Abstract:
According to one aspect, embodiments herein provide a CCD sensor comprising a pixel array having a first segment configured to produce a first tap signal responsive to receipt of electromagnetic radiation from a scene to be imaged, a second segment configured to produce a second tap signal responsive to receipt of the electromagnetic radiation from the scene, a region of interest including a portion of the first segment adjacent the second segment and a portion of the second segment adjacent the first segment, and a processor configured to receive the first and second tap signals, perform level correction on one of the first and second tap signals based on magnitudes of the first and second tap signals, and perform gain correction on one of the first and second tap signals based on a comparison between magnitudes of the first and second tap signals corresponding to the region of interest.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 61/770,505, entitled “METHOD AND APPARATUS FOR GAIN AND LEVEL CORRECTION OF BAYER COLOR MULTI-TAP CCD CAMERAS,” filed Feb. 28, 2013, which is hereby incorporated by reference in its entirety for all purposes. 
     
    
     FEDERALLY SPONSORED RESEARCH 
       [0002]    This invention was made with U.S. government support under a grant awarded by the Department of Defense (Grant No. Withheld). The U.S. government has certain rights in this invention. 
       BACKGROUND 
       [0003]    Charge coupled device (CCD) cameras are commonly used in digital imaging systems. Multi-tap CCD cameras split an image frame into two or more areas that are read out from the CDD camera in parallel. For example, an image frame of a multi-tap CCD camera may be divided into left and right halves, and the pixels in each half are read out from each half in parallel electrical paths. These multi-tap CCD cameras exhibit inherent response errors between the readouts of the different paths. For example, PVT (process, voltage, and/or temperature) variations in each path may produce errors. As a result, there may be differences in the gain and/or black levels of each group of individually read out pixels from each path. 
         [0004]    As a result of poor tap/channel matching between paths of the multi-tap CCD camera, the difference in the pixels read out from each path may result in a “split screen” visual effect when the pixels from each path are combined into a single image. In an image exhibiting a “split screen” visual effect, a relatively clean line of demarcation exists in the image between pixels read out from each half of the image frame, as a result of the differences in the pixels read out from each path. Digital image processing also typically performs contrast-enhancing and/or peaking operations on the image frame which may increase the error(s) between pixels from each path. 
         [0005]    Conventionally, errors between parallel readouts in multi-tap CCD cameras are addressed though single-point or multi-point factory calibration performed by the camera vendor. For example, one approach has been to attempt to minimize tap-based errors by performing a simple level calibration, assuming a generally-well balanced camera as an initial condition. Some factory calibration approaches manipulate digital controls of the analog to digital converters associated with the pixels. For example, U.S. Pat. No. 7,236,199 (hereinafter the &#39;199 patent) discloses performing automated tap balance within a CCD camera by controlling the analog to digital converter dynamically. In this method, gain resolution is limited by the control resolution and PVT variance of the analog to digital converter, and as a result, the minimum gain error is at least several digital counts. In addition, the method disclosed in the &#39;199 patent requires the CCD sensor to be shrouded to perform level calibration. 
       SUMMARY OF INVENTION 
       [0006]    Aspects and embodiments described herein are directed to methods and apparatus for providing tap balance, also referred to as tap/channel matching, in a multi-readout (or multi-tap) CCD sensor. According to certain embodiments, level (offset) and gain errors between readouts may be reduced to one digital count or less. This also addresses the undesirable “split screen” visual effect that can arise from poor tap/channel matching. 
         [0007]    For example, aspects and embodiments described herein provide “one shot” level correction and a continuous PID (proportional-integral-derivative) gain correction control loop. 
         [0008]    Through the control loop, the gain error may be accurately tracked with PVT variances. As discussed further below, in one embodiment, a method of channel matching includes measuring gradients at a tap seam within a predefined region, and producing a gain correction term based on the gradients. The method may also include filtering the scene data to address scene-based gradients that can contaminate the gain error, thereby achieving a more accurate gradient term. The method may also include selectively enabling gain correction only when scene motion is large enough to ensure new samples within the predefined region. 
