Abstract:
This invention concerns the spatial interpolation of colour images and, in particular, the reconstruction, or “de-mosaicing” of data from a single sensor-array electronic camera. Electronic cameras typically have an image sensor comprising a matrix of individual pixel sensors, each sensor being responsive to a colour component. In order to obtain colour component information for all pixels, in accordance with the disclosed embodiment, green component pixel values ( 203 ) are interpolated to obtain green component pixel values for all pixels ( 209 ). A difference value (B−G) ( 212 ) is formed from the green colour component values ( 207 ) and the original blue component values ( 204 ) at blue pixel locations, and the difference values (B−G) ( 212 ) are then interpolated to obtain difference values for all pixels ( 216 ). Blue component values for all pixels ( 225 ) can then be obtained from the difference values for all pixels ( 216 ) and the green component values for all pixels ( 209 ).

Description:
FIELD OF THE INVENTION 
       [0001]    This invention concerns the spatial interpolation of colour images and, in a particular example, the reconstruction, or “de-mosaicing” of data from a single sensor-array electronic camera. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is now common for electronic colour cameras to have an image sensor comprising a matrix of individual pixel sensors where each pixel sensor has a colour filter in front of it so that different sensors respond to different colour components. Typically red, green and blue filters are used, although other filter colours have been proposed. Often the green pixels are more numerous than either the red or blue pixels so that the spatial sampling frequency is highest for the green component. A well-known arrangement is the “Bayer Mask” pattern of red green and blue pixels in which half of the total number of pixels have green filters, one quarter have red filters and one quarter have blue filters. This pattern is shown in  FIG. 1 . 
         [0003]    Although spatial sub-sampling of colour components is not unusual in image processing, it is highly inconvenient if the samples from the respective colour components are not co-sited; and, many processes require a value for every component of every pixel in the image. There is therefore a need to spatially interpolate the colour component images from cameras using Bayer Mask, or similar, sensor patterns to obtain co-sited, fully-sampled pixel values. 
         [0004]    Many solutions to this problem have been proposed including simple, bilinear interpolation of individual colour components and complex, adaptive filtering schemes. The simpler systems suffer from lack of image sharpness or aliassing, and even complex systems can suffer from “false colour” effects when objects have structures or textures similar to the colour filter pattern of the camera. 
       SUMMARY OF THE INVENTION 
       [0005]    In accordance with one aspect of the invention, there is provided a method of spatially interpolating pixel values of an image, wherein first colour component pixel values of the image are spatially interpolated by making use of second colour component pixel values of the said image, the second colour component pixel values not being co-sited with the first colour component pixel values, comprising the steps of: spatially interpolating the second colour component pixel values so as to obtain, at least, second colour component pixel values co-sited with the first colour component pixel values; deriving an intermediate value from each interpolated second colour component pixel value and the respective co-sited first colour component pixel value; spatially interpolating the intermediate values to obtain intermediate values at, at least, the locations of the second colour component pixel values; and combining the said intermediate values with the said second colour component pixel values to obtain spatially interpolated pixel values of the first colour component. 
         [0006]    Additionally, third colour component pixel values of the image may also be spatially interpolated by making use of second colour component pixel values of the said image, the second colour component pixel values not being co-sited with the third colour component pixel values, wherein the step of spatially interpolating the second colour component pixel values includes obtaining second colour component pixel values co-sited with third colour component pixel values; further comprising the steps of: determining a second intermediate value from each interpolated second colour component pixel value and the respective co-sited third colour component pixel value; spatially interpolating the second intermediate values to obtain second intermediate values at, at least, the locations of the second colour component pixel values; and combining the said second intermediate values with the said second colour component pixel values to obtain spatially interpolated pixel values of the third colour component. 
         [0007]    In accordance with a second aspect of the invention, there is provided an apparatus for spatially interpolating pixel values of an image, wherein first colour component pixel values of the image are spatially interpolated by making use of second colour component pixel values of the said image, the second colour component pixel values not being co-sited with the first colour component pixel values, comprising: means for spatially interpolating the second colour component pixel values so as to obtain, at least, second colour component pixel values co-sited with the first colour component pixel values; combining means for forming an intermediate value from the interpolated second colour component pixel value and the respective co-sited first colour component pixel value; means for spatially interpolating the intermediate values to obtain intermediate values at, at least, the locations of the second colour component pixel values; and combining means for combining the said intermediate values with the said second component pixel values to obtain spatially interpolated pixel values of the first colour component. 
         [0008]    Preferably the second colour component pixel values are more numerous than first colour component pixel values. 
         [0009]    Suitably the intermediate values and/or the second intermediate values may be difference values. 
         [0010]    Suitably, the filter aperture used to spatially interpolate the said second colour component pixel values is larger than the filter aperture used to interpolate the said intermediate values. 
         [0011]    Advantageously, the filter aperture used to spatially interpolate the said second colour component pixel values has at least one negative coefficient value. 
