Abstract:
The apparatus and method invented are operating upon a digital image signal obtained from an image sensor. The sensor is covered with different colored filters and is only able to record the color transmitted through each specific filter into the photosite or pixel. This type of sensor is known as a color filter array or CFA sensor. The different colored filters are arranged in a predefined pattern across the sensor. To obtain a full color image the missing color information is estimated by a set of weighed values obtained by an inverted gradient function. The set of weighted values is found from the neighboring pixels in the four compass directions, north, east, west and south or is found horizontally and vertically. The surrounding pixels are corrected by the chrominance channel to better fit the center pixel in the luminance channel, prior to using the gradient functions. The chrominance channel is interpolated in a similar manner and is also corrected to better fit the center pixel before the inverted gradient functions are applied.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    (1) Field of the Invention  
           [0002]    The present invention relates generally to the field of electronic imaging, and more particularly, to a color-filter array (CFA) algorithm to estimate missing color information for a digital camera having a single color sensor.  
           [0003]    (2) Description of the Prior Art  
           [0004]    Color is the perceptual result of light in the visible region of the spectrum, having wavelengths in the region of 400 nm to 700 nm, incident upon the retina. The spectral distribution of light relevant to the human eye is often expressed in 31 components each representing a 10 nm band.  
           [0005]    The human retina has three types of color photoreceptors cone cells, which respond to incident radiation with somewhat different spectral response curves. Because there are exactly three types of color photoreceptor, three numerical components are necessary and sufficient to describe a color, providing that appropriate spectral weighting functions are used.  
           [0006]    The most common consumer cameras in digital imaging today are single sensor cameras. An image sensor is constructed as an array of tiny light-sensitive diodes called photosites or pixels. The photosites themselves usually do not extract color from light; they only convert photons from a wide spectral band into electrons. In order to record color images with a single sensor, the sensor is filtered so that different photosites receive different colored illumination. This type of sensor is known as a color-filter-array or CFA sensor. The different color filters are arranged in a predefined pattern across a sensor.  
           [0007]    The most common arrangement is the Bayer color-filter-array (CFA), which was invented at Kodak. The CFA of a color imager follows often the commonly used said “Bayer” pattern. The pattern of said Bayer CFA is showed in FIG. 1 prior art. Half of the total number of photosites is green (G), while a quarter of the total number is assigned to both red (R) and blue (B). In order to obtain this color information, the color image photosites are covered with a red, a green, or a blue filter, in a repeating pattern. This pattern, or sequence, of filters can vary, but the widely adopted “Bayer” pattern is a repeated 2×2 arrangement as shown in FIG. 1 prior art.  
           [0008]    This pattern, showed in FIG. 1 prior art, is based on the premise that the human eye derives most of the luminance data from the green content of a scene; and it is the resolution of this luminance data that is perceived as the “resolution” of an image. Therefore, by ensuring that more of the pixels are “green”, a higher resolution image can be created—compared with an alternating R-G-B color filter array with equal numbers of Red, Green and Blue photosites.  
           [0009]    Single-sensor cameras spatially sample the incoming image using a CFA. Consequently, each photosite only contains a single color value. In order to reconstruct the original full-color image a demosaicing step must be performed which interpolates the missing colors at each pixel. Goals in CFA demosaicing include color fidelity, spatial resolution, no false colors, no jagged edges, no aliasing artifacts, and computational practicality. There is often a trade-off between a sharp image and the so-called “zipper effect” or jagged edge look. Several interpolation methods that utilize the color information in different ways are in use.  
           [0010]    The challenge for the designer is to identify such an algorithm combining a high quality interpolation with a low computational effort.  
           [0011]    There are various patents available dealing with said problems:  
           [0012]    U.S. Pat. No. (6,366,318 to Smith et al.) describes a method for generating color image signals derived from an array of color image pixels, the array of color pixels are subsampled in order to generate an array of subsampled color pixels. An array of new, evenly spaced color pixels are interpolated from the subsampled color pixels using surrounding pixels of the same color, and the color image signals derived from the array of new color pixels are subsequently processed, e.g., in a color filter array interpolation. By providing this correction in a preprocessing step, the rest of the normal image processing hardware or software may be used on the resultant subsampled. CFA image without developing the characteristic blocky and wavy artifacts caused by an irregular subsampling pattern.  