         [0009]    Embodiments of the methods discussed herein may achieve tighter gain matching than is possible using conventional analog to digital controls (such as disclosed in the &#39;199 patent, for example, as discussed above) alone by providing higher resolution gain control. Additionally, unlike conventional methods such as disclosed in the &#39;199 patent, embodiments of the methods disclosed herein do not require the CCD sensor to be masked or shrouded to perform black level correction. Furthermore, embodiments of the methods disclosed herein may be used on CCD sensors with color filter arrays as well as on monochrome sensors. 
         [0010]    Aspects in accord with the present invention are direct to a Charge Coupled Device (CCD) sensor for capturing scene imagery, the CCD sensor comprising a pixel array comprising a first segment including a first plurality of pixels, the first segment configured to produce a first tap signal responsive to receipt of electromagnetic radiation from a scene to be imaged, a second segment including a second plurality of pixels, the second segment configured to produce a second tap signal responsive to receipt of the electromagnetic radiation from the scene, a region of interest including a portion of the first segment adjacent the second segment and a portion of the second segment adjacent the first segment, and a processor coupled to the first and second segments, wherein the processor is configured to receive the first tap signal from the first segment and the second tap signal from the second segment, perform level correction on one of the first tap signal and the second tap signal based on a magnitude of the first tap signal and a magnitude of the second tap signal, and perform gain correction on one of the first tap signal and the second tap signal based on a comparison between a magnitude of the first tap signal corresponding to the region of interest and a magnitude of the second tap signal corresponding to the region of interest. 
         [0011]    According to one embodiment, the first segment includes a first reference region, the second segment includes a second reference region, and the processor is further configured to receive at least one first reference tap signal from the first reference region, to receive at least one second reference tap signal from the second reference region, and to perform the level correction of the one of the first tap signal and the second tap signal based on a comparison between a magnitude of the at least one first reference tap signal and a magnitude of the at least one second reference tap signal. 
         [0012]    According to another embodiment, in performing the level correction of the one of the first tap signal and the second tap signal the processor is further configured to sum the magnitude of each first reference tap signal from the first reference region, sum the magnitude of each second reference tap signal from the second reference region, calculate a difference between the summed first reference tap signal magnitudes and the summed second reference tap signal magnitudes, and add the difference to the magnitude of the one of the first tap signal and the second tap signal. 
         [0013]    According to one embodiment, in performing the gain correction of the one of the first tap signal and the second tap signal the processor is configured to select pixel pairs, each pair including a pixel from the first segment within the region of interest and a pixel from the second segment within the region of interest, calculate a tap signal magnitude gradient for each pixel pair, and sum at least a portion of the tap signal magnitude gradients over the region of interest to generate an error term. 
         [0014]    According to another embodiment, the processor includes a Proportional-Integral Derivative (PID) controller configured to generate a gain correction term based on the error term, and wherein the processor is further configured to adjust a gain of the one of the first tap signal and the second tap signal based on the gain correction term. In one embodiment, the processor is further configured to compare each tap signal magnitude gradient of each pixel pair to a gradient threshold, and to only sum tap signal magnitude gradients less than the gradient threshold to generate the error term. In another embodiment, the pixel array is color filtered, and wherein the processor is further configured to select the pixel pairs from pixels of the same color in the first and second segments. In one embodiment, the pixel array is Bayer color filtered. 
         [0015]    According to one embodiment, the processor further includes a motion input line configured to be coupled to a motion system, and wherein the processor is further configured to receive motion signals from the motion system via the motion input line including information regarding motion of the scene imagery, and to perform gain correction of the one of the first tap signal and the second tap signal only when the motion signals indicate that the scene imagery has moved by more than a scene motion threshold. 
         [0016]    One aspect in accord with the present invention is directed to a method of providing tap balance in a multi-tap CCD camera, the CCD camera comprising a pixel array including a first segment including a first plurality of pixels, a second segment including a second plurality of pixels, and a region of interest including a portion of the first segment adjacent the second segment and a portion of the second segment adjacent the first segment, the method comprising receiving a readout from the first segment including a first tap signal representative of electromagnetic radiation received by the first segment from a scene to be imaged, receiving a readout from the second segment including a second tap signal representative of electromagnetic radiation received by the second segment from the scene, adjusting a magnitude of the first tap signal based on a magnitude of the second tap signal, and adjusting a gain applied to one of the first tap signal and the second tap signal based on a comparison between a magnitude of the first tap signal corresponding to the region of interest and a magnitude of the second tap signal corresponding to the region of interest. 
