Abstract:
A pixel array including circuitry for combining charges accumulated by individual pixels in the array enables addition and/or subtraction of individual pixel values, prior to their digitization, in the pixel array.

Description:
FIELD OF THE INVENTION 
     In various embodiments, the present invention relates to photo-sensing devices and methods and, in particular, to devices and methods suitable for compressive sampling. 
     BACKGROUND 
     Image capture and analysis now play a significant role in many industrial applications. For example, they may be employed in determining the orientation of a vehicle or platform, in vehicle navigation, and also in consumer products such as digital cameras and cellular phones. There is a trend toward increased resolution of the images captured, and storing a high-resolution image that includes millions of pixels typically requires a large amount of memory. But, some systems such as cameras in cell phones need to be small in size and, hence, permit the use of only a small on-board memory. Some other systems, such as those aboard satellites and spacecrafts, may be exposed to high radiation, requiring their on-board memory and/or processors to withstand high amounts of radiation. Such memory and processor components are relatively expensive compared to ordinary components of similar size and capacity that cannot withstand high radiation. It is, therefore, desirable to minimize the size of the memory and/or processor used in various imaging devices. 
     Typically, in image sensing, an image to be captured is divided into a matrix of pixels. The larger the required resolution, the smaller the size of each pixel and, correspondingly, the larger the number of pixels that must be sensed. The term “pixel” typically means a fraction of an image, but it is also commonly used to refer to the circuitry that senses the light received from a corresponding fraction of the image. Both meanings of the term are used herein. 
     Sensing a pixel generally includes measuring the intensity of the light corresponding to the pixel. In conventional imaging, each sensed pixel is digitized, i.e., the measured intensity of the light, generally expressed in the form of a voltage or current signal, is converted into a number that is stored in a memory cell. As such, high-resolution imaging, which requires a large number of pixels, typically requires a large memory. 
     Compressive sampling (which is also known as compressive sensing) is one approach that has been employed to meet the conflicting goals of high-resolution imaging and small-size memory. In certain cases, compressive sampling has decreased the required memory significantly, on a logarithmic scale. More specifically, in various existing approaches to compressive sampling, the total number of pixels P in an image are divided into N sets of M pixels, where M is typically much smaller than P. For example, M may be 10 or 100, while P can be as high as hundreds of millions. Then, for each set of M pixels, the values of some pixels are added to obtain a partial sum and the values of some pixels are subtracted from the partial sum to obtain a final pixel value corresponding to the set of M pixels. The pixels that are to be added or subtracted are determined according to a sampling function. For each set of M pixels, only the final pixel value is stored. As a result, compressive sampling generally only requires N memory cells, which is on the order of log P. 
     Although the final memory-storage requirement of compressive sampling can thus be substantially less than that of conventional imaging, compressive sampling still presents some challenges. As described above, the sampling functions require that a set of sensed pixels be added and/or subtracted. Unfortunately, the hardware used by many existing systems for compressive sampling requires that each pixel in the set of sensed pixels first be individually digitized so that the digitized values may then be added and/or subtracted by a processor. This requires a large temporary memory storage, significant processing capacity, and/or numerous interconnects for transferring the sensed pixels to a processor where the pixels may be digitized and processed (i.e., added or subtracted). In other words, the computational requirements of compressive sampling systems implemented significantly in the digital domain substantially undermine their benefits. Therefore, there is a need for improved systems and methods of sensing pixels that efficiently enable compressive sampling. 
     SUMMARY 
     In various embodiments, the present invention facilitates compressive sampling without requiring substantial temporary memory, processor capacity, or interconnects. This is achieved, in part, by performing addition and/or subtraction of pixels within the focal plane, without first digitizing the pixels. Unlike some methods in which pixel addition and/or subtraction is carried out by combining the currents flowing through one or more pixels, in various embodiments of the present invention the charges accumulated by the pixels are combined. The method by which the accumulated charges are combined makes the imaging system robust with respect to noise. 