           [0013]    U.S. Pat. No. (6,268,939 to Klassen et al.) discloses a method and apparatus for improving the efficiency of color correcting subsampled luminance and chrominance-based data. More specifically, the present invention is directed to a method of applying a full conversion between color spaces for one pixel in a selected pixel block. Chrominance values are assigned to the remaining pixels based upon their luminance values relative to the luminance value of the converted pixel. The invention may be used to convert between various types of color spaces, and may be used in the processing of both device dependent and device independent data.  
           [0014]    U.S. Pat. No. (5,506,619 to Adams et al.) describes an apparatus for processing a digitized image signal obtained from an image sensor having color photosites aligned in rows and columns that generate at least three separate color values but only one color value for each photosite location, a structure is provided which interpolates color values for each photosite location so that it has three different color values. The apparatus includes an image buffer for storing the digitized image signal and a processor operative with the buffer storing means for generating an appropriate color value missing from a photosite location by the interpolation of an additional color value for such photosite locations from color values of different colors than the missing color value at nearby photosite locations. The processor further includes a structure for obtaining Laplacian second-order values in at least two image directions from nearby photosites of the same column and row and which structure is responsive to the Laplacian second-order values for selecting a preferred orientation for the interpolation of the missing color value and for interpolating the missing color value from nearby multiple color values selected to agree with the preferred orientation.  
         SUMMARY OF THE INVENTION  
         [0015]    A principal object of the present invention is to achieve a method and an apparatus for a single sensor camera to estimate missing color information provided by a color filter array.  
           [0016]    A further object of the present invention is to achieve a system and an apparatus having a high quality interpolation with low computational costs  
           [0017]    Another further object of the present invention is to achieve a method and an apparatus providing a high quality color image.  
           [0018]    In accordance with the objects of this invention a method to obtain a full color image by estimating the missing color information of a single sensor camera has been achieved. Said method comprises providing an exposure unit comprising a color filter array (CFA), an analog/digital converter of the raw image, an interpolation unit, and a color processing unit. The steps of the method are to interpolate the luminance channel used as proxy for green color, to interpolate the chrominance channel used as proxy for red and blue, to correct the luminance channel, and to correct the chrominance channel.  
           [0019]    In accordance with said objects of the invention a method to obtain a full color image by estimating the missing color information of a single sensor camera has been achieved. Said method comprises providing an exposure unit comprising a C-M-Y color filter array (CFA), an analog/digital converter of the raw image, an interpolation unit, and a color processing unit. The steps of the method are to interpolate the luminance channel used as proxy for magenta, to interpolate the chrominance channel used as proxy for yellow and cyan, to correct the luminance channel, and to correct the chrominance channel.  
           [0020]    In accordance with the objects of this invention a method to obtain a full color image by estimating the missing color information of a single sensor camera has been achieved. Said method comprises providing an exposure unit comprising a Diagonal Bayer Pattern color filter array (CFA), an analog/digital converter of the raw image, an interpolation unit, and a color processing unit. The steps of the method are to interpolate the luminance channel used as proxy for green color, to interpolate the chrominance channel used as proxy for red and blue, to correct the luminance channel, and to correct the chrominance channel.  
           [0021]    In accordance with said objects of the invention a method to obtain a full color image by estimating the missing color information of a single sensor camera has been achieved. Said method comprises providing an exposure unit comprising a color filter array (CFA), an analog/digital converter of the raw image, an interpolation unit, and a color processing unit. The first steps of said method comprise to compute inverted and normalized gradient functions in the four compass directions using luminance and chrominance data, to interpolate the luminance considering luminance and chrominance data using results of previous step and to estimate chrominance at opposite chrominance locations using luminance and chrominance data considered. The next steps comprise to compute gradient functions, using chrominance and luminance data in specific directions, to interpolate chrominance considering chrominance and luminance data using results from previous step, to compute two gradient functions, horizontally and vertically, using information from all color channels, to correct the luminance channel horizontally and vertically if the gradient functions are not equal. The last steps comprise to estimate chrominance at opposite chrominance locations using enhanced luminance and chrominance data considered, to compute inverted and normalized gradient functions using enhanced chrominance and luminance data in specific directions, and to interpolate chrominance considering chrominance and enhanced luminance data using results from previous step.  