         [0017]    According to one embodiment, receiving a readout from the first segment includes receiving at least one first reference tap signal, receiving a readout from the second segment includes receiving at least one second reference tap signal, and adjusting the magnitude of the first tap signal includes adjusting the magnitude of the first tap signal based on a comparison between a magnitude of the at least one first reference tap signal and a magnitude of the at least one second reference tap signal. 
         [0018]    According to another embodiment, adjusting the magnitude of the first tap signal further includes summing the magnitude of each first reference tap signal, summing the magnitude of each second reference tap signal, calculating a level difference between the summed first reference tap signal magnitudes and the summed second reference tap signal magnitudes, and adding the level difference to the magnitude of the first tap signal. In one embodiment, adjusting the gain applied to one of the first tap signal and the second tap signal includes selecting pixel pairs, each pair including a pixel from the first segment within the region of interest and a pixel from the second segment within the region of interest, calculating a difference in magnitude between tap signals of each pixel pair over the region of interest, and summing at least a portion of the differences in magnitude between tap signals of each pixel pair over the region of interest to generate an error term. 
         [0019]    According to one embodiment, the method further comprises utilizing a PID control loop to generate a gain correction term based on the error term, and wherein adjusting the gain applied to one of the first tap signal and the second tap signal includes adjusting the gain based on the gain correction term. In one embodiment, utilizing a PID control loop to generate the gain correction term comprises integrating the error term over a prior period of time, applying an integral gain to the integrated error term to generate an integration control term, applying a proportional gain to the error term to generate a proportional control term, deriving a rate of change of the error term over time, applying a derivative gain to the rate of change to generate a derivative control term, and summing the integration control term, the proportional control term, and the derivative control term to generate a PID control term. In another embodiment, utilizing a PID control loop to generate the gain correction term further comprises adding the PID control term to a reference gain term to generate the gain correction term. 
         [0020]    According to another embodiment, the method further comprises comparing the difference in magnitude between tap signals of each pixel pair to a gradient threshold, wherein summing at least a portion of the differences in magnitude between tap signals over the region of interest to generate an error term includes only summing differences in magnitude less than the gradient threshold. In another embodiment, selecting pixel pairs includes selecting pixel pairs from pixels of the same color in the first segment and the second segment. 
         [0021]    According to one embodiment, the method further comprises monitoring motion of scene imagery captured by the multi-tap CCD camera, and adjusting the gain applied to one of the first tap signal and the second tap signal only when the scene imagery has moved by more than a scene motion threshold. 
         [0022]    Another aspect in accord with the present invention is directed to a CCD camera, the CCD camera comprising a pixel array comprising a first segment including a first plurality of pixels, the first segment configured to produce a first tap signal responsive to receipt of electromagnetic radiation from a scene to be imaged, and a second segment including a second plurality of pixels, the second segment configured to produce a second tap signal responsive to receipt of the electromagnetic radiation from the scene, a processor coupled to the first and second segments and configured to receive a first readout from the first segment including the first tap signal, to receive a second readout from the second segment including the second tap signal, and to combine the first tap signals and the second tap signals, and means for reducing level and gain errors between the first readout and the second readout to one digital count or less. 
         [0023]    Still other aspects, embodiments, and advantages of these exemplary aspects and embodiments, are discussed in detail below. Moreover, it is to be understood that both the foregoing information and the following detailed description are merely illustrative examples of various aspects and embodiments, and are intended to provide an overview or framework for understanding the nature and character of the claimed aspects and embodiments. Any embodiment disclosed herein may be combined with any other embodiment in any manner consistent with at least one of the objectives, aims, and needs disclosed herein, and references to “an embodiment,” “some embodiments,” “an alternate embodiment,” “various embodiments,” “one embodiment” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment. The appearances of such terms herein are not necessarily all referring to the same embodiment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    Various aspects of at least one embodiment are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of the invention. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. In the figures: 
           [0025]      FIG. 1  is a diagram of one example of a two-tap readout CCD sensor with a gain correction region of interest according to aspects of the invention; 
           [0026]      FIG. 2  is a block diagram illustrating one example of a two-tap readout CCD sensor level and gain calibration process according to aspects of the invention; and 
           [0027]      FIG. 3  is a diagram of one example of a color-filtered pixel array of a two-tap readout CCD sensor according to aspects of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    As described above, poor tap/channel matching between paths of a multi-tap CCD sensor may result in a “split screen” visual effect in a resulting image generated by the multi-tap CCD sensor. As also discussed above, prior attempts so address this problem have resulted in solutions with gain errors of at least several digital counts. Accordingly, aspects and embodiments described herein provide methods and apparatus for reducing level (offset) and gain error between readouts of a multi-readout (or multi-tap) CCD sensor to one digital count or less. 
