Patent Publication Number: US-10789684-B2

Title: Method, apparatus, and system for multilevel bilateral noise filtering

Description:
TECHNICAL FIELD 
     The invention relates to multilevel bilateral noise cleaning of digital images. Further, the invention relates to an apparatus and a system performing multilevel bilateral noise cleaning of digital images. 
     BACKGROUND 
     Digital cameras include image sensors to capture digital images. Image sensors detect a light intensity per pixel. To allow color information to be recorded, color filter arrays (CFA) may be bonded to the substrate of the image sensor which allocate a certain color to a certain pixel, and each pixel detects the light intensity for the specific color. 
     A typical pattern for a CFA used in digital cameras is a Bayer filter or Bayer CFA. A Bayer filter contains alternating rows of red and green filters and blue and green filters, wherein each row contains alternating red and green filters and blue and green filters, respectively. Green filters preferentially allow green light photons to pass to the detector (e.g., a photodiode) of the respective pixel of the image sensor. At the same time, red and blue light photons that arrive at the green filter are not transmitted by the filter and, therefore, not detected by the respective detector. Similarly, red filters preferentially allow red light photons and blue filters preferentially allow blue light photons to pass to the respective detector. 
     In a Bayer CFA, green filters occur at twice the spatial frequency of the red and blue filters. The green channel represents the luminance and the red and blue channels represent the chrominance of the digital image. When a Bayer filter is applied in front of an image sensor, the resulting image frame produced by the image sensor needs to be interpolated or demosaiced to reproduce all three colors for each pixel so that the original full-color resolution digital image can be displayed, printed or stored. 
     In digital cameras, interpolating or demosaicing may be performed by specialized image signal processors (ISPs), or by general purpose processors (CPUs) which execute image processing software programs. 
     Digital images typically contain noise from various sources that needs to be reduced. The need for reduction increases with International Organization for Standardization (ISO) sensitivity. 
     One of the noise components is fixed pattern noise (FPN), which has a spatial pattern that does not vary with time. Another noise component is temporal noise, which has a spatially varying pattern that varies with time. The overall noise characteristics in a digital image depend, for example, on the sensor type, pixel dimensions, temperature, exposure time, and ISO sensitivity. Image noise is also channel dependent, as discussed in Zhang, et. al.,  Multiresolution Bilateral Filtering for Image Denoising.    
     Various image denoising methods have been developed. One of these methods utilizes bilateral filtering. Bilateral filtering smooths images while preserving edges by a nonlinear combination of nearby image values. The method is generally described, e.g., in Tomasi et. al.,  Bilateral Filtering for Gray and Color Images.    
     While bilateral filtering is generally known for a number of applications including bilateral noise cleaning, it was first proposed as an intuitive tool, and parameters, such as the parameters controlling the fall-off of the weights in the spatial and intensity domains are typically selected by trial and error. See Zhang, et. al.,  Multiresolution Bilateral Filtering for Image Denoising.    
     In particular, existing noise cleaning procedures attempt to clean noise in a space where the noise has been mixed from either demosaicing or channel mixing as part of the color correction processing. As a result, noise is not necessarily uncorrelated for different pixels which results in unsatisfactory noise cleaning. Therefore, new approaches are needed which improve the effectiveness of bilateral noise filtering applied to a digital image. 
     SUMMARY 
     It is an object of the invention to provide a method, an image processing apparatus, and a computer-readable storage medium for noise filtering digital images that overcome the disadvantages of the related art. The object is achieved by providing multilevel bilateral noise filtering techniques that allow effective removal of noise in all spatial frequency bands. 
     Bilateral filters remove noise by creating a weighted average of neighborhood pixels defined by a kernel size and shape. According to an aspect of the invention, the kernel is centered upon a target pixel I(x,y). Two predictors are used when computing the weights for a target pixel mean, namely the distance of the kernel pixel from the target pixel, and the z-score of the target pixel defining a similarity or difference of a value of the kernel pixel to the value of the target pixel. 
     Bilateral filters act upon a group of pixels defined by the spatial weight. Typically, this size is only a few pixels wide and the image noise is broad band (spectrally flat). It has been determined that the bilateral filter is effective for high frequency noise, but because the support is only a few pixels, only high frequency noise is attenuated, leaving low frequency noise behind. 
     With regard to the distance of the kernel pixel from the target pixel, closest pixels to the target pixel have the most weight. With regard to the similarity or difference of a value of the kernel pixel to the value of the target pixel, pixels closest in value to the target pixel have the most weight. 
     At each target pixel I(x,y), weights are determined for each pixel, p within the kernel according to the following equation: 
     
