2), and amplifier  352 - 3  receives an image signal from p(0,4). During this time, column amplifiers  352 - 1  and  352 - 2  may be coupled together so that the image signals from p(0,0) and p(0,2) may be combined and fed to capacitor C 2  at the input of converter  354 - 2  by closing switch  362  (i.e., switches  354 - 1  and  354 - 3  may be turned off). The combined image signal of these green pixels in row 0 (labeled as signal  0 _S in  FIG. 6B ) may be equal to an amplified version of the average of the image signals obtained from p(0,0) and p(0,2). The second phase may correspond to the time period immediately following time t 4  in  FIG. 5 . 
       FIG. 6C  shows a third operational phase during which amplifier  352 - 1  receives a reset signal SHR from p(2,0) (i.e., a pixel in row 2, column 0), amplifier  352 - 2  receives a reset signal from p(2,2) (i.e., a pixel in row 2, column 2), and amplifier  352 - 3  receives an image signal from p(2,4) (i.e., a pixel in row 2, column 4). Note that a new row such as row 2 has been addressed while skipping intermediate row 1 to implement Y-Bin3 (e.g., only data from non-adjacent rows should be combined in the Y-Bin3 approach). During this time, column amplifiers  352 - 1  and  352 - 2  may be coupled together so that the reset signals from p(2,0) and p(2,2) may be combined and fed to capacitor C 1  at the input of converter  354 - 1  by closing switch  360  (i.e., switches  354 - 2  and  354 - 3  may be turned off). The combined reset signal of these green pixels in row 2 (labeled as signal  2 _R in  FIG. 6C ) may be equal to an amplified version of the average of the reset signals obtained from p(2,0) and p(2,2). The third phase may correspond to the time period immediately following time t 6  in  FIG. 5 . 
       FIG. 6D  shows a fourth operational phase during which amplifier  352 - 1  receives an image signal SHS from p(2,0), amplifier  352 - 2  receives an image signal from p(2,2), and amplifier  352 - 3  receives an image signal from p(2,4). During this time, column amplifiers  352  may be decoupled from data converters  354  (e.g., by temporarily breaking routing path  359 ). While path  359  is broken, sample-and-held reset signals  0 _R and  2 _R may be combined by closing switches  360  and  364 , and the combined reset signal may then be converted to digital signals using only first data converter  354 - 1  (i.e., converters  354 - 2  and  354 - 3  may be idle). The digital output signal generated in this way may be equivalent to the average of signals  0 _R and  2 _R. The fourth phase may correspond to the time period immediately following time t 7  in  FIG. 5 . 
       FIG. 6E  shows a fifth operational phase during which routing path  359  has been reestablished so that the image signals can be passed through to one of the data converters. During the fifth phase, column amplifiers  352 - 1  and  352 - 2  may be coupled together so that the image signals from p(2,0) and p(2,2) may be combined and fed to capacitor C 1  at the input of converter  354 - 1  by closing switch  360  (i.e., switches  354 - 2  and  354 - 3  may be turned off). 
     The combined image signal of these green pixels in row 2 (labeled as signal  2 _S in  FIG. 6E ) may be equal to an amplified version of the average of the image signals obtained from p(2,0) and p(2,2). The fifth phase may correspond to the time period immediately following time t 9  in  FIG. 5 . 
       FIG. 6F  shows a sixth operational phase during which column amplifiers  352  is decoupled from data converters  354  (e.g., by opening a switch at junction  359 ). While path  359  is open, sample-and-held image signals  0 _S and  2 _S may be combined by closing switches  360  and  362 ; the combined image signal may then be converted to digital signals using only first data converter  354 - 1  while converters  354 - 2  and  354 - 3  are inactive. The digital output signal generated in this way may be equivalent to the average of signals  0 _S and  2 _S. The sixth phase may correspond to the time period immediately following time t 10  in  FIG. 5 . 
     At the end of the six phases, the digital reset signal generated during the fourth phase and the digital image signal generated during the sixth phase may be used to compute a final output signal (e.g., by calculating the difference between the digital reset signal and the digital image signal). The final output signal generated in this way may represent one sub-sampled pixel output signal  304  described in connection with  FIG. 3 . 
     The example of  FIG. 3  shows only amplifiers  352  that are associated with columns 0, 2, and 4 and that are used to process green pixels. This is merely illustrative. Amplifiers  352  in other columns such as columns 1, 3, and 5 may be used to process red pixels (see, e.g.,  FIG. 4 ), blue pixels, or other types of pixels. In general, the arrangements in  FIGS. 6A-6F  may be used to process signals from pixels of any color from every other column (to support X-Bin3) and from every other row (to support Y-Bin3) in array  20 . 