         [0029]    It is to be appreciated that embodiments of the methods and apparatuses discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and apparatuses are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms. 
         [0030]      FIG. 1  illustrates one embodiment of a two-tap readout CCD sensor  100 . The two-tap readout CCD sensor  100  includes an array of pixels  101  and a processor  112 . According to one embodiment, the array of pixels  101  is a 2048×2048 array of pixels; however, in other embodiments, the array of pixels  101  may be of any size. The array of pixels  101  includes a left segment of pixels  102  and a right segment of pixels  104 , each coupled to the processor  112  via an electrical path  114  (e.g., a wire). According to one embodiment, each segment of pixels  102 ,  104  includes a 2048×1024 array of pixels; however, in other embodiments, the segments may include pixel arrays of any size. Each segment of pixels  102 ,  104  also includes a masked reference region  106 . According to one embodiment, each masked reference region  106  includes 2048 rows of pixels, each row including 20 pixels; however, in other embodiments, the masked reference regions  106  may be of any size and may be configured differently. According to one embodiment, the masked reference region  106  of each segment of pixels  102 ,  104  is located on a left or right side edge of the corresponding segment  102 ,  104 ; however, in other embodiments, the reference region  106  may be located on a different portion of the segment  102 ,  104  (e.g., on a top or bottom edge) of pixels. 
         [0031]    As light impinges on the pixel array  101 , each segment  102 ,  104  captures image data that represents the light incident on the pixels of the segment  102 ,  104 . Based on the captured image data, each segment  102 ,  104  generates tap signals representative of the light received by each segment  102 ,  104  at a given time. The tap signals are provided to the processor  112  via the paths  114 . For example, in one embodiment, photo diodes within the left segment  102  of the pixel array  101  generate left tap signals representative of light received by pixels of the left segment  102  and photo diodes within the right segment  104  of the pixel array  101  generate right tap signals representative of the light received by pixels of the right segment  104 . The left and right tap signals from each segment  102 ,  104  of the pixel array  101  are provided to the processor  112  via separate paths  114  and represent the light received by each segment  102 ,  104  of the pixel array  101  at a given time. Black reference tap signals generated from the masked reference region  106  of each segment  102 ,  104  are also provided to the processor  112 . 
         [0032]    According to one embodiment, the CCD sensor  100  also includes at least one Analog to Digital (A/D) converter  115  coupled between the processor  112  and each segment of pixels  102 ,  104 . The A/D converter(s)  115  receives analog tap signals from a segment of pixels  102 ,  104 , converts the analog tap signals into digital tap signals, and provides the digital tap signals to the processor  112  for processing. 
         [0033]    A gain correction Region Of Interest (ROI)  110  is defined in the CCD sensor  100  to include a portion of the left segment of pixels  102  adjacent the right segment  104  and a portion of the right segment of pixels  104  adjacent the left segment  102 . In one embodiment, the ROI  110  is two pixel columns wide. In another embodiment, the ROI  110  is four pixel columns wide. In other embodiment, the ROI  110  may be defined as any pixel column width. 
         [0034]    The processor operates on the tap signals (including the tap signals corresponding to the ROI  110  pixels and the black reference tap signals corresponding to the masked regions  106 ) received from each electrical path  114  (i.e., from each segment of pixels  102 ,  104 ) to generate a single image from the combination of the tap signals. The image may be provided to a display, memory, or some other external system. In generating the image, the processor  112  analyzes the received tap signals and calibrates the tap signals to reduce level (offset) and gain error between the tap signals of each segment  102 ,  104 . 
