Patent Publication Number: US-11665427-B2

Title: Still image stabilization/optical image stabilization synchronization in multi-camera image capture

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 15/696,583, filed Sep. 6, 2017, now allowed, which claims benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 62/383,992, filed Sep. 6, 2016, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to image processing techniques for use in camera systems and, in particular, multi-camera systems. 
     Multi-camera systems are finding application in a variety of imaging applications. For example, some multi-camera systems find use in stereoscopic imaging applications, which support 3D imaging. Other multi-camera systems find use for other imaging applications, where, for example, a first camera captures an image in a relatively wide field of view and a second camera captures the same image in a narrower, zoomed field of view. In either case, the images captured by the multi-camera systems typically are used by other processes. It is desirable that the images output by the multi-camera system have the highest image quality possible. 
     There is a need for a synchronized image capture and processing protocol to improve image quality in the output of multi-camera imaging systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a functional block diagram of an image capture system according to an embodiment of the present disclosure. 
         FIG.  2    illustrates an exemplary set of image data that may be generated by the system of  FIG.  1   . 
         FIG.  3    illustrates a method according to an embodiment of the present disclosure. 
         FIG.  4    illustrates application of the method of  FIG.  3    to an exemplary set of image data. 
         FIG.  5    illustrates a method according to an embodiment of the present disclosure. 
         FIG.  6    illustrates application of the method of  FIG.  5    to an exemplary set of image data. 
         FIG.  7    illustrates a fusion unit according to an embodiment of the present disclosure. 
         FIG.  8    illustrates an exemplary computer system suitable for use with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure provide techniques for managing image capture and processing in a multi-camera imaging system. In such a system, a pair of cameras each may output a sequence of frames representing captured image data. The cameras&#39; output may be synchronized to each other to cause synchronism in the image capture operations of the cameras. The system may assess image quality of frames output from the cameras and, based on the image quality, designate a pair of the frames to serve as a “reference frame pair.” Thus, one frame from the first camera and a paired frame from the second camera will be designated as the reference frame pair. The system may adjust each reference frame in the pair using other frames from their respective cameras. The reference frames also may be processed by other operations within the system, such as image fusion. 
       FIG.  1    is a functional block diagram of an image capture system  100  according to an embodiment of the present disclosure. The system  100  may include a pair of cameras  120 ,  130  and an image processor  140 . The cameras  120 ,  130  each may capture image data representing a local environment and output the captured image data to the image sensor  140  as a sequence of frames. The image sensor  140  may perform processing operations on the frames output from the cameras  120 ,  130 , discussed herein, to improve their image quality. 
     The cameras  120 ,  130  may have different image capture characteristics from each other by, for example, having different numbers of pixels, different zoom characteristics, and/or different focal lengths. For example, in one implementation, both cameras  120 ,  130  may have fixed optical systems (predetermined lens types, fixed focal lengths, etc.) that are different from each other. In another implementation, one (or both) of the cameras  120 ,  130  may accommodate different lenses, may operate according to variable focus positions, and/or may permit selection of variable optical filters during operation; again, the optical characteristics of the cameras  120 ,  130  may be different from each other, either in design or in selection of operational settings. 
     Image capture operations of the cameras  120 ,  130  also may operate according to variable operational settings, which may cause them either to be the same or different from each other. The cameras  120 ,  130  may include image sensors that convert incident light to electrical signals. The cameras&#39; image sensors may differ from each other, as noted, by the number of pixel circuits provided for each. The image sensors also may operate according to operational parameters that, for example, determine the exposure times of the images generated respectively by the sensors and other operational modes (pixel binning) that determine the number of pixel values to be generated by the cameras&#39; sensors. Again, in some operations, the operational settings for the cameras  120 ,  130  may be selected to be different from each other. 
     As indicated, the cameras  120 ,  130  may operate in a synchronized fashion. In one embodiment, synchronization may cause the cameras  120 ,  130  to generate frames at a predetermined frame rate (e.g., 60 frames/sec., 30 frames/sec.) notwithstanding operational settings differences between the cameras  120 ,  130 . In one embodiment, the cameras  120 ,  130  may be controlled so that the exposure operations for each frame from one camera (say, camera  120 ) that is to be paired with a frame from the other camera  130  at least partially overlap. For example, if a first camera  120  generates frames with exposure times that are longer than the exposure times for paired frames from the second camera  130 , it may be advantageous to control exposure timing to cause the exposure times of the second camera  130  to be contained entirely within the exposure times of the paired frames from the first camera  120 . It may be advantageous to control exposure timing to cause predetermined events in each camera&#39;s exposures (e.g., the starting times of exposure, midpoints of exposure, or end times of exposure) temporally coincide. The cameras  120 ,  130  may operate according to variable exposure times, which may render it impractical to cause predetermined events to coincide on every exposure; in such cases, it may be sufficient to cause the exposures of the two cameras  120 ,  130  at least to partially overlap for paired frames. 