       
         
           
             
               
                 
                   
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     For a target pixel I(x,y) centered in a kernel, Δr is the distance in pixels, as measured from the original CFA, from I(x,y) to another pixel in the kernel. Parameter r is a Gaussian radius parameter. Δν is a code value difference between the target pixel I(x,y) and another pixel in the kernel. σ is the standard deviation of the target pixel with intensity I(x,y). The evaluation is performed for all I(p) values within the kernel. 
     The pixel value difference is scaled by the standard deviation of I(x,y). Scaling the pixel value difference by the standard deviation of I(x,y) is a further enhancement to implementations of the bilateral filter in the related art. This ensures that the strength of the noise cleaning is consistent with the expected noise in every portion of the digital image. The relationship between image code values and the noise (standard deviation) is stored in a noise table that is provided to the noise filter. This noise table σ[I(x,y)] is computed from knowledge of the specific capture system parameters or empirically measured from calibrated image data. In the noise table σ[I(x,y)], the standard deviation at every integer value of I(x,y) is recorded. 
     These modifications to the bilateral filter(s) significantly improve the effectiveness of the noise cleaning process in the image processing apparatus and thereby the overall functionality of the image processing apparatus in which they are used. 
     It has been determined that noise cleaning is most effective when the noise is un-correlated. In the context of a digital camera processing path, most un-correlated noise occurs before CFA interpolation and before the matrix mixing is performed in color correction. Therefore, the bilateral filter(s) according to an aspect of the invention is/are adapted to operate directly in a pre-interpolated CFA space, i.e., before the digital image is interpolated or demosaiced and the distances used are consistent with the spatial gaps that exist for any given color in the image record. In other words, according to an aspect of the invention, a new arrangement is provided in which the bilateral filtering is performed on color plane images that are generated by decomposing an image frame received from an image sensor that includes a color mosaic having a CFA pattern. 
     The z-score predictor requires an estimate of the standard deviation of the image data and the standard deviation may be determined empirically from the device and not estimated from the data. 
     According to a first aspect of the invention, a method for multilevel bilateral noise filtering of digital images is provided. The method includes receiving from an image sensor an image frame including a color mosaic having a CFA pattern, decomposing the image frame into at least one color plane image for each of a plurality of color planes, sequentially and separately reducing noise in the at least one color plane image by performing multilevel bilateral noise filtering of the at least one color plane image, and reconstructing the at least one color plane image to generate a noise filtered image frame. 
     According to a second aspect of the invention, an image pre-processing apparatus is provided which includes one or more processors in communication with an image sensor and one or more memory devices in communication with the one or more processors. The one or more processors are configured to receive from the image sensor an image frame including a color mosaic having a CFA pattern, decompose the image frame into at least one color plane image for each of a plurality of color planes, sequentially and separately reduce noise in the at least one color plane image by performing multilevel bilateral noise filtering of the at least one color plane image, and reconstruct the at least one color plane image to generate a noise filtered image frame. 
     According to a third aspect of the invention, a non-transitory computer readable storage medium is provided. The non-transitory computer readable storage medium is encoded with software including computer executable instructions that when executed by one or more processors cause the one or more processors to receive from the image sensor an image frame including a color mosaic having a CFA pattern, decompose the image frame into at least one color plane image for each of a plurality of color planes, sequentially and separately reduce noise in the at least one color plane image by performing multilevel bilateral noise filtering of the at least one color plane image, and reconstruct the at least one color plane image to generate a noise filtered image frame. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the drawings, wherein: 
         FIG. 1  shows a schematic illustration of an image processing apparatus according to an exemplary embodiment of the invention; 
         FIG. 2  shows an illustration of a plane separation according to an exemplary embodiment of the invention; 
         FIG. 3  shows a detailed schematic illustration of an image processing apparatus according to an exemplary embodiment of the invention; 
         FIG. 4  shows a schematic illustration of a method for noise filtering digital images according to an exemplary embodiment of the invention; 
         FIG. 5  shows a schematic illustration of a method for frequency splitting according to an exemplary embodiment of the invention; 
         FIG. 6  shows a detailed illustration of the method for frequency splitting according to an exemplary embodiment of the invention; and 
         FIG. 7  is a flow chart depicting operations performed by the image processing apparatus to noise filter digital images according to an exemplary embodiment of the invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  shows an image processing apparatus  100  configured for multi-level bilateral noise filtering of digital images. The image processing apparatus  100  includes an image sensor  110 , an image pre-buffer  120  connected to the image sensor  110  and to image processor  130  and Image Signal Processor (ISP)  140 . Image processing apparatus  100  further includes memory  150  which is in communication with image processor  130 . The image processing apparatus  100  further includes digital image storage medium  160  which is in communication with ISP  140 . 
     The image sensor  110  can be operated in a live view mode and in a still capture mode. In both modes, the full active area of the image sensor  110  is utilized, and an image frame is generated and outputted by the image sensor  110  to the image pre-buffer  120 . 
     The memory  150  and the digital image storage medium  160  are non-transitory computer readable storage media, for example, solid-state drives (SSD), but are not limited thereto. Any other non-transitory computer readable storage medium or a plurality of non-transitory computer readable storage media can be also utilized as the memory  150  or the digital image storage medium  160 . 
     Reference is now made to  FIG. 2  (with further reference to  FIG. 1 ).  FIG. 2  shows an illustration of plane separation. A CFA, and in particular a Bayer CFA may be bonded to image sensor  110 . As a result, the image frames generated by the image sensor  110  include a color mosaic with a Bayer CFA pattern.  FIG. 2  shows a portion of an image frame  210  which is received from the image sensor  110  and stored in image pre-buffer  120 . As shown in  FIG. 2 , the image frame  210  includes alternating rows of red and green pixels and of blue and green pixels. Pixels of the same color represent a color plane or color channel. The Bayer CFA pattern is decomposed into three color plane images, namely, the red color plane image  220 , the green color plane image  230 , and the blue color plane image  240  by the image processor  130  prior to the noise cleaning to permit each color plane to be cleaned independently. The green channel represents the luminance of the digital image and the red and blue channels represent the chrominance of the image frame  210 . As shown in  FIG. 2 , the green plane has a structure that is different from the red and blue planes with regard to the distance between adjacent pixels of the same color. For an image frame with a certain pixel width and a pixel height, the dimensions of the red and blue planes can be described as follows: for a dimension of the image frame  210  defined by pixel width×pixel height, the dimension of the red color plane image  220  and of the blue color plane image  240  is pixel width/2×pixel height/2, and the dimension of the green color plane image  230  is pixel width/2×pixel height. A distance Δr in pixels from a kernel pixel used for noise reduction, as described in more detail below, is calculated for all red plane rows and blue plane rows based on the following equation:
 