         [0035]    For example,  FIG. 2  is a block diagram illustrating one example of a two-tap readout CCD sensor level and gain calibration process  200  which may be implemented by the processor  112 . As shown in  FIG. 2 , the process  200  includes a two-stage control loop that actively performs a level correction stage  202  and then a gain correction stage  204 . In the level correction stage  202 , the reference tap signals corresponding to the masked reference regions  106  of each segment  102 ,  104  are compared to determine the level difference between the reference tap signals of the left segment  102  and the reference tap signals of the right segment  104 . The reference tap signal levels of the segment  102  or  104  which generated the lower level reference tap signals are increased to match the reference tap signal levels of the other segment (i.e., the segment with the higher level reference tap signals). By matching the reference tap signal levels of the two segments, the inherent difference in reference tap signal levels (as seen in the level difference between the black reference tap signals of each segment  102 ,  104 ) may be compensated for. 
         [0036]    For example, in the level correction stage  202 , reference tap signals generated by each segment  102 ,  104  for the same period of time are received by the processor  112  (as indicated in block  206 ). The processor  112  sums the magnitude of all of the black reference tap signals corresponding to the masked reference region  106  of the left segment  102  (as indicated in block  208 ). The processor  112  sums the magnitude of all of the black reference tap signals corresponding to the masked reference region  106  of the right segment  104  (as indicated in block  210 ). The processor  112  calculates the difference between the summed magnitude of the black reference tap signals corresponding to the masked reference region  106  of the left segment  102  and the summed magnitude of the black reference tap signals corresponding to the masked reference region  106  of the right segment  104  (as indicated in block  212 ). The calculated magnitude difference is added to the tap signals of the segment  102  or  104  which initially generated the lower level black reference tap signals to increase the tap signal levels and match them with the tap signal levels of the other segment (as indicated in block  214 ). The tap signals from each segment  102 ,  104  (including the level adjusted tap signals) are provided to the gain correction stage  204  (as indicated in block  216 ). 
         [0037]    As described above, the tap signal levels of the segment  102  or  104  which initially generated the lower level black reference tap signals are increased to match the tap signal levels of the other segment; however, in other embodiments, the tap signal levels of the segment  102  or  104  which initially generated the higher level black reference tap signals may be decreased to match the tap signal levels of the other segment. 
         [0038]    As also described above, the sensor  100  includes masked reference regions  106  that generate black reference tap signals. However, in another embodiment, the masked reference regions  106  of the segments  102 ,  104  may be removed and the sensor  100  may instead be shuttered to generate black reference tap signals from any portion of each segment of pixels  102 ,  104 . 
         [0039]    In the gain correction stage  204 , tap signals associated with the ROI  110  are analyzed and based on this analysis (described in greater detail below), the gain applied to tap signals of a segment  102 ,  104  by the processor  112  may be adjusted to reduce the gain error between the tap signals of the segments  102 ,  104 . For example, in one embodiment, the gain correction is implemented as a PID controller  218  which accepts as its input the absolute value of an error term between the two segments  102 ,  104 . This error term, referred to herein as the error gradient term, is generated by calculating the difference in magnitude (i.e., a gradient) between tap signals of pixel pairs (between the two segments  102 ,  104 ) and summing this magnitude over all pixel pairs within the ROI  110 . A multiplicative gain correction term is generated at the output of the PID controller, and then applied to subsequent frames of pixel information (i.e., tap signals) from a segment  102 ,  104  to adjust the gain applied to tap signals of the segment  102 ,  104  and reduce the gain error between the tap signals of the segments  102 ,  104 . The control loop may be run continuously, or may be stopped when the error term is less than a defined threshold. 
         [0040]    For instance, as shown in the gain correction stage  204  of  FIG. 2 , tap signals from each segment  102 ,  104  (including any level adjusted tap signals) are received from the level correction stage  202  (as indicated in block  222 ). The processor  112  analyzes tap signals associated with pixels within the ROI  110  and identifies pixel pairs between the first segment  102  and the second segment  104  in the ROI  110  (as indicated in block  224 ). According to one embodiment, each pixel pair includes one pixel chosen from the ROI  110  within the first segment  102  and a pixel chosen from the ROI  110  within the second segment  102 . The processor  112  calculates the difference in magnitude between the tap signals of each pixel pair and sums each difference across the ROI  110  (as indicated in block  226 ). The resulting sum of differences between each pixel pair is the error gradient term  220 . 