     In some variations, the cameras  120 ,  130  may be mounted within a device  110  in a manner so the cameras have fields of view that are generally aligned with each other such that the fields of the first and second cameras at least partially overlap but, owing to their different characteristics, not identical to each other. 
     The image processor  140  may control certain operations of the cameras  120 ,  130 . For example, the image processor  140  may control timing of image capture operations by the cameras or it may control exposure times of pixels within the cameras  120 ,  130 . The image processor  140  also may control timing of read operations. Thus, the image sensor  140  may read frame data from the cameras  120 ,  130  at times that it dictates to the cameras  120 ,  130 . 
     In an embodiment, the cameras  120 ,  130  and image processor  140  may be provided within a processing device  110 , such as a smartphone, a tablet computer, a laptop computer, a desktop computer, a portable media player or the like. 
       FIG.  2    illustrates an exemplary set of image data that may be transferred from the cameras to the image processor. As illustrated, a first camera may output a first sequence of frames F 1 . 1 -F 1 .N to the processor and a second camera may output a second sequence of frames F 2 . 1 -F 2 .N. Image capture operations may be controlled to synchronize timing of images captured by the two cameras as closely as possible within limitations imposed by the cameras&#39; hardware and the image capture settings that the image processor selects. As noted, the cameras  120 ,  130  may operate according to exposure settings that may cause frames&#39; exposure times either to be the same or to be different between the cameras. 
     In the example of  FIG.  2   , the frames F 1 . 1 -F 1 .N output by the first camera are shown as being captured using a common pixel exposure setting E 1 .  FIG.  2    also illustrates that the frames F 2 . 1 -F 2 .N output by the second camera are captured using a variable exposure setting—frames F 2 . 1 -F 2 . 3  are captured using an exposure setting E 2  while frame F 2 .N is captured using an exposure setting E 3 . In this example, the image processor  140  may coordinate timing of the image capture settings so the pixel exposure operations in the two cameras at least partially overlap notwithstanding differences in the duration of the pixel exposure operations. 
     As illustrated in  FIG.  2   , the cameras  1  and  2  may output frames to the image sensor in paired relationships based on the timing of the frames&#39; capture. Frames F 1 . 1  and F 2 . 1  are paired together (shown as P 1 ), frames F 1 . 2  and F 2 . 2  are paired together as pair P 2 , etc., based on their exposure times 
     In an embodiment, an image processor may perform image-processing operations on frame data output from a pair of cameras to derive an aggregate image having improved image quality as compared to the images output from either of the cameras by themselves.  FIG.  3    illustrates a method  300  that may be performed by an image processor according to the embodiment. 
     The method  300  may assess image quality of frames output from the cameras (box  310 ) and, based on the image quality, designate a pair of the frames to serve as a “reference frame pair” (box  320 ). Thus, one frame from the first camera (say, frame F 1 . 2  in  FIG.  2   ) and a counterpart frame F 2 . 2  from the second camera will be designated as the reference frame pair. The method  300  may adjust content of the reference frame F 1 . 2  from the first camera using other frames F 1 . 1 , F 1 . 3 -F 1 .N output by the first camera (box  330 ). The method also may adjust content of the reference frame F 2 . 2  from the second camera using other frames F 2 . 1 , F 2 . 3 , F 2 .N output by the second camera (box  340 ). Optionally, the method  300  may merge the denoised reference frames together to form a final frame (box  350 ). 