Δ r =√{square root over ((2·ΔRows) 2 +(2·ΔColumns) 2 )}  (4)
 
     The distance Δr in pixels from a kernel pixel for even green plane rows is calculated based on the following equation:
 
Δ r =√{square root over (ΔRows 2 +(2·ΔColumns) 2 )}  (5)
 
     The distance Δr in pixels from a kernel pixel for odd green plane rows is calculated based on the following equation:
 
Δ r =√{square root over (ΔRows 2 +(2·ΔColumns+1) 2 )}  (6)
 
     As a result, the distances Δr as measured on the extracted red, blue, and green planes are consistent with the CFA pixel distances of the image frame  210 . 
     After decomposing the image frame  210  into the red, green, and blue color plane images, the image processor  130  sequentially and separately reduces noise in each of the color planes by performing multilevel bilateral noise filtering of the red, green, and blue color plane images  220 ,  230 , and  240 , respectively. Thereafter, the image processor  130  reconstructs the red, green, and blue color plane images  220 ,  230 , and  240 , respectively, to generate a noise filtered image frame that is stored by the image processor  130  in the image pre-buffer  120 . Subsequently, the noise filtered image frame is transmitted to ISP  140  where it is interpolated or demosaiced to a viewable image and stored in digital image storage medium  160 . 
     Reference is now made to  FIG. 3 , which shows a more detailed schematic illustration of an image processing apparatus  300  for processing digital images according to an exemplary embodiment of the invention. As shown in  FIG. 3 , the image processing apparatus  300  includes an image pre-processing device  310  to which image sensor  110  is connected, and an image main processing device  330 . The image main processing device  330  is in communication with image pre-processing device  310 , display  140 , and storage medium  160 . 
     The image main processing device  330  in the exemplary embodiment of the image processing apparatus  300  shown in  FIG. 3  has a system on chip (SoC) architecture and integrates all components necessary to process an image frame received from an image sensor to generate digital images that can be displayed, printed or stored. Thus, image main processing device  330  includes image processor  342  which may be implemented, for example, as a digital signal processor (DSP) or a graphics processing unit (GPU). Image main processing device  330  further includes a first ISP  338  and data transceiver  332  configured to receive and transmit image frames to be stored in still image pre-buffer  334 . In addition, an image data receiver  336  is provided which is configured to receive image subframes to be processed by the ISP  338 . The image subframes processed by the ISP  338  are stored in still image post-buffer  340 . A display controller  352  is provided which performs operations to allow the image frame captured by the image sensor  110  to be visible on the entire display  140 . The display controller  352  is connected to the display  140  via display data transmitter  354 . To store still image frames in a graphics image format or image frames in a raw image format in the storage medium  160 , a storage controller  356  and a storage interface  358  are provided. 
     The image pre-processing device  310  includes a data transceiver  312  and a first imager data transmitter  314 . Data transceiver  312  and data transceiver  332  form a data interface between the image pre-processing device  310  and the image main processing device  330 . The data transceiver  312 ,  332  may be a high-speed serial computer expansion bus standard interface, such as a Peripheral Component Interconnect Express (PCIe) standard interface, but is not limited thereto. 
     Like the data transceiver  312 ,  332 , the imager data transmitter  314  together with the imager data receiver  336  form another interface (i.e., an imager data interface) between the image pre-processing device  310  and the image main processing device  330 . Data transceiver  312  and imager data transmitter  314  are controlled by receive DMA (RDMA) controller  316  and transmit DMA (TDMA) controller  318 . RDMA controller  316  is in communication with imager data transmitter  314  via first in first out (FIFO) buffer  320 . Image pre-processing device  310  also includes image data receiver  322  and pixel processor  324  which is in communication with transmit DMA controller  318  via FIFO buffer  326 . 