           [0022]    In accordance with said objects of the invention an apparatus to obtain a full color image by estimating the missing color information of a single sensor camera has been achieved. Said apparatus comprises an exposure unit comprising a color filter array (CFA), an analog/digital converter of the raw image, an interpolation unit, wherein said interpolation unit is estimating the missing color information by a set of weighted values obtained by gradient functions and wherein the set of weighted functions is found from neighboring pixels in specific directions, and a color processing unit.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    In the accompanying drawings forming a material part of this description, there is shown:  
         [0024]    [0024]FIG. 1 prior art shows the basic layout of a R-G-B color array  
         [0025]    [0025]FIG. 2 shows a functional block diagram of the components of single sensor CFA camera having an interpolation unit.  
         [0026]    [0026]FIG. 3 shows a flowchart of the major steps of the method invented.  
         [0027]    [0027]FIG. 4 shows a flowchart of the detailed steps of a embodiment of the method invented.  
         [0028]    [0028]FIG. 5 shows a flowchart of the detailed steps of an alternative embodiment of the method invented  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    The preferred embodiments disclose an apparatus and a method to achieve high quality images by using a high quality interpolation of chrominance and luminance values combined with a low computational cost.  
         [0030]    The RGB color space primarily indicates the levels of Red, Green and Blue in a photosite location and thus is a measure of the spectrum. An RGB color space image such as one resulting from color interpolating of a Bayer pattern image also has both “chrominance” and “luminance” information. Luminance refers generally to the level of brightness in an image or photosite, whereas chrominance refers to the hue or placement of color in the color spectrum and saturation or intensity of that hue. A one-color per photosite image is equivalent to a sum of luminance and opponent chromatic signals. In the case of a regular CFA arrangement, luminance and chrominance are well localized in the spatial frequency domain, providing a way to estimate them by appropriate frequency selections. In a RGB image, the Green color plane associated pixels contain most of the luminance information while Blue and Red associated pixels contain most of the chrominance information. This is why a Bayer CFA consists of twice as much “green color filters” than “red or blue color filters”. The fact, that the luminance, approximated by green image data, is more accurately estimated, is the reason to interpolate the chrominance channels, approximated by red and blue color channels, after the luminance channel. Based on this, the luminance is interpolated as accurate as possible to avoid that data from that channel will produce wrong modifications in the chrominance channels.  
         [0031]    A fundamental part of the invention is that luminance is used as a proxy for the green channel and chrominance is used as a proxy for the red and blue channel. The fact, that the luminance is more accurate, is the reason to interpolate the chrominance channels after the luminance channel. Based on this the luminance is interpolated as accurate as possible to avoid that data from that channel will produce wrong modifications in the chrominance channels.  
         [0032]    [0032]FIG. 2 shows the basic components of the front end  20  of a single sensor CFA invented. Said front end  20  comprises two parts, a recording unit  21  and a processing unit  22 . Said recording unit  21  comprises two modules, an exposure unit  23  and an analog/digital converter of the raw image  24 , said processing unit  22  comprises two modules, an interpolation unit  25  and a color-processing unit  26 . Said interpolation unit  25  is a key part of the invention.  
         [0033]    [0033]FIG. 3 shows a functional block diagram of the interpolation method invented Said method comprises two basic steps. The first basic step  30  comprises the interpolation of the luminance channel  300  and the interpolation of the chrominance channels  301 , the next basic step  31  comprises the correction of the luminance channel  310  followed by the correction of the chrominance channel  311 .  