     Identification of reference frames may occur in a variety of ways. In a first embodiment, the method  300  may assess image quality of both frames F 1 . 1  F 2 . 1 , F 1 . 2 , F 2 . 2 , etc. in each pair P 1 , P 2 , etc. and may select the pair (say P 2 ) of frames having the highest overall image quality. A metric may be developed for a target image quality to be achieved, and the quality of the frames from each camera may be compared against the target image quality value. The metric may be based in measurable characteristics of image content, for example, image sharpness, image brightness, dynamic range of image content, etc., from which estimates of each frame output by the cameras  120 ,  130  ( FIG.  1   ) may be generated. The quality values of the frames output by each camera may be weighted equally for such purposes or may be assigned weights that more heavily weight frames output from one camera (say, camera  120 ) over frames from the other camera  130 . In one embodiment, quality assessments may be made based solely on output from a single one of the cameras  120 . Additionally or alternatively, the quality of the frames from the first camera may be used to make a preliminary selection of frames that will be candidate reference frame pairs (e.g., only frames from the first camera meeting a first quality threshold, or a predetermined number of frames from the first camera within a period of time having highest estimated quality), and then the quality of the second camera&#39;s frames may be used to select a reference frame pair from among the candidate frames. 
     In another embodiment, the method  300  may assess image quality from a set of frames output by a single camera that is designated by operational parameters in which the system operates. For example, in a system having a pair of cameras with different zoom properties (e.g., a wide angle camera and a telephoto camera), operator zoom controls may cause the output of one of the cameras to be designated. An operator zoom operation that selects close in image content for image capture may cause the telephoto camera to be designated. By contrast, an operator zoom operation that selects a wide field of view for image capture may cause the wide angle camera to be designated. In such an embodiment, when one of the cameras is designated, the image quality assessment may be derived from the frames output by the designated camera. 
     Image quality assessment may be performed from sharpness data that is output by one or more of the cameras that indicate sharpness of content in each frame. In such an embodiment, the cameras may perform image stabilization assessments that indicate a degree of sharpness in each frame. Frames having relatively higher sharpness scores may be considered to have higher quality than other frames having relatively lower sharpness scores. The cameras may output metadata to the image processor that identify the sharpness scores of the respective frames. 
     In another embodiment, image quality assessment may be performed from an analysis of image content of each frame. An image processor  140  may evaluate characteristics of the frames output from the cameras, for example, by the frames&#39; relative sharpness, exposure conditions and/or image content. For example, frames having relatively higher sharpness scores may be considered to have higher quality than other frames having relatively lower sharpness. As another example, exposure data may provide an indication how well a candidate image captures the dynamic range of a scene (i.e. a very bright image may be saturated in many areas of an image, whereas a very dark image removes the details in the shadows); an image histogram may indicate how well the current image is exposed. Alternatively, when coupled with face detection, frames identified as having faces in which eyes are determined to be open (e.g., not blinking) or in which faces are determined to be smiling may be identifies as having higher quality than other frames in which these conditions do not appear. The image quality may be estimated based on a single factor or a weighted combination of a plurality of these factors. 
     In a further embodiment, image quality assessment may be performed from an analysis of data from sources outside the camera. For example, image quality may be assessed from data of a motion sensor  150  ( FIG.  1   ) provided in a common device  100  with the cameras  120 ,  130 . Data from the motion sensor  150  may indicate, for example, a degree of motion that occurs during capture of each of the frames F 1 . 1 -F 1 .N, F 2 . 1 -F 2 .N, which may indicate an amount of motion blur that likely is present in each frame&#39;s image content. Images that are captured during times of low motion may be designated as having relatively higher image quality than images that are captured during times of low motion. Image quality may be assessed based on data from a light sensor, which may indicate a type of ambient light (e.g., natural light, artificial light) that is present during image capture. 
     Additionally, image quality assessment may be performed to disqualify certain frames output based on image capture characteristics. As discussed, image capture may be performed with different exposure settings. In some applications, individual frames may have long exposure times, for example ¼ of a second or longer. Such frames typically exhibit good signal to noise characteristics for image content that does not move but they also exhibit heavy blur artifacts for moving image content. Such techniques may be performed with cameras that perform optical image stabilization in which frames are generated according to alternating frame exposure times, such as a “short,” “short,” “long” exposure cadence. Frames captured according to the long exposure times may be disqualified from serving as reference frames. Alternatively, the system may disqualify frame pairs from serving as reference frames when exposure times of either frame in a pair exceeds a threshold amount of time. In another embodiment, the system may prioritize some frame pairs over others based on, for example, differences in exposure times among the frames in each pair. In one embodiment, when candidate frame pairs are generated with variable exposure times—some where the frames in each pair have similar exposure times and others where the frames in each pair have different exposure times from each other—the system may prioritize the candidate pairs for selection with similar exposure times. 