     The first and second imager data interfaces  314 ,  336  and  346 ,  348  according to the exemplary embodiment shown in  FIG. 3  are Mobile Industry Processor Interface (MIPI) Camera Serial Interface (CSI) image data interfaces. The imager data interfaces, however, are not limited to MIPI CSI and any other serial interfaces can also be utilized instead. 
     In the exemplary embodiment shown in  FIG. 3 , the image pre-processing device  310  is implemented as a field-programmable gate array (FPGA). However, the image pre-processing device  310  may also be implemented as an application-specific integrated circuit (ASIC). Similarly, the image processor  342  may also be implemented as a separate FPGA or a separate ASIC. 
     Image pre-processing device  310  further includes image downsizer  328  that is connected via FIFO buffer  344  to a second imager data transmitter  346  that forms together with a second imager data receiver  348  a second imager data interface. The imager data receiver  348  is connected to a second ISP  350 . 
     Noise filtering is performed by image processor  342 . An image frame  210  generated by the image sensor  110  is stored in still image pre-buffer  334  before it is decomposed by image processor  342  into color plane images in which noise is sequentially and separately reduced by the image processor  342  before the color plane images are reconstructed to generate a noise filtered image frame that is stored in still image pre-buffer and thereafter transmitted to ISP  338  via data transceiver  332  and  312  and imager data transmitter  314  and imager data receiver  336  where it is interpolated or demosaiced to a viewable image and stored in digital image storage medium  160 . 
     Referring now to  FIG. 4  (with continued reference to  FIGS. 1 to 3 ).  FIG. 4  is a schematic illustration of a method for noise filtering digital images according to an exemplary embodiment of the invention. More specifically,  FIG. 4  illustrates a method for sequentially reducing noise in a color plane by separately performing multilevel bilateral noise filtering by image processors  130 ,  342 . The method shown in  FIG. 4  generates noise filtered color plane images  405  for each color plane, which are subsequently combined to generate a noise filtered image that is demosaiced by ISPs  140 ,  338  and stored in storage medium  160 . 
     To perform multilevel bilateral noise filtering, a plurality of noise cleaning levels is defined.  FIG. 4  illustrates a first, second and highest noise cleaning level. However, the method is not limited thereto. Instead, the number of noise cleaning levels may be increased to four or more noise cleaning levels. 
     At the first level shown in  FIG. 4 , the color plane image  410  is bilaterally filtered by bilateral filter  415 . To bilaterally filter the color plane image  410 , a kernel of a target pixel is defined. The kernel has a square width size and includes the target pixel and kernel pixels. Subsequently, each pixel of the at least one color plane image is processed as the target pixel by determining a weighted averaged pixel value for the target pixel. The weighted averaged pixel value for the target pixel is determined by applying weights to values of each kernel pixel and by subsequently averaging the weighted kernel pixel values. 
     A weight for a kernel pixel is determined based on a distance of the kernel pixel from the target pixel and based on a z-score defining a similarity of the value of the kernel pixel to the value of the value of the target pixel. The distance of the kernel pixel from the target pixel is determined to be consistent with spatial gaps in the color mosaic having the CFA pattern. 
     The weight for the kernel pixel is determined in accordance with the following equation: 
     