     In general, charge accumulated by a pixel is proportional to the light received from the corresponding image pixel and, hence, the charge represents the image-pixel value. Therefore, a combination of the accumulated charges can be equivalent to adding and/or subtracting individual image-pixel values. While only one type of light-sensing element can be used for adding pixel values, the subtraction of pixels is typically performed using two different kinds of light-sensing elements. One type accumulates positive charges, while the other type accumulates negative charges. Combining like charges results in an addition of the sensed image-pixel values, while combining positive and negative charges results in a subtraction of the sensed image-pixel values. 
     In a pixel array, which may also be referred to as a sensor array, a sensor matrix, or a focal plane, various control signals permit selection of a set of pixels in the array required for addition and/or subtraction. After the charges accumulated by the pixels are combined (e.g., added or subtracted) a signal corresponding to the net charge is produced. This signal represents the processed (i.e., sampled) pixel value corresponding to the selected set of pixels. Only this value for each set of pixels is digitized and stored. Thus, because only a few pixel values are digitized, the demands on the analog to digital converter are dramatically reduced. This allows the processor to use significantly less power compared to the power required to digitize the sensed pixels. Moreover, relatively more time is available for each digitization so that these can be performed with greater accuracy, resulting in improved imaging performance. 
     In one aspect, embodiments of the invention feature a system for adding and/or subtracting pixels. The system includes a plurality of photo-sensitive pixels. Each photo-sensitive pixel outputs a charge to a common charge-sharing line. The system also includes circuitry, in electrical communication with the charge-sharing line, for producing an output signal related to the charge present on the charge-sharing line. In some embodiments, each photo-sensitive pixel includes a photo-sensitive element, which may be a p-type semiconductor photo diode, or an n-type semiconductor photo diode. 
     Each photo-sensitive pixel may also include a reset switch. The reset switch of each photo-sensitive element may be controlled by a common reset input, or, in the alternative, the reset switch of at least one photo-sensitive pixel may be controlled by a first reset input, and the reset switch of at least one other photo-sensitive pixel may be controlled by a second reset input. 
     In some embodiments, each photo-sensitive pixel of the system for adding and/or subtracting pixels includes a transfer switch in electrical communication with the common charge-sharing line. The transfer switch of each photo-sensitive pixel may be controlled by a common transfer input. Alternatively, the transfer switch of at least one photo-sensitive pixel may be controlled by a first transfer input, and the transfer switch of at least one other photo-sensitive pixel may be controlled by a second transfer input. 
     In a second aspect, embodiments of the invention feature a method for adding and/or subtracting pixels. The method includes outputting, by each of a plurality of photo-sensitive pixels, a charge to a common charge-sharing line, and producing an output signal related to the charge present on the charge-sharing line. 
     In some embodiments, the method includes resetting, for a reset period, a photo-sensitive element in at least one photo-sensitive pixel. The photo-sensitive element may be reset according to a reference voltage, and, as a result, a reference signal related to the reference voltage may be produced during the reset period. The method may also include comparing, after the reset period, the output signal with the reference signal. 
     In some embodiments, outputting the charge, by each of the plurality of photo-sensitive pixels, includes accumulating charge for a predetermined duration at a photo-sensitive element in the photo-sensitive pixel, and transferring the accumulated charge to the common charge-sharing line. The performances of the accumulating and transferring steps may overlap in part, or, in the alternative, the performance of the charge accumulating step may substantially precede the performance of the transferring step. Moreover, the transferring step may be performed substantially simultaneously at each photo-sensitive pixel. 
     In some embodiments, the charge transferring step is performed at a first set of photo-sensitive pixels substantially prior to performing the charge transferring step at a second set of photo-sensitive pixels. The step of producing the output signal may include producing a first output signal following the performance of the charge transferring step by the first set of photo-sensitive pixels, and producing a second output signal following the performance of the charge transferring step by the second set of photo-sensitive pixels. Arithmetic operations may also be performed on the first and second output signals. In some embodiments, the second set of photo-sensitive pixels includes all the photo-sensitive pixels in the first set. 
     These and other objects, along with advantages and features of the embodiments of the present invention herein disclosed, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. As used herein, the term “substantially” means±10%, and in some embodiments±5%. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
         FIG. 1  schematically illustrates a pixel array for performing addition and/or subtraction of image pixels in accordance with one embodiment of the invention; 
         FIG. 2  schematically illustrates a pixel array for performing addition and/or subtraction of image pixels in accordance with another embodiment of the invention. 