     The principles of the present disclosure find application with cameras that perform high dynamic range (colloquially, “HDR”) image capture techniques, in which a frame sequence is captured according to a pattern of variable exposure times. 
       FIG.  4    illustrates application of the method  300  of  FIG.  3    to the exemplary frame sequence of  FIG.  2   . As indicated, upon analysis of the frames F 1 . 1 -F 1 .N and F 2 . 1 -F 2 .N, one frame pair F 1 . 2 , F 2 . 2  may be designated as the reference frame pair. Content of the other images in each sequence may be used to adjust image content of the reference frame pair. Thus, content of frames F 1 . 1  and F 1 . 3 -F 1 .N may be used to adjust frame F 1 . 2 , yielding a first adjusted frame  410 . Similarly, content of frame F 2 . 1  and F 2 . 3 -F 2 .N may be used to adjust frame F 2 . 2 , yielding a second adjusted frame  420 . The adjusted frames  410 ,  420  may be merged, yielding a final frame  430 . 
     Image adjustment also may be performed in a variety of ways. In one embodiment, content of each frame in a reference pair may be denoised using content of other frames output by the same camera. Returning to  FIG.  2   , if frame pair P 3  were selected as a reference frame pair, content of frame F 1 . 3  may be adjusted based on content of some of the other frames F 1 . 1 , F 1 . 2 , and/or F 1 .N from camera  1  and content of frame F 2 . 3  may be adjusted based on content of some of the other frames F 2 . 1 , F 2 . 2 , and/or F 2 .N from camera  2 . For example, content of frames F 1 . 1 , F 1 . 2 , and/or F 1 .N may be compared to content of frame F 1 . 3  to identify content of frame F 1 . 3  that likely is noise and remove it, or to select parameters of denoising filters that are to be applied to frame F 1 . 3 . In one embodiment, for example, motion estimation analyses may be applied to a sequence of frame F 1 . 3  to estimate spatial region(s) of the frame that have high motion content and region(s) that have low motion content. Similarly, spatial complexity estimates may be performed on regions identified in the frames F 1 . 1 -F 1 .N. Parameters may be selected for denoising filters based on one or more of the region-based motion and/or complexity estimates and a denoising filter may be applied to the region(s) of frame F 1 . 3 . Similar processes may be applied to frame F 2 . 3  from the reference frame pair P 3 , using other frames F 2 . 1 , F 2 . 2 , and/or F 2 .N for analyses. In application, it is not necessary all frames F 2 . 1 , F 2 . 2 , and/or F 2 .N from a given camera (camera  2 ) be used to select image adjustments to be applied to a frame F 2 . 3 ; some frames (e.g., frame F 2 .N) may be disqualified from use based on their exposure times or if they are estimated to have low image quality themselves. 
     In another embodiment, image adjustment may be performed as upsampling, which may increase resolution of a given frame. For example, resolution of frame F 1 . 3  may be increased using content from frames F 1 . 1 , F 1 . 2 , and/or F 1 .N from camera  1 . 
     Image processing of a reference frame pair P 3  may be performed in a variety of ways. In a first embodiment, the frames F 1 . 3 , F 2 . 3  of the pair P 3  may be fused together to form a single composite image. Where the two frames F 1 . 3 , F 2 . 3  have fields of view that overlap only partially, image fusing may cause a single image to be created from a merger of the two fields of view. In some applications, the two frames F 1 . 3 , F 2 . 3  may have overlapping fields of view (e.g. a view of one frame F 1 . 3  may be contained entirely within the field of view of a second frame F 2 . 3  but have higher resolution than the second frame F 2 . 3 ), in which case image fusing may cause a single image to be created from a merger of the two fields of view at the higher resolution using upsampling of low resolution portions of the second frame F 2 . 3  where frame F 1 . 3  does not provide image content. 
     Image fusion may be performed according to any of a number of techniques. Typically, fusion involves a registration process involving a comparison among image data between the denoised reference frames  410 ,  420  to identify correspondence between image content of the frames  410 ,  420 . Thereafter, a warp process may be applied to one of the frames, to align the corresponding content of each frame with each other. Thereafter, content of the frames may be merged together. Merger of the frames may occur according to transform domain fusion techniques and/or spatial-domain fusion techniques. Exemplary transform domain fusion techniques include Laplacian pyramid based techniques, curvelet transform based techniques, discrete wavelet transform based techniques, and the like. Exemplary spatial domain transform techniques include weighted averaging, Brovey method and principal component analysis techniques. A final frame  430  may be output from merger of the frames. 