       
         
           
             
               
                 
                   
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     wherein ThrottleR_ML defines a Gaussian radius control, ThrottleS_ML defines a pixel difference control, Δr defines a distance in pixels from the kernel, I(x,y) defines the target pixel centered in the kernel, Δν defines a code value difference, σ[I(x,y)] defines a noise table, and level defines the noise cleaning level. Thus, the bilateral filter  415  in  FIG. 4  determines the weights for the first level, the bilateral filter  425  determines the weights for the second level and the bilateral filter  435  determines the weights for the highest level, i.e., the third level in the exemplary embodiment shown in  FIG. 4 . 
     The bilaterally filtered color plane image is provided to frequency splitter  420 . Frequency splitter  420  divides the bilaterally filtered color plane image into a first level low frequency color plane image and a first level high frequency color plane image. Frequency splitting is described in further detail in conjunction with  FIGS. 5 and 6  below. 
     The first level low frequency color plane image is bilaterally filtered by second level bilateral filter  425  and thereafter provided to second level frequency splitter  430  where it is divided into a second level low frequency color plane image and a second level high frequency color plane image. 
     At the highest level in  FIG. 4 , the second level low frequency color plane image is bilaterally filtered by bilateral filter  435  and upsized by upsizer  440 . Pixel values of the upsized color plane image output by upsizer  440  are added to respective pixel values of the second level high frequency color plane image, and the resulting color plane image is upsized again by upsizer  445  before the pixel values of the upsized color plane image output by upsizer  444  are added to respective pixel values of the first level high frequency color plane image generated by frequency splitter  420  to generate noise filtered color plane image  405 . 
       FIG. 5  shows a schematic illustration of a method for frequency splitting. The method is performed by frequency splitter  420  on a bilaterally filtered first level color plane image  410 . However, frequency splitter  430  operates similarly on the bilaterally filtered second level low frequency color plane image. 
     Generally, frequency splitting is an invertible process that creates a low frequency image and a residual high frequency image. Downsize and upsize operations are bi-linear. The bilateral filter described is effective in cleaning noise within the spatial frequency domain of the kernel size. However, support for larger spatial frequencies is minimal. Filtering lower frequency noise is achieved by downsizing the image by some factor and re-filtering. The downsizing process splits the image data into two spatial frequency bands: low and high. There are many methods that can be used for separating an image into high and low spatial frequency bands. One such method is described below in further detail. It is also advantageous to the extent possible to match the frequency split with the spatial frequency support of the bilateral filter. This insures uniform noise cleaning across all spatial frequencies. 
     As shown in  FIG. 5 , the bilaterally filtered color plane image  505  is provided to Gaussian filter  510  with a radius  2 . The bilaterally filtered color plane image  505  filtered by Gaussian filter  510  is two times downsized by downsizer  515  to generate the first level low frequency color plane image. The color plane image downsized by downsizer  515  is two times upsized by upsizer  520  and respective pixel values are subtracted from corresponding pixel values of the color plane image  505  to generate the first level high frequency color plane image. The first level low and high frequency color plane images are provided to the next level(s) to be subsequently processed by the highest noise cleaning level  525  where the respective low frequency color plane image is provided to bilateral filter  530  to be upsized by upsizer  535 . The color plane image upsized by upsizer  535  is added to the high frequency color plane image(s) from the previous level(s). If more than two levels are present, additional upsizers (not shown) may be provided, similar to the upsizer  445  in  FIG. 4  to match the size of the low and high frequency color plane images. As a result, noise filtered color plane image  540  is generated. 