         FIG. 3  schematically illustrates a pixel array for performing addition and/or subtraction of image pixels in accordance with an embodiment of the invention similar to that illustrated in  FIG. 1 , but in which all pixels are reset by a single switch; and 
         FIG. 4  schematically illustrates a pixel array for performing addition and/or subtraction of image pixels in accordance with an embodiment of the invention similar to that illustrated in  FIG. 2 , but in which all pixels are reset by a single switch 
     
    
    
     DESCRIPTION 
     The pixel array  100  shown schematically in  FIG. 1  includes photo-sensitive pixels  110 ,  140 , i.e., circuitry for sensing light. Although only two photo-sensitive pixels  110  and two photo-sensitive pixels  140  are shown, the pixel array  100  may in fact include many more or fewer photo-sensitive pixels  110 ,  140 . The pixel  110  includes a p-type photodiode  112  as a photo-sensitive element. The photodiode  112  in each pixel  110  corresponds to an image pixel, i.e., a fraction of the image to be sensed. More than one photodiode may also correspond to the same image pixel. 
     The cathode  114  of the p-type photodiode  112  is connected to a supply voltage denoted as Vdd. Vdd can be, for example, 3V or 5V. The anode  116  of the p-type photodiode  112  is connected to a charge-sharing line  102 . More generally, as illustrated in the pixel array  100  of  FIG. 1 , the charge-sharing line  102  is connected to the anode  116  of each p-type photodiode  112  in each photo-sensitive pixel  110 . 
     The anode  116  is also connected to a terminal  120  of a reset switch  118 . The reset switch  118  also includes a control terminal  122 , and a terminal  124  connected to a reference voltage denoted as Vmid. Typically, the reference voltage Vmid is less than the supply voltage Vdd. For example, if Vdd is 5V, Vmid can be 3.5V or 2.5V, and if Vdd is 3V, Vmid can be 1.5V or 1.2V. The control terminal  122  of the reset switch  118  in each pixel  110  is connected to a signal denoted as RESET−. Although the reset switch  118  of the pixel  110  is depicted as a p-type metal oxide semiconductor field-effect transistor (p-MOSFET), other types of switches including n-type MOSFETs (n-MOSFETs), transistors, and other non-semiconductor switches may also be used. 
     The pixel  140  includes an n-type photodiode  142  that can operate as a photo-sensitive element. The cathode  146  of the n-type photodiode  142  in each pixel  140  is connected to the charge sharing line  102 . The anode  144  of the n-type photodiode is connected to a certain reference voltage, e.g., ground voltage or 0V, typically denoted as GND. 
     The cathode  146  is also connected to a terminal  150  of a reset switch  148 . In a similar fashion to the reset switch  118  in the pixel  110 , the reset switch  148  includes a control terminal  152 , and a terminal  154  connected to the reference voltage Vmid. It should be understood, however, that although  FIG. 1  shows the reference voltage Vmid connected to all pixels  110 ,  140 , the reset switches of different pixels may in fact be connected to different reference voltages (e.g., Vmid1, Vmid2, etc.), having different values, such as 2.5V and 3V. The control terminal  152  of the reset switch  148  in each pixel  140  is connected to a signal denoted as RESET+. The reset switch  148  of the pixel  140  is depicted as an n-MOSFET, but, again, other types of switches such as p-MOSFETs, transistors, and other non-semiconductor switches may also be used. 
     The charge-sharing line  102  is connected to a GATE terminal  162  of a driver  160 . The driver  160  is configured as a source follower, i.e., it produces an output at a DRAIN terminal  164  that is proportional to the voltage at the GATE terminal  162 . The voltage at the GATE terminal  162  is determined by the charge accumulated on the charge-sharing line  102 . A SOURCE terminal  166  of the driver  160  is connected to a supply voltage, e.g., Vdd. 