     In another embodiment, image processing may compare content of the two frames to develop analytical information. For example, content of the frames F 1 . 3 , F 2 . 3  of the reference frame pair P 3  may be compared to generate a disparity map, which may form the basis for further analytical processes such as depth information. In this embodiment, disparity map data may be output in conjunction with one of the frames (say frame F 1 . 3 ) as a composite image. 
     In another embodiment, an image processor may perform image processing operations on frame data output from a pair of cameras to derive an aggregate image having improved image quality as compared to the images output from either of the cameras by themselves.  FIG.  5    illustrates a method.  500  that may be performed by an image processor according to the embodiment. 
     The method  500  may assess image quality of frames output from the cameras (box  510 ) and, based on the image quality, designate a pair of the frames to serve as a reference frame pair (box  520 ). Thus, one frame from the first camera (say, frame F 1 . 2  in  FIG.  2   ) and a counterpart frame F 2 . 2  from the second camera will be designated as a reference frame pair. The method  300  may process the reference frames to form a composite frame (box  530 ) and may adjust the composite frame using other frames F 1 . 1 , F 1 . 3 -F 1 .N and/or F 2 . 1 , F 2 . 3 -F 2 .N output from the cameras  1  and  2 . 
     The image quality assessment techniques described hereinabove in connection with  FIG.  3    also may find application with the embodiment of  FIG.  5   . 
       FIG.  6    illustrates application of the method  500  of  FIG.  5    to the exemplary frame sequence of  FIG.  2   . As indicated, upon analysis of the frames F 1 . 1 -F 1 .N and F 2 . 1 -F 2 .N, one frame pair F 1 . 2 , F 2 . 2  may be designated as the reference frame pair. The reference frames F 1 . 2 , F 2 . 2  may be processed, yielding a composite image  610 . Content of the other images in each sequence may be used to adjust image content of the composite image  610 , yielding a final frame  620 . 
     Image processing may be performed in a variety of ways. In a first embodiment, the frames F 1 . 2 , F 2 . 2  of the pair P 3  may be fused together to form a single composite image. Where the two frames F 1 . 2 , F 2 . 2  have fields of view that overlap only partially, image fusing may cause a single image to be created from a merger of the two fields of view. In some applications, the two frames F 1 . 2 , F 2 . 2  may have overlapping fields of view (e.g. a view of one frame F 1 . 2  may be contained entirely within the field of view of a second frame F 2 . 2  but have higher resolution than the second frame F 2 . 2 ), in which case image fusing may cause a single image to be created from a merger of the two fields of view at the higher resolution using upsampling of low resolution portions of the second frame F 2 . 2  where frame F 1 . 2  does not provide image content. 
     Image fusion may be performed according to any of a number of techniques. Typically, fusion involves a registration process involving a comparison among image data between the reference frames F 1 . 2 , F 2 . 2  to identify correspondence between image content of the frames F 1 . 2  F 2 . 2 . Thereafter, a warp process may be applied to one of the frames, to align the corresponding content of each frame with each other. Thereafter, content of the frames may be merged together. Merger of the frames may occur according to transform domain fusion techniques and/or spatial-domain fusion techniques. Exemplary transform domain fusion techniques include Laplacian pyramid based techniques, curvelet transform based techniques, discrete wavelet transform based techniques, and the like. Exemplary spatial domain transform techniques include weighted averaging, Brovey method and principal component analysis techniques. A final frame  610  may be output from merger of the frames. 
     In another embodiment, image processing may compare content of the two frames to develop analytical information. For example, content of the frames F 1 . 2 , F 2 . 2  of the reference frame pair P 3  may be compared to generate a disparity map, which may form the basis for further analytical processes such as depth information. In this embodiment, disparity map data may be output in conjunction with one of the frames (say frame F 1 . 2 ) as a composite image. 
