Abstract:
A method for reading out pixel values for a two dimensional array, the method includes the steps of periodically combining at least two pixel values of a same first color in a first dimension of the two dimensional array; reading out the combined pixel values of the first color; and reading out pixels of a second color in the first dimension of the two dimensional array.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to the field of subsampling of pixels of an image sensor. More specifically, the invention relates to subsampling pixels of an image sensor that changes pixel sensitivity to account for scene illuminant changes.  
       BACKGROUND OF THE INVENTION  
       [0002]     Subsampling image sensors for reducing spatial resolution is performed for purposes such as increasing processing speed and the like. Referring to  FIG. 1 , there is shown a Bayer color filter array (CFA) used for creating color images on the image sensor. To reduce resolution, prior art image sensors are subsampled by skipping pixels (designated by the X) as shown in  FIG. 2 . Alternatively, the prior art reduces resolution by combining pixels as shown in  FIG. 3 . In this regard, the individual colors are combined both vertically and horizontally as shown by the curved lines. The resulting subsampled image for both  FIG. 2  and  FIG. 3  have sampling characteristics that are not optimal for representing the subsampled image. Specifically, the pixels that comprise the subsampled image are not evenly spaced but are instead grouped together. This results in a perceived lower resolution image than might be anticipated by the amount of subsampling done.  
         [0003]     Consequently there exists a need for better subsampling of an image. Also, since subsampling is often needed to reduce spatial resolution and increase speed for the purpose of generating a sequence of images for video, there is also a need to improve the sensitivity of one color relative to another color to account for changes in scene illuminant, for example, if the scene is illuminated with tungsten light. Therefore, a further need exists to accomplish subsampling in a manner that also changes the sensitivity of specific colors to match the scene illuminant.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the present invention is directed to a method for reading out pixel values for a two dimensional array, the method comprising the steps of periodically combining at least two pixel values of a same first color in a first dimension of the two dimensional array; reading out the combined pixel values of the first color; and reading out pixels of a second color in the first dimension of the two dimensional array.  
         [0005]     These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.  
       ADVANTAGEOUS EFFECT OF THE INVENTION  
       [0006]     The present invention has the following advantage of permitting selective combining and subsampling of colors based on lighting conditions in order to improve the inherent white balance, or chromatic sensitivity, of the image sensor, while also improving the spatial sampling pattern of the resulting reduced resolution image. It also includes the advantage of permitting extended dynamic range in the reduced resolution image when certain combining and subsampling approaches are used. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a Bayer color filter array pattern;  
         [0008]      FIG. 2  is a prior art decimation arrangement for a Bayer color filter array pattern that skips pixels;  
         [0009]      FIG. 3  is a prior art decimation arrangement that combines pixels;  
         [0010]      FIGS. 4   a  and  4   b  are decimation arrangements that skip some pixels and combine some pixels in the vertical direction to favor certain colors;  
         [0011]      FIGS. 5   a  and  5   b  are decimation arrangements that skip some pixels and combine some pixels in the horizontal direction to favor certain colors;  
         [0012]      FIGS. 6   a  and  6   b  are decimation arrangements that skip some pixels and combine some pixels in both vertical and horizontal directions to favor certain colors;  
         [0013]      FIGS. 7   a  and  7   b  are alternative decimation arrangements that skip some pixels and combine some pixels in the vertical direction to favor alternative colors from  FIGS. 4   a  and  4   b;    
         [0014]      FIGS. 8   a  and  8   b  are alternative decimation arrangements that skip some pixels and combine some pixels in the horizontal direction to favor alternative colors from  FIGS. 5   a  and  5   b;    
         [0015]      FIGS. 9   a  and  9   b  are alternative decimation arrangements that skip some pixels and combine some pixels in both vertical and horizontal directions to favor alternative colors from  FIGS. 6   a  and  6   b ; and  
         [0016]      FIGS. 10   a  and  10   b  are decimation arrangements that skip some pixels and combine some pixels in both the vertical and horizontal directions provide both combined and non-combined green results. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     The invention will be described in the context of a commonly known Bayer color filter array pattern. It is to be understood that the present invention is applicable to both types of image sensors, CCDs (charge-coupled devices) and active pixel (also commonly known as CMOS) image sensors. It is also noted that a two-dimensional array can be retrieved from any image whether on a sensor or in electronic memory. Specifically, the invention will first describe subsampling in the y direction that also improves the sensitivity of the subsampled blue pixels ( FIGS. 4   a  and  4   b ), then will describe subsampling in the x direction with similar improvement in blue sensitivity ( FIGS. 5   a  and  5   b ), and then will describe subsampling in both y and x directions with still further improvement in blue sensitivity ( FIGS. 6   a  and  6   b ).  