         [0041]    One aspect to be accounted for in determining the appropriate error gradient term  220  is filtering out scene information from sensor tap imbalance. Large-magnitude gradients occurring within the ROI  110  are capable of contaminating the resulting error gradient term  220 . For example, if an object is sensed in the right segment&#39;s  104  portion of the ROI  110  but is not sensed in the corresponding left segment&#39;s  102  portion of the ROI  110  (i.e., in only one pixel of a pixel pair), the difference in magnitude between the tap signals of the pixel pair would be relatively high due to the presence of the object in half the scene and not because of a difference in gain. This relatively large difference in magnitude would contribute to the calculation of the error gradient term  220  and would result in the error gradient term  220  being erroneously high. 
         [0042]    Therefore, according to certain embodiments, the processor operates on active scene data and rejects large gradient inputs within the ROI  110 . In one embodiment, by thresholding the error gradient term  220 , the effects of scene information may be filtered out from the error gradient term  220  calculation. For example, as indicated at block  226 , upon calculating the difference in magnitude (i.e., the gradient) between the tap signals of each pixel pair across the ROI  110 , the processor  112  also compares the difference in magnitude between the tap signals of each pixel pair to a gradient threshold  228 . If a difference in magnitude between the tap signals of a pixel pair is greater than the gradient threshold  228 , then the processor  112  will not utilize the difference in magnitude in the summing of the differences in magnitude. Accordingly, pixel pairs with corresponding tap signals having a magnitude difference greater than the gradient threshold do not contribute to the calculation of the error gradient term  220  that is fed to the PID controller  218 . This thresholding may limit the impact of scene-based gradients when calculating the error gradient term  220 . 
         [0043]    The error gradient term  220  is provided to the PID controller  218  (as indicated in block  230 ). The PID controller  218  integrates the error gradient term  220  over a prior period of time and at step  234  the integrated error gradient term is multiplied by a constant integral term coefficient (K_i)  235  (as indicated in block  232 ). The PID controller  218  also multiples the current error gradient term  220  by a constant proportional term coefficient (K_p) (as indicated in block  236 ). The PID controller  218  further derives the rate of change of the error gradient term  220  over time (as indicated in block  238 ). The rate of change is multiplied by a constant derivative term coefficient (K_d)  241  (as indicated in block  240 ). The integrated error gradient term multiplied by K_i  235 , the current error gradient term  220  multiplied by K_p  237 , and the rate of change of the error gradient term multiplied by K_d  241  are summed together to generate a PID control term (as indicated in block  242 ). The PID control term is combined with a reference (or nominal) gain term  245  to generate a gain correction term  246  (as indicated in block  244 ). 
         [0044]    The gain correction term  246  is applied to subsequent tap signals from a segment  102 ,  104  of the pixel array  101  to adjust the gain applied to tap signals of the segment  102 ,  104  and reduce the gain error between the tap signals of the segments  102 ,  104  (as indicated in block  248 ). According to one embodiment, the gain correction term  246  is applied to tap signals of a segment  102 ,  104  that currently has a gain which must be increased to match the gain of the other segment  102 ,  104 . In such an embodiment, the gain correction term increases the gain applied to tap signals of the lower gain segment  102 ,  104 . According to another embodiment, the gain correction term  246  is applied to tap signals of a segment  102 ,  104  that currently has a gain which must be decreased to match the gain of the other segment  102 ,  104 . In such an embodiment, the gain correction term decreases the gain applied to tap signals of the higher gain segment  102 ,  104 . 
         [0045]    According to one embodiment, the gain control stage  204  may run continuously to match the gains of the two segments  102 ,  104 . However, in another embodiment, the gain control stage  204  may be stopped when the error gradient term  220  is less than a defined minimum threshold. 
         [0046]    According to another embodiment, by using knowledge of system motion of the sensor  100 , the gain control stage  204  may be selectively enabled only when scene motion is large enough to ensure new samples within the ROI  110  in subsequent iterations of the control loop. 
         [0047]    For example, a situation may arise where a non-moving object (e.g., a corner of a building) is sensed in the right segment&#39;s  104  portion of the ROI  110  but not in the corresponding left segment&#39;s  102  portion of the ROI  110 . If the gain control stage  204  continuously attempts to perform its gain adjustment process with an error gradient term based on a large magnitude gradient resulting from the continuous sensing of the non-moving object in only one half of the ROI  110 , the resulting error gradient term would continue to be erroneously high and proper gain matching of the segments  102 ,  104  would not occur. 