     The DRAIN terminal  164  of the driver  160  is connected to a select switch  170  having a SELECT input  172 . When the SELECT input  172  is activated, the select switch  170  turns “on” and a signal (e.g., a voltage signal) at the DRAIN terminal  164  is substantially reproduced at the output terminal  174  of the select switch  170 . The embodiment of the pixel array  100  depicted in  FIG. 1  employs n-MOSFETS as the driver  160  and select switch  170 . This, however, is for illustration only, and other types of devices such as p-MOSFETs, transistors, and other non-semiconductor devices may also be used as the driver  160  and/or select switch  170 . Moreover, the device used for the driver  160  need not be of the same type as the device used for the select switch  170 . 
     To perform image sensing, the pixel array  100  is exposed to a view to be imaged, such as a view from a camera. The reset switches  118 ,  148  are turned on substantially simultaneously by activating the respective select signals RESET− and RESET+. The signal RESET− is activated by setting it to a voltage that is substantially less than the voltage at the terminal  124 , i.e., Vmid. For example, the signal RESET− may be set to 0V. The signal RESET+ is activated by setting it to a voltage substantially the same as the voltage Vmid at the terminal  154 , e.g., 3.5V. 
     The reset signals are held active for a predetermined reset duration, on the order of a few nanoseconds. During this time, the photodiodes  112 ,  142  are “reset,” i.e., the voltages at the anode  116  of the photodiode  112  and at the cathode  146  of the photodiode  142  are set to be approximately equal to Vmid. Accordingly, the voltage on the charge-sharing line  102  is also approximately equal to Vmid. After the reset duration, the reset switches  118 ,  148  are turned off substantially simultaneously by deactivating the respective select signals RESET− and RESET+. 
     Each photodiode  112 ,  142  that is exposed to a portion of the view to be imaged (i.e., an image pixel) accumulates a charge proportional to the intensity of the light incident upon the photodiode from the image pixel. In particular, the n-type photodiodes  142  accumulate and discharge electrons at the cathode  146 . Because the number of electrons discharged at each cathode  146  is proportional to the intensity of the light from the associated image pixel, the number of electrons (i.e., the accumulated charge) at each cathode  146  represents the value of the associated image pixel. As all cathodes  146  are connected to the charge-sharing line  102 , the electrons from all cathodes  146  are accumulated, i.e., “added” on the charge-sharing line  102 . Thus, the charge on the charge-sharing line  102  due to the accumulated electrons is related to the sum of the image-pixel values associated with the photodiodes  142 . 
     The p-type photodiodes  112  accumulate and discharge holes at the anode  116 . In a similar fashion to the case of the n-type photodiodes  142 , the number of holes discharged at the anode  116  of each p-type photodiode  112  is proportional to the intensity of the light from the image pixel associated with the photodiode  112 . Accordingly, the number of holes (i.e., the accumulated charge) at each anode  116  represents the value of the associated image pixel. All anodes  116  are also connected to the charge-sharing line  102 , so that the holes from all anodes  116  are accumulated, i.e., “added” on the charge-sharing line  102 . Thus, the charge on the charge-sharing line  102  due to the accumulated holes is related to the sum of the image-pixel values associated with the photodiodes  112 . 
     In the pixel array  100 , all anodes  116  and cathodes  146  are connected to the charge-sharing line  102 . As a result, the holes collectively discharged at all anodes  116  and the electrons collectively discharged at all cathodes  146  combine on the charge-sharing line  102 . An electron-hole combination nullifies the charges associated with each individual electron and hole in the pair and, as a result, the charge on the charge-sharing line  102  can be expressed as the difference between the sum of charges due to all holes and the sum of charges due to all electrons. In effect, the charge on the charge-sharing line  102  represents the sum of pixel values associated with the p-type photodiodes  142  minus the sum of pixel values associated with the n-type photodiodes  112 . Thus, the net charge on the charge-sharing line  102  corresponds to a pixel value obtained by adding the pixel values associated with the p-type photodiodes  142  and subtracting therefrom the pixel values associated with each n-type photodiode  112 . 