         [0018]     Referring to  FIG. 4   a , there is shown a top view of a two dimensional image sensor of the present invention. The sensor is covered and spanned by a color filter array, typically a Bayer color filter, for creating pixels containing different colors. The present invention is directed to subsampling the image sensor to create a smaller resolution image that exhibits a different color balance than that of a non-subsampled image or prior art subsampled images. For example, the non-subsampled image may be better suited for capturing images in daylight and the subsampled image may be better suited for capturing images in incandescent light. To achieve this, the image sensor of the present invention periodically combines at least two pixel values of a same first color, the color blue in column  2 , in the y direction of the two dimensional array. It is further noted that green is combined in column  1 . The combined pixel values of the first color are read out of the sensor, via a horizontal shift register in a CCD image sensor and by addressable column circuits in a CMOS image sensor. The non-combined pixels are also read out in the y direction of the two dimensional array The non-combined colors are red in column  1  and green in column  2 .  
         [0019]     The second color is periodically skipped or not read out. The second color is red in column  1  and green in column  2 . This is accomplished by not addressing these pixels in a CMOS image sensor and by a fast dump gate in a CCD image sensor. The periodicity is formed by skipping the color between the combined colors. The skipped colors are indicated by Xs in  FIG. 4   a.    
         [0020]     Referring to  FIG. 4   b , subsampling by the above described method produces an image having combined first colors and second colors that are uniform in spacing.  
         [0021]     Referring to  FIGS. 5   a  and  5   b , it is seen that the image may alternatively be subsampled in the x direction by repeating the above described process with column  1  being analogous to row  1  and column  2  analogous to row  2 . Furthermore, it is noted that the subsampling in the x and y directions may be done separately or combined as in  FIGS. 6   a  and  6   b  for producing the desired subsampled image.  FIGS. 6   a  and  6   b  have the further advantage of additional improvement in blue sensitivity, because additional blue pixels from the original image are combined.  
         [0022]     The invention has been described in a manner that improves the sensitivity to blue light. Referring to  FIGS. 7   a  and  7   b , there is shown an alternative embodiment analogous to  FIGS. 4   a  and  4   b  but with improved sensitivity to red light instead of blue light. In this regard, the image sensor of the present invention periodically combines at least two pixel values of a same first color, the red in column  1 , in the y direction of the two dimensional array. It is further noted that green is combined in column  2 . The combined pixel values of the first color are read out to the sensor. The non-combined pixels are also read out in the y direction of the two dimensional array The non-combined colors are blue in column  2  and green in column  1 .  
         [0023]     The second color is periodically skipped or not read out. The second color is blue in column  2  and green in column  1 . The periodicity is formed by skipping the color between the combined colors. The skipped colors are indicated by Xs in  FIG. 7   a.    
         [0024]     Similar to the correspondence between  FIGS. 4   a  and  4   b  and  7   a  and  7   b , there are corresponding subsampling arrangements but with improved sensitivity to red light:  FIGS. 8   a  and  8   b  corresponding to  FIGS. 5   a  and  5   b , and  FIGS. 9   a  and  9   b  corresponding to  FIGS. 6   a  and  6   b . Improved sensitivity to blue light may be suited to capturing images illuminated by incandescent light, whereas improved sensitivity to red light may be suited to capturing images illuminated by fluorescent light. Choosing one subsampling scheme over another may be performed automatically by sensor circuitry or by algorithmic examination of the sensor scene data. The selectivity may be performed manually by simply permitting the user to override the automatic function and select the appropriate light based on the sensed lighting conditions.  
         [0025]     Referring to  FIGS. 4   b ,  5   b ,  7   b , and  8   b , it is seen that in addition to the combined blue or red color there are also both a non-combined green result and a combined green result. These two green results effectively provide two different green sensitivities in the resulting image. The two green sensitivities can be used to extend the dynamic range of the captured image, with the non-combined green pixels being effective at capturing and reproducing bright areas of the image, the combined green pixels being effective at capturing and reproducing dark areas of the image, and areas of the image that are between the light and dark areas may be effectively represented by both the non-combined and the combined green pixels. Furthermore, referring to  FIGS. 10   a  and  10   b , selectively combining and not combining green pixels in both the horizontal and vertical directions (analogous to combining blue pixels and not combining red pixels in  FIGS. 6   a  and  6   b  or to combining red pixels and not combining blue pixels in  FIGS. 9   a  and  9   b ) provides a further increase in dynamic range based on the green pixels while not affecting the speed of the red and blue combined pixels relative to each other.  
         [0026]     The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.