         [0012]    Advantageously, the filter aperture used to interpolate the said intermediate values has no negative coefficient values. 
         [0013]    Advantageously, the filter used to spatially interpolate the said second colour component pixel values has a non-variables-separable spatial frequency response. 
         [0014]    Advantageously, the shape of the spatial frequency response of the filter used to spatially interpolate the said second colour component pixel values is related to the pattern of its input pixel values, so that the direction of lowest spatial frequency response of the said filter corresponds to the direction of lowest spatial frequency of the said pattern of input pixel values. 
         [0015]    Suitably, the second colour component pixel values are interpolated by a diagonal filter. 
         [0016]    Suitably, the second colour component is Green and the first colour component is Blue. 
         [0017]    Suitably, the second colour component is Green and the first colour component is Red. 
         [0018]    The pixels may be derived from a Bayer Mask. 
         [0019]    In yet a further aspect, the present invention consists in apparatus comprising a monochromatic image sensor array, a filter arrangement disposed in relation to the sensor array to provide colour discrimination and a processor constructed and adapted to spatially interpolate pixel values in a method as above defined. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    An example of the invention will now be described with reference to the drawings in which: 
           [0021]      FIG. 1  shows the known “Bayer Mask” pattern. 
           [0022]      FIG. 2  shows a colour component pixel interpolator according to an embodiment of the invention. 
           [0023]      FIG. 3   a  shows a filter aperture and coefficients of a two-dimensional spatial interpolator. 
           [0024]      FIGS. 3   b ,  3   c  and  3   d  show three alternative spatial interpolator filter apertures and coefficients. 
           [0025]      FIG. 4  shows diagrammatically the arrangement of a sensor array and a Bayer Mask. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    The invention will now be described in the context of the spatial interpolation of pixel values from a Bayer mask pattern as shown in  FIG. 1 . Clearly, the invention is also applicable to other patterns of spatial sub-sampling of colour components. As is well understood, the Bayer Mask  101  is positioned in front of a monochromatic image sensor  102 , with pixel values of the sensor being taken to a processor  103 , which, inter alia, generates the required output pixel values. 
         [0027]    Referring to  FIG. 2 , input data ( 201 ), representing the values of colour component pixels arranged in the pattern of  FIG. 1 , is demultiplexed ( 202 ) into three sets of primary colour component data: green pixel data ( 203 ), blue pixel data ( 204 ) and red pixel data ( 205 ). 
         [0028]    The green pixel data ( 203 ) is passed to a two-dimensional spatial interpolator, or filter, ( 206 ) which interpolates green-component values ( 207 ) for the “missing” pixels, i.e. the red- and blue-component pixels in the data ( 201 ) which were not demultiplexed to the data ( 203 ). 
         [0029]    The interpolator ( 206 ) calculates the required green-component value for each pixel missing a green-component value from a weighted sum of input pixels with green-component values according to a filter aperture extending at least two pixel positions away from the position of the output pixel. It is important that the interpolator filter has a relatively flat pass-band and it will therefore preferably include some negative coefficients. A suitable filter aperture and its coefficients are shown in  FIG. 3   a , and the extent of the filter aperture is shown ( 301 ). 
         [0030]    It is also advantageous to exploit the difference between the sampling patterns for the respective colour components. In the “Bayer” pattern of  FIG. 1  the green pixels have a diagonal pattern whereas the blue and red pixels both have horizontal and vertical patterns. The interpolator ( 206 ) has green input pixels and therefore, preferably, has a diagonal spatial stop-band, i.e. the cut-off frequencies for horizontal and vertical spatial frequencies are higher than the cut-off frequencies for diagonal spatial frequencies. It is also advantageous if this filter has a “non-variables-separable” spatial frequency response, i.e. a response which cannot be obtained by convolution of a horizontal filer with a vertical filter. 
         [0031]    Returning to  FIG. 2 , the original green pixel values ( 203 ) are combined with the interpolated green pixel values ( 207 ) in a multiplexer ( 208 ) to produce green-component pixel values ( 209 ) for all positions of the mask. 
         [0032]    In an alternative embodiment the interpolator ( 206 ) could also output filtered versions of its input pixels, and the multiplexer ( 208 ) would not be needed because all the pixel values ( 209 ) would be output from the interpolator ( 206 ). In this case a suitable interpolation filter aperture could be obtained by convolving the filter of  FIG. 3   a  with a low-pass filter having a relatively high cut-off frequency and slow rate of cut. 
         [0033]    The advantage of this alternative is that all the pixels ( 209 ) are filtered with substantially the same spatial frequency response. The filter characteristic to be applied (in the interpolator ( 206 )) to those pixels ( 209 ) which are co-sited with pixels ( 203 ) should be selected to have substantially the same frequency response (particularly at lower frequencies) as that applied to the non-co-sited pixels ( 209 ). This avoids impairments to the image due to differences between these two types of pixel. 