     If more electrons were discharged at the cathodes  146  than holes discharged at the anodes  116 , the net charge on the charge-sharing line  102  would be negative, causing the voltage on the charge-sharing line  102  to decrease from the reset value Vmid. Conversely, if more holes than electrons were discharged, the net charge would be positive, causing the voltage on the charge-sharing line  102  to increase from Vmid. The net voltage on the charge-sharing line  102 , denoted as Vpix, is also present at the GATE  162  of the driver  160 . Therefore, the voltage at the DRAIN terminal  164  changes according to Vpix. 
     After a pre-determined duration of accumulating the above-described charges, called the integration time (typically on the order of a few milliseconds), the select switch  170  is activated by activating the SELECT input  172 . Then, an output signal, denoted as Vout, is produced at the output terminal  174  having a voltage substantially the same as the voltage at the DRAIN terminal  164 . As the voltage at the DRAIN terminal  164  is related to Vpix, which represents the addition and subtraction of various image pixels, the signal Vout also represents addition and subtraction of the image pixels. 
     Advantageously, in the pixel array  100 , the value of an individual image pixel is merely represented as a charge at an anode  116  or at a cathode  146 . Individual pixel values are not digitized, and are not sent to a processor for analog-to-digital conversion. This substantially decreases the required processor capacity and the number of interconnects that would otherwise be required to deliver the charges sensed at each pixel  110 ,  140  to the processor for the analog-to-digital conversion. Moreover, a digitized value corresponding to each individual image pixel is not stored in a memory cell. Only the signal at the output  174  (i.e., Vout) is digitized and stored in memory. Thus, the size of the memory that is required to store the compressively sampled signals is substantially smaller than the memory that would otherwise be required to store each pixel value individually. 
     As mentioned, in the pixel array  100 , circuitry including the driver  160  and the switch  170  produces a voltage at the output terminal  174  that is related to the voltage Vpix at the GATE terminal  162 , but, in doing so, the circuitry generally introduces some noise (denoted as Vnoise). In addition, the voltage at the terminal  174  may also be offset by a threshold voltage Vth, a parameter associated with the driver  160  and selector-switch  170  circuitry. Typically, the noise and the threshold voltage are not known and, hence, the value corresponding only to the addition and/or subtraction of pixels cannot easily be extracted from the output signal Vout at the terminal  174 . 
     Often, the noise and the threshold voltage offset values are not substantial, and do not significantly affect the accuracy of the added and/or subtracted pixel values obtained from the output signal Vout. In these cases, the pixel array  100  works just fine. In some very high-resolution image-sensing applications, however, it is desirable to mitigate or eliminate the effect of the noise and the threshold voltage. Some methods, such as obtaining a pixel value from the output signal Vout after resetting the photodiodes  112 ,  142 , storing that value in a temporary memory, reading a pixel value corresponding to Vpix, as described above, and subtracting from this pixel value the value stored in the temporary memory can mitigate the effects of noise and threshold-voltage offset. The temporary memory storage and the subtraction operation, however, can significantly increase the system size and/or power consumption. The pixel array  200  illustrated in  FIG. 2  can be employed, however, to avoid or mitigate the noise and threshold-voltage offset effects without substantially increasing the system size, power consumption, or cost. 
     In the pixel array  200 , the pixel  210  includes a transfer switch  280  having a transfer control input  282 . The anode  116  of the p-type photodiode  112  is connected to a terminal  284  of the transfer switch  280 . A terminal  286  of the transfer switch  280  is connected to the charge-sharing line  102  and to the terminal  120  of the reset switch  118 . The transfer control input  282  of each transfer switch  280  in each pixel  210  is connected to a signal TG+. The transfer switch  280  is illustrated as being a p-MOSFET, but it may also be another type of field-effect transistor (FET), transistor, or a non-semiconductor switch. 
     The pixel  240  includes a transfer switch  290  having a transfer control input  292 . The cathode  146  of the n-type photodiode  142  is connected to a terminal  294  of the transfer switch  290 . A terminal  296  of the transfer switch  290  is connected to the charge-sharing line  102  and to the terminal  150  of the reset switch  148 . The transfer control input  292  of each transfer switch  290  in each pixel  240  is connected to a signal TG−. The transfer switch  290  is illustrated as being an n-MOSFET, but it may also be another type of FET, transistor, or a non-semiconductor switch. In the pixel array  200 , one or more of the p-type photodiode  112  and/or one or more of the n-type photodiode  142  can be pinned photodiodes. Moreover, the pixels  210 ,  240  can be back-illuminated pixels and/or photogate pixels. 