     Image adjustment  540  also may be performed in a variety of ways. In one embodiment, content of each frame in a reference pair may be denoised using content of other frames output by the same camera. Thus, content of frame F 1 . 2  may be adjusted based on content of some of the other frames F 1 . 1 , F 1 . 3 , and/or F 1 .N from camera  1  and content of frame F 2 . 2  may be adjusted based on content of some of the other frames F 2 . 1 , F 2 . 3 , and/or F 2 .N from camera  2 . For example, content of frames F 1 . 1 , F 1 . 3 , and/or F 1 .N may be compared to content of frame F 1 . 2  to identify content of frame F 1 . 2  that likely is noise and remove it, or to select parameters of denoising filters that are to be applied to frame F 1 . 2 . In one embodiment, for example, motion estimation analyses may be applied to a sequence of frame F 1 . 2  to estimate spatial region(s) of the frame that have high motion content and region(s) that have low motion content. Similarly, spatial complexity estimates may be performed on regions identified in the frames F 1 . 1 -F 1 .N. Parameters may be selected for denoising filters based on one or more of the region-based motion and/or complexity estimates and a denoising filter may be applied to the region(s) of frame F 1 . 2 . Similar processes may be applied to frame F 2 . 2  from the reference frame pair, using other frames F 2 . 1 , F 2 . 3 , and/or F 2 .N for analyses. In application, it is not necessary all frames F 2 . 1 , F 2 . 3 , and/or F 2 .N from a given camera (camera  2 ) be used to select image adjustments to be applied to a frame F 2 . 2 ; some frames (e.g., frame F 2 .N) may be disqualified from use based on their exposure times or if they are estimated to have low image quality themselves. 
     In another embodiment, image adjustment may be performed as upsampling, which may increase resolution of a given frame. For example, resolution of frame F 1 . 2  may be increased using content from frames F 1 . 1 , F 1 . 3 , and/or F 1 .N from camera  1 . 
     Image processing of a reference frame pair may be performed in a variety of ways. In a first embodiment, the frames F 1 . 2 , F 2 . 2  of the pair may be fused together to form a single composite image. Where the two frames F 1 . 2 , F 2 . 2  have fields of view that overlap only partially, image fusing may cause a single image to be created from a merger of the two fields of view. In some applications, the two frames F 1 . 2 , F 2 . 2  may have overlapping fields of view (e.g. a view of one frame F 1 . 2  may be contained entirely within the field of view of a second frame F 2 . 2  but have higher resolution than the second frame F 2 . 2 ), in which case image fusing may cause a single image to be created from a merger of the two fields of view at the higher resolution using upsampling of low resolution portions of the second frame F 2 . 2  where frame F 1 . 2  does not provide image content. 
       FIG.  7    illustrates a fusion unit  700  according to an embodiment of the present disclosure. The fusion unit may include a plurality of frequency decomposition units  710 - 712 ,  720 - 722 , . . . ,  730 - 732 , a plurality of layer fusion units  740 - 746  and a merger unit  750 . The frequency decomposition units  710 - 712 ,  720 - 722 , . . . ,  730 - 734  may be arranged as a plurality of layers, each layer generating filtered versions of the data input to it. A first chain of frequency decomposition units  710 ,  720 , . . . ,  730  may be provided to filter data of a first image (called the “primary” image, for convenience) and a second chain of frequency decomposition units  712 ,  722 , . . . ,  732  may be provided to filter image data of a second image (called the “secondary” image, again for convenience). The primary image data may correspond one of the images output from the image registration process, for example, either the warped image or the non-warped image, and the secondary image data may correspond to the other of the images from image registration process. Each layer of the frequency decomposition units  710 - 712 ,  720 - 722 ,  730 - 732  may have a layer fusion unit  740 ,  742 ,  744 , . . .  746  associated with it. 
     The layer fusion units  740 - 746  may output image data of their associated layers. Thus, the layer fusion unit  740  may be associated with the highest frequency data from the primary image and the warped secondary image (no frequency decomposition), a second layer fusion unit  742  may be associated with a first layer of frequency decomposition, and a third layer fusion unit  744  may be associated with a second layer of frequency decomposition. A final layer fusion unit  746  may be associated with a final layer of frequency decomposition. Each layer fusion unit  740 ,  742 ,  744 , . . .  746  may receive the primary image layer data, the secondary image layer data and the weight layer data of its respective layer. Output data from the layer fusion units  740 - 746  may be input to the merger unit  750 . 