         [0034]    [0034]FIG. 4 shows a more detailed functional block diagram compared to the overview of FIG. 3. Step  300  of FIG. 3 is divided into two steps, steps  40  and  41  of FIG. 4. The algorithm invented begins in step  40  with an examination of the neighborhood considered, where it looks for high spatial frequency information in the four compass directions; north, south, east and west. This examination involves gradient information not only from the luminance channel, but also from the color channel that is to be interpolated. Since we want to interpolate along edges and not across them, we form an inverted gradient function, f grad , which weights the gradient functions so that the smallest gradient carries the greatest weight, while the greatest gradient carries the least weight. The intention of weighting the gradients is to find a value well representing the surrounding neighbors. In order to simplify the interpolation equations, the function f grad  is normalized, represented by F n . In order to get an even more accurate estimation of the missing luminance value, the chrominance values are used for correction to get a better fit between the different color channels.  
         [0035]    First the gradient functions are defined as:  
         Δ W=| 2 L   (x−1,y)   −L   (x−3,y)   −L   (x+1,y)   |+|C   (x,y)   −C   (x−2,y) |,  
         Δ E=| 2 L   (x+1,y)   −L   (x−1,y)   −L   (x+3,y)   |+|C   (x,y)   −C   (x+2,y) |,  
         Δ N=| 2 L   (x,y−1)   −L   (x,y−3)   −L   (x,y +1)   |+|C   (x,y)   −C   (x,y−2) |,  
         Δ S=| 2 L   (x,y+1)   −L   (x,y−1)   −L   (x,y+1)   |+|C   (x,y)   −C   (x,y+2) |  (1 )  
         [0036]    wherein L (i,j)  is the luminance, approximated by green and C (i,j)  is the chrominance, approximated by red and blue.  
         [0037]    Next, these gradients are inverted:  
                             f     grad                       (   x   )       =     1   x             for                x     ≠   0     ,     and             (   2   )                         f     grad                       (   x   )       =   1                               for                   x     =   0     ,                   (   2   )                               
 
           f grad( x )=1 for  x= 0  (2)  
         [0038]    wherein x are the gradient functions of the equations (1) shown above.  
         [0039]    Next, the inverted gradient functions are normalized:  
                 F   n          (   x   )       =         f   grad          (   x   )             f   grad          (     Δ                 W     )       +       f   grad          (     Δ                 E     )       +     fgrad        (     Δ                 N     )       +       f   grad          (     Δ                 S     )                   (   3   )                               
 
         [0040]    wherein x are the gradient functions of the equations (1) shown above. The next step  41  is the interpolation of missing luminance values utilizing the normalized inverted gradient functions F n (x) as shown above:  
                           L     (     x   ,   y     )       =           F   n          (     Δ                 W     )            L     (       x   -   1     ,   y     )         +         F   n          (     Δ                 E     )            L     (       x   +   1     ,   y     )         +                       F   n          (     Δ                 N     )            L     (     x   ,     y   -   1       )         +         F   n          (     Δ                 S     )            L     (     x   ,     y   +   1       )                 ++                    C     (     x   ,   y     )       -         F   n          (     Δ                 W     )            C     (       x   -   2     ,   y     )         -         F   n          (     Δ                 E     )            C     (       x   +   2     ,   y     )         -                     F   n          (     Δ                 N     )            C     (     x   ,     y   -   2       )         -         F   n          (     Δ                 S     )            C     (     x   ,     y   +   2       )                 2       ,           (   4   )                               
 
         [0041]    wherein L (i,j)  is the luminance, approximated by green and C (i,j)  is the chrominance, approximated by red or blue.  