         [0034]    A multiplexer ( 210 ) selects the interpolated green-component values for the those pixels at the blue positions of the mask. These selected values are subtracted ( 211 ) from the blue-component of the original blue-component pixels ( 204 ) to give Blue minus Green (B−G) values for these positions ( 212 ). A spatial interpolator ( 213 ) interpolates (B−G) values for the remaining pixel positions i.e. for the original green-component pixels ( 203 ) and original red-component pixels ( 205 ). 
         [0035]    The interpolator ( 213 ) calculates the required (B−G) values for the remaining pixel positions from a weighted sum of (B−G) values at the original blue-component pixel positions ( 212 ) according to a filter aperture that is smaller than that of the interpolator ( 206 ), and preferably extends no more than one pixel position away from the position of the output pixel. Typical filter apertures are shown in  FIGS. 3   b ,  3   c  and  3   d , and their respective extents are shown at ( 302 ), ( 303 ) and ( 304 ) respectively. 
         [0036]    The filter of  FIG. 3   b  is suitable for interpolating (B−G) values at original green-component pixel positions in the blue/green rows of the mask. In this case there are only two available (B−G) values no more than one pixel position away from the output position, and they are averaged to obtain the output value. 
         [0037]    The filter of  FIG. 3   c  is suitable for interpolating (B−G) values at original red-component pixel positions in the mask; in this case there are four available (B−G) values no more than one pixel position away from the output position, and they are averaged to obtain the output value. 
         [0038]    The filter of  FIG. 3   d  is suitable for interpolating (B−G) values at original green-component positions in the red/green rows of the mask; in this case there are two available (B−G) values no more than one pixel position away from the output position, and they are averaged to obtain the output value. 
         [0039]    The interpolated values ( 214 ) are input to a multiplexer ( 215 ) which combines them with the (B−G) values ( 212 ) at the original blue component pixel locations to give (B−G) values for every pixel position ( 216 ). 
         [0040]    In an alternative embodiment the interpolator ( 213 ) could also output filtered versions of its input pixels, and the multiplexer ( 215 ) would not be needed because all the pixel values ( 216 ) would be output from the interpolator ( 213 ). The necessary filter could be derived from the filters shown by convolution with a low-pass filter as described above. As explained above (for the interpolator ( 206 )) the objective is to apply substantially similar spatial frequency responses to all the pixel values ( 216 ). 
         [0041]    (R−G) values for every pixel position are derived in an exactly analogous way to the above-described derivation of the (B−G) values. A multiplexer ( 217 ) selects those of the interpolated green component values ( 207 ) for those pixels at red positions of the mask. These green component values ( 207 ) are subtracted ( 218 ) from the red component of the original red input pixels ( 205 ) to give (R−G) values ( 219 ) at the original red-component pixel positions. The (R−G) component values ( 219 ) are interpolated ( 220 ) to provide additional (R−G) component values ( 221 ) at the original blue- and green-component pixel positions. The interpolated (R−G) component values ( 221 ) are combined ( 222 ) with the (R−G) values at the original red-component pixels ( 219 ) to give (R−G) values for every pixel position ( 223 ). 
         [0042]    The operation of interpolator ( 220 ) is the same as that of the interpolator ( 212 ) except for the fact that its input samples are located at red positions of the mask and interpolated output samples are required at blue and green mask positions. The alternative embodiment, in which all the samples ( 223 ) are filtered in the interpolator ( 220 ) so that there are no substantial differences in the applied spatial frequency characteristics, is equally suitable. 
         [0043]    Finally, the (B−G) values for every pixel ( 216 ) are added ( 224 ) to the green values ( 209 ) to give a blue value output for every pixel ( 225 ); and the (R−G) values for every pixel ( 223 ) are added ( 226 ) to the green values ( 209 ) to give a red value output for every pixel ( 227 ). 
         [0044]    The invention described in the exemplary embodiment above thus provides a simple and effective method of spatial interpolation of pixel colour component values. 
         [0045]    The invention has been described by way of example and other embodiments of the inventive concept are possible. For example there is no need for any of the output pixels ( 209 ) ( 225 ) and ( 227 ) to be co-sited with any of the input pixels ( 201 ) provided that the interpolators ( 206 ) ( 213 ) and ( 220 ) produce values at the required output pixel locations. Although it is important that co-sited pixels are available for input to the subtractors ( 211 ) and ( 217 ), there is no need for the pixel positions for which subtraction takes place to correspond to output pixel positions. 
         [0046]    The green pixels ( 203 ) could be interpolated to a higher resolution than the total number of pixels in the mask, and the number of interpolated colour difference values increased to obtain R, G and B values at this higher resolution. 
         [0047]    Alternatively the invention could be used to provide a down-sampled output in which the output is at a lower resolution. In this case the characteristics of the interpolators ( 206 ) ( 213 ) and ( 220 ) will preferably need to be chosen to reject the alias components that would otherwise result. 
         [0048]    In further embodiments, more or less than three primary colour components could be used; only one lower resolution component could be interpolated; and colours other than red green and blue could be used as primaries.