       FIG. 6  shows a detailed illustration of the method for frequency splitting performed by frequency splitter  420 . As shown in  FIG. 6 , blue color plane image  240  is bilaterally filtered in the first level by bilateral filter  415  and provided as bilaterally filtered image  605  to frequency splitter  420 . Some pixels of bilaterally filtered image  605  are numbered as B N1  to B N18  to illustrate how these pixels are processed by the frequency splitter  420  to obtain blue color low and high frequency color plane images of the blue color plane image of the first level. Frequency splitters of higher levels operate analogously. 
     The pixels B N1  to B N18  are Gaussian filtered and result in Gaussian filtered pixels B N1 ′ to B N18 ′ of Gaussian filtered color plane image  610 . The Gaussian filtered pixels are grouped and the Gaussian filtered color plane image  610  is downsized to color plane image  615  by calculating average values of groups of four pixels of the Gaussian filtered color plane image  610 . For example, pixel B D1 ′ of downsized image  615  represents an average value of pixels B N1 ′, B N2 ′, B N6 ′, and B N7 ′ of Gaussian filtered color plane image  610 . Similarly, pixel B D2 ′ of downsized image  615  represents an average value of pixels B N3 ′, B N4 ′, B N8 ′, and B N9 ′. Pixels B D2 ′, B D6 ′, and B D7 ′ are pixels of the first level low frequency blue color plane image (LF Image) provided to the next noise reduction level. 
     To generate the first level high frequency blue color plane image, each of the pixels B D1 ′, B D2 ′, B D6 ′, and B D7 ′ of the downsized image  615  is upsized twice, i.e., the pixels are duplicated twice to generate upsized color plane image  620 . The pixel values of the upsized color plane image  620  are subtracted from the pixel values of the bilaterally filtered image  605  which results in the first level high frequency color plane image (HF Image) that is provided to the next noise reduction level. 
     Reference is now made to  FIG. 7  (with continued reference to  FIGS. 1, 2, and 3 ), which illustrates a flow chart for method  700  for noise filtering digital images. Method  700  is performed by image processors  130  and  342  for each color plane image. Method  700  begins at step  710  where the image processors  130  and  342  receive an image frame including a color mosaic having a CFA pattern via image pre-buffer  120  (or still image pre-buffer  334 , respectively) from the image sensor  110 . Method  700  continues to step  720  at which the image frame is decomposed by the image processors  130  and  342  into at least one color plane image for each of a plurality of color planes, i.e., in red color plane image  220 , green color plane image  230 , and blue color plane image  240 . 
     At step  730 , noise in the at least one color plane image is sequentially and separately reduced by performing multilevel bilateral noise filtering of the at least one color plane image. The method concludes with step  740  at which the at least one color plane image is reconstructed to generate a noise filtered image frame. 
     Referring back to  FIG. 1 , memory  150  is described in more detail. According to an exemplary embodiment of the invention, memory  150  is a non-transitory computer readable storage media encoded with software including computer executable instructions that when executed by image processor  130  cause the image processor  130  to receive from the image sensor  110  an image frame including a color mosaic having a CFA pattern, decompose the image frame into at least one color plane image  220 ,  230 , and  240  for each of a plurality of color planes, sequentially and separately reduce noise in the at least one color plane image by performing multilevel bilateral noise filtering of the at least one color plane image; and reconstruct the at least one color plane image to generate a noise filtered image frame, e.g., noise filtered color plane image  405 . Image processor  130  may be implemented as a single processor or as a plurality of processors. 
     In summary, techniques for multi-level bilateral noise filtering of digital images are provided which are more effective than existing noise cleaning methods, in particular in high frequency bands. The multi-level approach is effective in removing noise from the remaining frequency bands. Scaling the noise cleaning at each pixel allows for the proper amount of noise cleaning in shadows and highlights of the digital images. By arranging the multi-level bilateral noise filters before the ISPs in which the noise filtered image frames are demosaiced, more efficient noise filtering can be performed since the noise in the CFA space is less correlated than the noise that has been mixed from either demosaicing, or channel mixing as part of the color correction processing. As a result, the overall functionality of the image processing apparatus and the quality of the resulting full color images can be significantly improved. 
     It is understood that the foregoing description is that of the exemplary embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.