         [0042]    Step  301  of FIG. 3, comprising the interpolation of chrominance channels, is divided into the three steps  42 ,  43 , and  44  of FIG. 4. Said chrominance channels are approximated by red and blue. The assumption that the green channel is a good proxy for luminance and the red and blue channel for chrominance is further utilized and thus the interpolation in the chrominance channels is done in the same manner. Step  42  comprises the estimation of chrominance at opposite chrominance locations using luminance and chrominance data considered. The next equation shows for example:  
               C     (     x   ,   y     )       =       L     (     x   ,   y     )       +         (       C     (       x   -   1     ,     y   -   1       )       -     L     (       x   -   1     ,     y   -   1       )         )     +     (       C     (       x   +   1     ,     y   -   1       )       -     L     (       x   +   1     ,     y   -   1       )         )       4     +         (       C     (       x   -   1     ,     y   +   1       )       -     L     (       x   -   1     ,     y   +   1       )         )     +     (       C     (       x   +   1     ,     y   +   1       )       -     L     (       x   +   1     ,     y   +   1       )         )       4               (   5   )                               
 
         [0043]    wherein L (i,j)  is the luminance, approximated by green and wherein C (i,j)  is the chrominance, approximated by red and blue.  
         [0044]    In the following step  43  the inverted and normalized gradient functions are computed in the four compass directions using chrominance and luminance data. In the above shown equation (5) the chrominance channels are populated to the same structure as the luminance had before the interpolation. Said previous step  42  is enhanced now by introducing an examination of the gradients in the four compass directions: north, south, east and west. The gradient functions are formed as:  
         Δ W=| 2 C   (x−1,y)   −C   (x−3,y)   −C   (x+1,y)   |+|L   (x,y)   −L   (x−1,y) |,  
         Δ E=| 2 C   (x+1,y)   −C   (x−1,y)   −C   (x+3,Y)   |+|L   (x,y)   −L   (x+1,y) |,  
         Δ N =| 2 C   (x,y−1)   −C   (x,y−3)   −C   (x,y+1)   |+|L   (x,y)   −L   (x,y−1) |,  
         Δ S=| 2 C   (x,y+1)   −C   (x,y−1)   −C   (x,y+3)   |+|L   (x,y)   −L   (x,y+1) |  (6)  
         [0045]    wherein C (i,,j)  is the chrominance, approximated by red or blue and wherein L (i,j)  is the luminance, approximated by green.  
         [0046]    These gradient functions are inverted and normalized now equivalent to the inversion and normalization shown in equation (2) and (3). The next step  44  comprises interpolating the chrominance channels by the interpolation at luminance locations utilizing the inverted gradient function F n (x) as:  
           C   (x,y)   =F   n (Δ W ) C   (x−1,y)   +F   n (Δ E ) C   (x+1,y)   +F   n (Δ N ) C   (x,y−1)   +F   n (Δ S ) C   (x,y+1)   ++L   (x,y)   F   n (Δ W ) L   (x−1,y)   −F   n (Δ E ) L   (x+1,y)   −F   n (Δ N ) L   (x,y−1)   −F   n (Δ S ) L   (x,y+1)   (7)  
         [0047]    wherein C (i,j)  is the chrominance, approximated by red or blue and wherein L (i,j)  is the luminance, approximated by green.  
         [0048]    Step  310  of FIG. 3, comprising the correction of the computed values of the luminance channel, is divided into the three steps  45 ,  46 , and  47  of FIG. 4. In step  45  the correction starts by defining horizontal gradient function ΔH and vertical gradient function ΔV. Said correction contains gradient information from all color channels. The equivalent equations are:  
         Δ H=| 2 L   (x,y)   −L   (x−1,y)   −L   (x+1,y)   |+|C   (x−1,y)   R   −C   (x+1,y)   R   |+|C   (x−1,y)   B   −C   (x+1,y)   B | and  
         Δ V=| 2 L   (x,y)   −L   (x,y−1)   −L   (x,y+1)   |+|C   (x,y−1)   R   −C   (x,y+1)   R   |+|C   (x,y−1)   B   −C   (x,y+1)   B |  (8)  
         [0049]    wherein L (i,j)  is the luminance, approximated by green, C (i,j)   R  is the red chrominance channel and C (i,j)    B  is the blue chrominance channel.  