     In operation of the pixel array  200 , both the reset switch  118  and the transfer switch  280  in pixel  210  are turned on substantially simultaneously during the reset period. The transfer switch  280  is turned on by settling the signal TG+ to a voltage less than Vmid (e.g., 0 v). After resetting the photodiode  112 , the reset switch  118  and the transfer switch  280  are turned off substantially simultaneously. The p-type photodiode  112  then accumulates charge according to the intensity of the light received from the image pixel associated with the photodiode  112 . Accordingly, the photodiode  112  discharges holes at the anode  116 , as described above. But, in this case, because the transfer switch  280  is turned off, these holes are not transferred to the nodes  286 ,  120  (collectively referred to as a floating-diffusion node) or to the charge-sharing line  102 . The reset switch  118  is then turned on a second time, setting the voltage at the nodes  286 ,  120  substantially equal to Vmid, and is thereafter again turned off. 
     The pixel  240  is operated in a similar fashion, where both the transfer switch  290  and the reset switch  148  are turned on substantially simultaneously, thereby resetting the n-type photodiode  142 , and then off substantially simultaneously so that the electrons discharged at the cathode  146  are not transferred to the nodes  296 ,  150 . The reset switch  148  is then turned on a second time (substantially at the same time at which the reset switch  118  is turned on a second time), setting the voltage at the nodes  296 ,  150  substantially to Vmid, and is thereafter again turned off. 
     Once the voltage at nodes  286 ,  120  and  296 ,  150  are set substantially equal to Vmid, the select switch  172  is turned on. A first reading of the signal at the terminal  174 , denoted as Vout_ref, is then related to Vmid. In particular, Vout_ref can be expressed as Vmid+Vth+Vnoise. 
     Subsequently, the transfer switches  280 ,  290  are turned on. As a result, the electrons accumulated at cathodes  146  are transferred to the charge-sharing line  102  through switches  290 , and the holes accumulated at anodes  116  are also transferred to the charge-sharing line  102  through switches  280 . As described above with reference to  FIG. 1 , the electrons and holes combine on the charge-sharing line  102  so as to represent a charge corresponding to a pixel value obtained by adding the individual pixel values associated with the p-type photodiodes  142  and subtracting therefrom the individual pixel values associated with each n-type photodiode  112 . 
     Thus, the voltage on the charge-sharing line  102  after turning on the transfer switches  280 ,  290  corresponds to Vpix, as described above. A second reading of the signal at the terminal  174 , denoted as Vout_pix, is related to Vpix, and may be expressed as Vpix+Vmid+Vth+Vnoise. The difference between Vout_pix and Vout_ref substantially eliminates the effect of Vth and Vnoise, i.e., the effect of the unknown threshold voltage and noise. Accordingly, the difference between the two readings of the output signal at the terminal  174  virtually yields only Vpix, which corresponds to the addition and/or subtraction of individual pixels. 
     Although in  FIGS. 1 and 2  all p-type photodiodes  112  share common control inputs RESET− and/or TG+, and all n-type photodiodes  142  share common control inputs RESET+ and/or TG−, this is for illustrative purposes only. In other embodiments, only a subset of the p-type photodiodes  112  share a common reset control, or a common transfer control, or both. Similarly, only a subset of the n-type photodiodes  142  may share a common reset control, or a common transfer control, or both. In yet other embodiments, each photodiode may have a distinct pair of reset and transfer control inputs. 
     In a pixel array  300 , depicted in  FIG. 3 , the pixels  310 ,  340  comprise p-type photodiodes  312  and n-type photodiodes  342 , respectively, and each pixel  310 ,  340  shares a common reset switch  318 . In this embodiment, the photodiodes  312 ,  342  may not be reset individually, but because fewer switches (e.g., transistors) are required compared to the pixel array  100  (shown in  FIG. 1 ), the pixel array  300  can be smaller than the pixel array  100 . Similarly, in the pixel array  400  shown in  FIG. 4 , the pixels  410 ,  440  share a common reset switch  418 , and, hence, the photodiodes included therein may not be reset individually. But, the pixel array  400  can be smaller than the pixel array  200  shown in  FIG. 2  due to the use of fewer switches. 