       FIG. 7  is a flow chart of illustrative steps involved in performing the 3×3 sub-sampling operation illustrated in  FIGS. 5 and 6 . At step  400 , reset values SHR from row 0 may be sampled by corresponding column amplifiers. These sampled signals may be held temporarily at the input capacitors (sometimes referred to as ADC sampling capacitors) of a first group of A/D converters  354 . For example a reset signal O_R may be computed by taking the average of the reset signal from p(0,0) and the reset signal from p(0,2). Step  400  may correspond to  FIG. 6A . 
     At step  402 , the signal values SHS from row 0 may be sampled by the corresponding column amplifiers. These sampled signals may be held temporarily at the input capacitors of a second group of A/D converters  354 . For example, an image signal  0 _S may be computed by taking the average of the image signal from p(0,0) and the image signal from p(0,2). Step  402  may correspond to  FIG. 6B . 
     At step  404 , reset values SHR from row 2 may be sampled by the corresponding column amplifiers. These sampled signals may be held temporarily at the input capacitors of a third group of A/D converters  354 . For example, a reset signal  2 _R may be computed by taking the average of the reset signal from p(2,0) and the reset signal from p(2,2). Step  404  may correspond to  FIG. 6C . 
     At step  406 , selected A/D converters  354  may be used to convert the sum of the signals generated from steps  400  and  404 . A division of two may be built into the digital conversion or may be applied after conversion. A digital reset signal may be generated in this way. Step  406  may correspond to  FIG. 6D . 
     At step  408 , image values SHS from row 2 may be sampled by the corresponding column amplifiers. These sampled signals may be held temporarily at the input capacitors of the first group of A/D converters  354 . For example, an image signal  2 _S may be computed by taking the average of the image signal from p(2,0) and the image signal from p(2,2). Step  408  may correspond to  FIG. 6E . 
     At step  410 , selected A/D converters  354  may be used to convert the sum of the signals generated from steps  402  and  408 . A division of two may be built into the digital conversion or may be applied after conversion. A digital image signal may be generated in this way. Step  410  may correspond to  FIG. 6F . A final pixel output may then be computed based on the digital reset and image signals. 
     The steps of  FIG. 7  are merely illustrative and do not serve to limit the scope of the present invention. If desired, the steps of  FIG. 7  may be used to generate digital signals for each group of 3×3 image pixels across the entire image pixel array. Implementing 3-by-3 sub-sampling using this approach reduces undesired demosaic artifacts without introducing extraneous control circuitry that is otherwise not needed for normal operation of the image sensor. 
       FIG. 8  shows in simplified form a typical processor system  300 , such as a digital camera, which includes an imaging device  200  (e.g., an imaging device  200  such as imaging sensor  16  that includes column control circuitry  222  operable to implement the 3×3 sub-sampling operation described in  FIGS. 3-7 ). The processor system  300  is exemplary of a system having digital circuits that could include imaging device  200 . Without being limiting, such a system could include a computer system, still or video camera system, scanner, machine vision, vehicle navigation, video phone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system, and other systems employing an imaging device. 
     The processor system  300 , for example a digital still or video camera system, generally includes a lens  396  for focusing an image on pixel array  100  when a shutter release button  397  is pressed, central processing unit (CPU)  395 , such as a microprocessor which controls camera and one or more image flow functions, which communicates with one or more input/output (I/O) devices  391  over a bus  393 . Imaging device  2000  also communicates with the CPU  395  over bus  393 . The system  300  also includes random access memory (RAM)  392  and can include removable memory  394 , such as flash memory, which also communicates with CPU  395  over the bus  393 . Imaging device  2000  may be combined with the CPU, with or without memory storage on a single integrated circuit or on a different chip. Although bus  393  is illustrated as a single bus, it may be one or more busses or bridges or other communication paths used to interconnect the system components. 
     Various embodiments have been described illustrating imaging systems with 3×3 sub-sampling capabilities. A system may include an image sensor module with an array of image sensor pixels and one or more lenses that focus light onto the array of image sensor pixels (e.g., image pixels arranged in rows and columns). 
     The image sensor may also include a plurality of column amplifiers operable to produce first reset signals during a first time period, to produce first image signals during a second time period, to produce second reset signals during a third time period, and to produce second image signals during a fourth time period in that order. The first reset signals may be generated using column amplifiers in non-adjacent columns. The first reset signals and the second reset signals may correspond to reset data accessed from the image sensor pixels in non-adjacent rows. Similarly, the first image signals may be generated using column amplifiers in non-adjacent columns. The first image signals and the second image signals may correspond to image data accessed from the image sensor pixels in non-adjacent rows. 
     The image sensor may further include a data converter operable to produce a digital reset signal by converting a first combined signal that includes both the first and second reset signals during a fifth time period that occurs after the third time period and before the fourth time period and to produce a digital image signal by converting a second combined signal that includes both the first and second image signals during a sixth time period that occurs after fourth time period. A final output signal may then be computed based on the digital reset signal and the digital image signal (e.g., by calculating the difference between the digital reset signal and the digital image signal). 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. The foregoing embodiments may be implemented individually or in any combination.