         [0050]    In step  46  both gradient functions for ΔH and ΔV are compared. In the following step  47  both gradients are evaluated and in case both gradients are not equal an additional correction of the luminance data in horizontal and vertical direction is performed. In case both gradients are equal said correction will be skipped. For said correction of the luminance channel the following equations are applied (9):  
           L     (     x   ,   y     )       =           2        L     (     x   ,   y     )         +     L     (       x   -   1     ,   y     )       +     L     (       x   +   1     ,   y     )         4     +         2        C     (     x   ,   y     )     R       +     2        C     (     x   ,   y     )     B       -     C     (       x   -   1     ,   y     )     R     -     C     (       x   -   1     ,   y     )     B     -     C     (       x   +   1     ,   y     )     R     -     C     (       x   +   1     ,   y     )     B       16                                for          Δ                 H     &lt;     Δ                 V               L        (     x   ,   y     )       =           2        L     (     x   ,   y     )         +     L     (     x   ,     y   -   1       )       +     L     (     x   ,     y   +   1       )         4     +         2        C     (     x   ,   y     )     R       +     2        C     (     x   ,   y     )     B       -     C     (     x   ,     y   -   1       )     R     -     C     (     x   ,     y   -   1       )     B     -     C     (     x   ,     y   +   1       )     R     -     C     (     x   ,     y   +   1       )     B       16                   for                   Δ                 H     &gt;     Δ                 V                           
 
         [0051]    wherein L (i,j)  is the luminance, approximated by green, C (i,j)   R  is the red chrominance and C (i,j)   R  is the blue chrominance.  
         [0052]    Step  311  of FIG. 3, comprising the correction of the computed values of the chrominance channel, is divided into the two steps  48  and  49  of FIG. 4. The correction of the chrominance channels is performed following the same interpolation principle as used when interpolating the luminance channel.  
         [0053]    In step  48  the gradient functions are redefined compared to equation (6):  
         Δ W=| 2 C   (x−1,y)   −C   (x−2,y)   −C   (x+1,y)   |+|L   (x,y)   −L   (x−1,y) ,  
         Δ E=| 2 C   (x+1,y)   −C   (x−1,y)   −C   (x+2,y)   |+|L   (x,y)   −L   (x+1,y) |,  
         Δ N=| 2 C   (x,y−1)   −C   (x,y−2)   −C   (x,y+1)   |+|L   (x,y)   −L   (x,y−1) |,  
         Δ S=| 2 C   (x,y+1)   −C   (x,y−1)   −C   (x,y+2)   |+|L   (x,y)   −L   (x,y) |  (10)  
         [0054]    Using the modified gradient functions computed according to the above shown equations 10, the chrominance values are corrected in step  49  by interpolation using equation (7) as in step  44 .  
         [0055]    [0055]FIG. 5 describes an alternative method how to perform the interpolation and correction of the chrominance channels replacing steps  43 ,  44  and steps  48 - 49  shown in FIG. 4. The interpolation of the luminance channel, as shown in steps  40  and  41  of FIG. 4, remains unchanged. Step  42 , comprising an estimation of chrominance at opposite chrominance locations using equation (5), remains unchanged as well. Step  43  of FIG. 4, the computation of gradient functions, is replaced by step  51 . Instead of using the inverted weighted function to determine how the surrounding pixels in the four compass directions will be utilized, described in step  43 , a horizontal gradient function ΔH and a vertical gradient function ΔV are calculated in the alternative approach. Said gradient functions are formed as:  
                   Δ                 H     =              C     (       x   -   1     ,   y     )       -     C     (       x   +   1     ,   y     )              +                2        L     (     x   ,   y     )         -     L     (       x   -   2     ,   y     )       -     L     (       x   +   2     ,   y     )         2                          and                
          Δ                 V     =              C     (     x   ,     y   -   1       )       -     C     (     x   ,     y   +   1       )              +              2        L     (     x   ,   y     )         -     L     (     x   ,     y   -   2       )       -     L     (     x   ,     y   +   2       )         2              ,           (   11   )                               
 
         [0056]    wherein C (i,j)  is the chrominance, approximated by red or blue and wherein L (i,j)  is the luminance, approximated by green.  