         [0048]    Accordingly, if the gain control stage  204  is selectively enabled only when scene motion is large enough (i.e., above a scene motion threshold) to ensure new samples within the ROI  110 , such a situation may be avoided as the gain control stage  202  would only be enabled when adequate motion of the sensor  100  resulted in new samples within the ROI  110  and continuous attempts at gain calibration based on the same non-moving object would not occur. 
         [0049]    For example, in one embodiment, the processor  112  includes a motion control input  113  that is configured to be coupled to a motion control system that controls and/or senses the motion of the sensor  100 . For example, the motion control system may include a gyroscope, a servomechanism, a positioning system (e.g., a GPS), a motor, a pump, an actuator, or any other device or system common in a motion control system. 
         [0050]    According to one embodiment, the motion control system controls the motion of the sensor itself  100  (e.g., via an actuator, motor, servomechanism, etc) and provides information regarding the motion of the sensor  100  to the processor  112  via the motion control input  113 . In another embodiment, the motion control system merely monitors the motion of the sensor  100  (e.g., as the sensor  100  is moved by a user or the area to which the sensor  100  is attached (e.g., a vehicle) moves) and provides the motion information of the sensor  100  to the processor  112 . In one embodiment, the gain control stage  204  is only enabled when the processor  112  receives signals from a motion control system via the motion control input  113  that indicates that the scene imagery received by the sensor  100  (e.g. due to movement of the sensor  100  or the area to which the sensor  100  is attached) has moved an adequate amount (i.e., above a scene motion threshold such as 2 pixels). 
         [0051]    Many traditional CCD sensors operate with color filters in the optical path. For example, a pixelated Bayer (red, green, blue) filter array may be placed in front of the CCD sensor  100 . 
         [0052]    The Bayer color filter array (including multiple different colors) may further affect the calculation of the error gradient term  220 . Therefore, according to at least one embodiment, the processor  112  may account for the presence of a color filter in the error gradient term calculations of the gain control stage  204 . For example, in the gain control stage  204 , based on a color processing mode  251 , the processor  112  may be configured to only utilize pixels (and corresponding tap signals) of a certain color from the pixel array  101  in its error gradient term calculation (as indicated in block  250 ). For instance, according to one embodiment, in a green color processing mode  251 , only green pixels from the pixel array  101  are used by the processor  112  in the error gradient term  220  calculation. 
         [0053]    For example, in a Bayer filtered pixel array  300  (as seen in  FIG. 3 ), green pixels (each marked with an “X”) cover fifty percent of the array  300 . Therefore, for a ROI  110  having a width of two pixel columns, the green pixels  302  in adjacent rows may be used as the pixel pairs for the error gradient term  220  calculation. For a ROI  110  having a width of four pixel columns, the green pixels  304  of the same row may be used as the pixel pairs for the error gradient term  220  calculation. In other examples, pixels of colors other than green may be used, or different pixel pairs may be selected for the error gradient term  220  calculation. Additionally, although the process may be simplified by using only pixels of a single color, other embodiments may use all pixels or pixels of more than one color. 
         [0054]    As described above, the level and gain calibration process is utilized with a two-tap (i.e., segment) CCD sensor; however, in other embodiments, the CCD sensor may include any number of taps or segments. 
         [0055]    As also described above, the pixel array  101  is split into two half segments  102 ,  104 ; however, in other embodiments, the pixel array  101  may be divided in any other way and the segments  102 ,  104  may be defined in any other configuration. 
         [0056]    As discussed above, level (offset) and gain errors between readouts of a multi-tap CCD camera may be reduced to one digital count or less. By measuring gradients at a tap seam between pixel pairs of a region of interest and summing the gradients, an error gradient term may be generated. The error gradient term is fed to a PID controller to produce a gain correction term that is used to adjust gain applied to pixels of a tap and to reduce gain error between taps. In addition, by rejecting large gradient inputs within the region of interest and/or selectively enabling gain adjustment only when scene motion is large enough to ensure new samples within the region of interest, an error gradient term (and resulting gain correction term) may be achieved that results in gain errors of one digital count or less. 
         [0057]    Having described above several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.