     Each layer fusion unit  740 ,  742 ,  744 , . . .  746  may determine whether to fuse the primary image layer data and the secondary image layer data based on a degree of similarity between the primary image layer data and the secondary image layer data at each pixel location. If co-located pixels from the primary image layer data and the secondary image layer data have similar values, the layer fusion unit (say, unit  742 ) may fuse the pixel values. If the co-located pixels do not have similar values, the layer fusion unit  742  may not fuse them but rather output a pixel value taken from the primary image layer data. 
     The merger unit  750  may combine the data output from the layer fusion units  740 - 746  into a fused image. The merger unit  750  may scale the image data of the various layers to a common resolution, then add the pixel values at each location. Alternatively, the merger unit  750  may weight the layers&#39; data further according to a hierarchy among the layers. For example, in applications where secondary image data is expected to have higher resolution than primary image data, correspondingly higher weights may be assigned to output data from layer fusion units  740 - 742  associated with higher frequency layers as compared to layer fusion units  744 - 746  associated with lower frequency layers. In application, system designers may tailor individual weights to fit their application needs. 
       FIG.  7    illustrates structure of an exemplary layer fusion unit  760  according to an embodiment of the present disclosure. The layer fusion unit  760  may include an adder  462 , a selector  764  and a comparison unit  766 . The adder  762  may generate pixel-wise sums of the image data input to from the primary and secondary images. In this manner, the adder  762  may generate fused image data at each pixel location. 
     The selector  764  may have inputs connected to the adder  762  and to the primary image data that is input to the layer fusion unit  760 . A control input may be connected to the comparison unit  766 . The selector  764  may receive control signals from the comparison unit  766  that, for each pixel, cause the selector  764  to output either a pixel value received from the adder  762  or the pixel value in the primary image layer data. The selector&#39;s output may be output from the layer fusion unit  760 . 
     As indicated, the layer fusion unit  760  may determine whether to fuse the primary image layer data and the secondary image layer data based on a degree of similarity between the primary image layer data and the secondary image layer data at each pixel location. The comparison unit  766  may determine a level of similarity between pixels in the primary and the secondary image level data. In an embodiment, the comparison unit  766  may make its determination based on a color difference and/or a local high frequency difference (e.g. gradient difference) between the pixel signals. If these differences are lower than a predetermined threshold then the corresponding pixels are considered similar and the comparison unit  766  causes the adder&#39;s output to be output via the selector  764  (the image data is fused at the pixel location). 
     In an embodiment, the comparison threshold may be set based on an estimate of a local noise level. The noise level may be set, for example, based properties of the cameras  120 ,  130  ( FIG.  1   ) or based on properties of the image capture event (e.g., scene brightness). In an embodiment, the threshold may be derived from a test protocol involving multiple test images captured with each camera. Different thresholds may be set for different pixel locations, and they may be stored in a lookup table (not shown). 
     In another embodiment, the image processing techniques described herein may be performed by a central processor of a computer system.  FIG.  8    illustrates an exemplary computer system  800  that may perform such techniques. The computer system  800  may include a central processor  810 , a pair of cameras  820 ,  830  and a memory  840  provided in communication with one another. The cameras  820 ,  830  may perform image capture according to the techniques described hereinabove and may store captured image data in the memory  840 . Optionally, the device also may include a display  850  and a coder  860  as desired. 
     The central processor  810  may read and execute various program instructions stored in the memory  840  that define an operating system  812  of the system  800  and various applications  814 . 1 - 814 .N. The program instructions may perform image processing according to the techniques described herein. As it executes those program instructions, the central processor  810  may read from the memory  840 , image data created by the cameras  820 ,  830  and it may perform quality assessment operations, frame selection, denoising operations, and image fusion operations as described hereinabove. 
     As indicated, the memory  840  may store program instructions that, when executed, cause the processor to perform the image fusion techniques described hereinabove. The memory  840  may store the program instructions on electrical-, magnetic- and/or optically-based storage media. 
     The image processor  120  ( FIG.  1   ) and the central processor  810  ( FIG.  8   ) may be provided in a variety of implementations. They can be embodied in integrated circuits, such as application specific integrated circuits, field programmable gate arrays, digital signal processors and/or general purpose processors. The image processor  120  and/or the central processor  810  each may be provided either as a single processor integrated circuit or as a distributed system of processor integrated circuits. The image processor  120  and the central processor  810  may be provided as a common processor integrated circuit, if convenient. 
     Several embodiments of the disclosure are specifically illustrated and/or described herein. However, it will be appreciated that modifications and variations of the disclosure are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the disclosure.