         [0057]    The next step  52  to get fully populated chrominance channels is the interpolation of the remaining chrominance pixel values correspondent to step  44  of FIG. 4. In this alternative approach the equation (7) used in step  44  is modified in step  52  using the values of ΔH and ΔV as computed according to the equation (11) above. The modified equations to calculate the remaining pixel values are:  
                   C     (     x   ,   y     )       =       L     (     x   ,   y     )       +         (       C     (       x   -   1     ,   y     )       -     L     (       x   -   1     ,   y     )         )     +     (       C     (       x   +   1     ,   y     )       -     L     (       x   +   1     ,   y     )         )       4         ,         if          Δ                 H     &lt;     Δ                 V                   or                
              C     (     x   ,   y     )       =       L     (     x   ,   y     )       +         (       C     (     x   ,     y   -   1       )       -     L     (     x   ,     y   -   1       )         )     +     (       C     (     x   ,     y   +   1       )       -     L     (     x   ,     y   +   1       )         )       4         ,         if          Δ                 H     &gt;     Δ                 V                   or                
              C     (     x   ,   y     )       =       L     (     x   ,   y     )       +         (       C     (       x   -   1     ,   y     )       -     L     (       x   -   1     ,   y     )         )     +     (       C     (       x   +   1     ,   y     )       -     L     (       x   +   1     ,   y     )         )       4     +                +         (       C     (     x   ,     y   -   1       )       -     L     (     x   ,     y   -   1       )         )     +     (       C     (     x   ,     y   +   1       )       -     L     (     x   ,     y   +   1       )         )       4           ,                    if          Δ                 H     =     Δ                 V       ,             (   12   )                               
 
         [0058]    wherein C (i,j)  is the chrominance, approximated by red or blue and wherein L (i,j)  is the luminance, approximated by green.  
         [0059]    The following steps  45  to  47 , covering the correction of the luminance channel, remain unchanged.  
         [0060]    The computation of the gradient functions as part of the correction of the chrominance, described in step  48  of FIG. 4, can also be replaced alternatively with step  53  and is taking advantage of the enhanced luminance channel calculated in steps  45 ,  46  and  47 .  
         [0061]    The gradient functions shown in equation (11) are redefined in step  53  as follows:  
                 Δ                 H     =              C     (       x   -   1     ,   y     )       -     C     (       x   +   1     ,   y     )              +                2        L     (     x   ,   y     )         -     L     (       x   -   1     ,   y     )       -     L     (       x   +   1     ,   y     )         2                          and                
              Δ                 V     =              C     (     x   ,     y   -   1       )       -     C     (     x   ,     y   +   1       )              +              2        L     (     x   ,   y     )         -     L     (     x   ,     y   -   1       )       -     L     (     x   ,     y   +   1       )         2              ,             (   13   )                               
 
         [0062]    wherein C (i,j)  is the chrominance, approximated by red or blue and wherein L (i,j)  is the luminance, approximated by green.  
         [0063]    In this alternative approach equation (12) is used again in step  54  to perform the final correction of chrominance pixel values using both luminance and chrominance values of opposite locations. In said approach the gradient functions of equation (13) are used now.  
         [0064]    Furthermore it is possible to use the alternative steps  51  and  52 , describing the interpolation of the chrominance channel, combined with the correction of the chrominance channel described in steps  48  and  49 . It is also possible to perform the interpolation of the chrominance channel as described in steps  43  and  44  and to do the correction of the chrominance channel using the alternative steps  53  and  54 .  
         [0065]    Another alternative approach deals with the interpolation of the luminance channel modifying step  40 - 41  shown in FIG. 4 and in FIG. 5. Step  40  comprises the computation of inverted and normalized gradient functions in four compass directions. The computation of the gradient functions in the four compass directions as described in step  40  using equation (1) is replaced by a computation of the gradient functions in horizontal and vertical directions:  
                 Δ                 H     =              L     (       x   -   1     ,   y     )       -     L     (       x   +   1     ,   y     )              +                2        C     (     x   ,   y     )         -     C     (       x   -   2     ,   y     )       -     C     (       x   +   2     ,   y     )         2                          and                
              Δ                 V     =              L     (     x   ,     y   -   1       )       -     L     (     x   ,     y   +   1       )              +              2        C     (     x   ,   y     )         -     C     (     x   ,     y   -   2       )       -     C     (     x   ,     y   +   2       )         2              ,             (   14   )                               
 
         [0066]    wherein C (i,j)  is the chrominance, approximated by red or blue and wherein L (i,j)  is the luminance, approximated by green.  
         [0067]    In this alternative method the inversion of the gradients, as performed in equation (2), and the normalization of the gradient, as performed in equation (3), are obsolete and the interpolation of the luminance, shown in step  41 , is performed using the following equations (15), replacing equation (4):  
                   L     (     x   ,   y     )       =       C     (     x   ,   y     )       +         (       L     (       x   -   1     ,   y     )       -     C     (       x   -   1     ,   y     )         )     +     (       L     (       x   +   1     ,   y     )       -     C     (       x   +   1     ,   y     )         )       4         ,     
              if          Δ                 H     &lt;     Δ                 V                   or                
              L     (     x   ,   y     )       =       C     (     x   ,   y     )       +         (       L     (     x   ,     y   -   1       )       -     C     (     x   ,     y   -   1       )         )     +     (       L     (     x   ,     y   +   1       )       -     C     (     x   ,     y   +   1       )         )       4         ,     
                if          Δ                 H     &gt;     Δ                 V                   or          
          L     (     x   ,   y     )           =       C     (     x   ,   y     )       +         (       L     (       x   -   1     ,   y     )       -     C     (       x   -   1     ,   y     )         )     +     (       L     (       x   +   1     ,   y     )       -     C     (       x   +   1     ,   y     )         )       4     +                +         (       L     (     x   ,     y   -   1       )       -     C     (     x   ,     y   -   1       )         )     +     (       L     (     x   ,     y   +   1       )       -     C     (     x   ,     y   +   1       )         )       4           ,                    if          Δ                 H     =     Δ                 V       ,             (   15   )                               
 
         [0068]    wherein C (i,j)  is the chrominance, approximated by red or blue and wherein L (i,j)  is the luminance, approximated by green.  
         [0069]    The different methods described above are not limited to a R-G-B CFA only. The complementary Bayer pattern is another CFA which can be used:  
                                                       M   C           Y   M                      
 
         [0070]    Said complementary Bayer pattern comprises three colors, being complementary to the R-G-B pattern: Cyan, Magenta, and Yellow (CMY), as shown in the table above. There are two different ways to use the methods described above providing a CMY CFA. The first alternative is to convert the CMY pattern into a RGB pattern and proceed exactly as described. The other alternative is to use said CMY CFA directly by using luminance as proxy for magenta and chrominance as proxy for yellow and cyan and proceed as described above.  
         [0071]    The Diagonal Bayer Pattern and its complementary version are other alternatives of CFAs to be used with the invention presented. The table below shows such a Diagonal Bayer Pattern with the colors Red, Green and Blue.  
                                                               G   B   G   R                      
 
         [0072]    [0072]                                                               R   G   B   G           G   R   G   B           R   G   B   G                        
         [0073]    Using said Diagonal Bayer Pattern the equation (5) in step  43  has to be replaced by the following equation:.  
                 C     (     x   ,   y     )       =       L     (     x   ,   y     )       +         (       C     (       x   -   1     ,     y   +   1       )       -     L     (       x   -   1     ,     y   +   1       )         )     +     (       C     (       x   +   1     ,     y   -   1       )       -     L     (       x   +   1     ,     y   -   1       )         )       2         ,           (   16   )                               
 
         [0074]    wherein C (i,j)  is the chrominance, approximated by red or blue and wherein L (i,j)  is the luminance, approximated by green. The complementary version of said Diagonal Bayer pattern can be easily implemented by using luminance as proxy for magenta and chrominance as proxy for yellow and cyan and proceed as described above.  
         [0075]    While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.