     In another embodiment according to the invention, a pixel array includes only one type of photodiode (e.g., only p-type photodiodes or only n-type photodiodes). One set of pixels, designated as “A” pixels, shares one pair of reset and transfer control signals, namely R1 and T1. Another set of pixels, designated as “B” pixels, shares another pair of reset and transfer control signals, namely R2 and T2. All pixels “A” and “B,” however, are connected to the same charge-sharing line and driver/selector circuitry. Initially, the “A” and “B” pixels are reset by activating the R1, T1, R2, and T2 signals, as described above with reference to  FIG. 2 , and a signal Vout_ref is measured. Then, R1, R2, T1, and T2 are deactivated, and, after the integration time, only T1 is activated. A value corresponding to the sum of all “A” pixels, denoted as Vout_Apix, is measured. Soon thereafter, T2 is activated, allowing the charges accumulated by the “B” pixels to be combined with the charge accumulated by all “A” pixels, which is already present on the charge-sharing line. Accordingly, a value corresponding to the sum of all “A” and “B” pixels, denoted as Vout_A+Bpix, is measured. By digitizing Vout_Apix and Vout_A+Bpix, and by subtracting the digitized value of Vout_Apix from Vout_A+Bpix, a value corresponding to the sum of all “B” pixels can be obtained. From these values, a value corresponding to the sum of all “A” pixels minus the sum of all “B” pixels can also be computed. 
     In an alternative embodiment, after measuring Vout_Apix, as described above, the voltage at the charge-sharing line that is common to the “A” and “B” pixels is reset, e.g., to Vmid, by activating and deactivating the R1 and R2 signals. Then T2 is activated as before, but now only the charges accumulated by the “B” pixels are combined at the charge-sharing line. Accordingly, a value corresponding to the sum of all “B” pixels, denoted as Vout_Bpix, is measured. By digitizing Vout_Apix and Vout_Bpix, a value corresponding to the sum of all “A” pixels minus the sum of all “B” pixels and/or the sum of all “B” pixels minus the sum of all “A” pixels can be computed. The photodiodes corresponding to the “A” and “B” pixels can be of the same type or can be of different types, i.e., the “A” pixels may correspond to p-type or n-type photodiodes, and, accordingly, the “B” pixels may correspond to n-type or p-type photodiodes. 
     In yet another embodiment according to the invention, a pixel array includes more than one charge-sharing line, where each line is connected to a different set of pixels in the array. Each charge-sharing line may also be associated with a driver/selector circuitry such as that including the driver  160  and the select switch  170  shown in  FIGS. 1 and 2 . Accordingly, the pixels in the set associated with each charge-sharing line may be added and/or subtracted, and a signal representing the sampled (i.e., processed) pixel value for the associated set may be output from the driver/selector circuitry. These signals can be digitized, and additional arithmetic operations can be performed on the digitized values resulting in additions and/or subtractions, or other functions, of the sums of pixel values from each set. 
     In general, a pixel array or focal plane can be configured as an M×N matrix where N is the number of charge-sharing lines and output signals, and M is the number of pixels in a set associated with each charge-sharing line. For example, a 2×2 pixel array includes two charge-sharing lines C1, C2, two pixels p11, p12 connected to line C1, and two pixels p21, p22 connected to line C2. All four pixels may be of the same type (i.e., accumulating electrons, or accumulating holes), or p11, p12 may be of one type and p21, p22 may be of a different type. Alternatively, p11, p21 may be of one type and p12, p22 may be of a different type. Other configurations of pixel arrays, e.g., 4×2, 4×4, 8×2, 8×4, 8×8, etc., are also within the scope of the invention. The numbers M and N can be selected to be substantially larger than 8 (e.g., 64 or larger) so as to meet the requirements of a suitable sampling function. 
     While the invention has been particularly shown and described with reference to specific embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced.