Abstract:
The invention is directed to a method and a device for providing higher performance imaging capture using a novel pattern of a color filter array that enables one to achieve higher sampling rate for luminance than for chrominance. A majority of the elements in the CFA are neutral elements. Gray scale images with high spatial resolution and high light sensitivity can be acquired from image samplings at these neutral elements. The remaining elements of the CFA are color filtered ones. Color elements cluster to form a repeating block pattern. Color images with low spatial resolution but low color artifacts can be acquired from image samplings at these color filter elements. The color images are transformed into a luminance-chrominance color space, and the luminance components are replaced with the gray scale images to regain high spatial resolution.

Description:
CROSS-RELATED APPLICATION 
       [0001]    Under 35 U.S.C. 119(e)1), this application claims the benefit of provisional application serial number, 60/758,361, filed Jan. 12, 2006, entitled, “Color filter array with neutral elements and color image formation.” 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates generally to the field of electronic photography, and in particular to electronic imaging apparatus having a single imaging sensor and a color filter array. 
       BACKGROUND OF INVENTION 
       [0003]    A color filter array (CFA) is a required component in single-chip color imaging devices. Its pattern, consisting of color filter elements, allows “color-blind” sensors such as Charge Coupled Devices (CCD), complimentary metal oxide semiconductor (CMOS) and charge-injection device (CID) sensors to capture vivid full-color images. The most commonly adopted CFA pattern is the Bayer pattern as cited in U.S. Pat. No. 3,971,065 and incorporated herein by reference, which consists of 50% of green elements and 25% of elements for red and blue respectively. 
         [0004]    In Bayer&#39;s patent, the CFA was initially claimed to be comprised of one type of luminance element (Y), and two types of chrominance elements (C 1  and C 2 ). Such a pattern was employed based on the recognition of human visual system&#39;s relatively greater ability to discern luminance detail. However, as the exact chrominance filters as required by Bayer have not yet been invented, green filters are commonly used to substitute for Y, red and blue filters for C 1  and C 2  respectively. 
         [0005]    Some other patents (U.S. Pat. Nos. 4,663,661, 5,374,956, 6,917,381) have proposed CFA with more than 50% of green elements, attempting to achieve higher resolution in luminance. However, the green component is not exactly the same as the luminance component. For reddish and bluish scenes, the luminance estimation based only upon green elements would be far from the true value. In these cases, special CFAs may be favorable. For instance, a CFA comprising 50% red elements has been proposed for applications of imaging internal human body organs (U.S. Pat. No. 6,783,900 B2), as these images are usually reddish. 
         [0006]    Previous patents such as U.S. Pat. Nos. 5,323,233, 5,914,749, 6,476,865B1 and 6,714,243B1 disclosed CFA patterns wherein partial elements have no spectral selectivity to directly detect luminance. In patents U.S. Pat. Nos. 6,476,865B1, and 6,714,243B1, 25% of elements are neutral. In patents U.S. Pat. Nos. 5,323,233 and 5,914,749 incorporated herein by reference, the proportion of luminance elements is as high as 50%. 
         [0007]    However, the 50% sampling rate of luminance is not necessarily the optimal design for a human visual system. In image files saved in JPEG format, the most popular image format at present, more than 50% of data bits are for luminance. For example, the average amount of luminance data in high quality JPEG images is about 72%, and the number in low quality JPEG images is about 85% (G. Luo, “A novel color filter array with 75% transparent elements” Proceedings SPIE Vol. 6502, (Jan. 29, 2007), Appendix. In press). In other words, about 28% or 15% of data are of chrominance. The quality of JPEG images is not obviously attenuated when so little data bits are used for chrominance. There is a need therefore for a CFA pattern that may better match human visual system, popular image formats as well as address color artifacts that sparsely chrominance sampling may cause. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is directed to a method and a device for providing higher performances of image capturing and rendering than conventional methods and devices. 
         [0009]    An object of the present invention is to provide CFA patterns that can achieve higher sampling rate for luminance than for chrominance. A majority of the CFA&#39;s are neutral elements without color selectivity. These elements can be neutral density filters, which only reduce the intensity of light, or completely transparent, which do not cause light energy loss. Gray scale images with high spatial resolution and high light sensitivity can be acquired from image samplings at these neutral elements. 
         [0010]    Another object of the invention is to provide CFA patterns that can yield low color artifacts in output images in spite of sparse sampling of chrominance. The remaining elements of the CFA other than neutral elements are color filtered ones. Color elements cluster to form a repeating block pattern, and each block includes several types of color filter elements that are necessary for calculation of at least one color pixel. Color images with low spatial resolution but low color artifacts can be acquired from image samplings at these color filter elements. 
         [0011]    A further object of the invention is to provide a color image formation method to combine luminance and chrominance information. The gray scale images and the color images mentioned above are combined to form output images in a luminance-chrominance color space, such as CIE Lab or HSB (hue-saturation-brightness). The process is to first transform the color images to one of the luminance-chrominance color spaces, e.g. Lab, and then to replace the luminance component (e.g. L component in the Lab model) with the gray scale images acquired from neutral elements. 
         [0012]    Another object of the invention is to provide a color image capturing apparatus comprising CFA means, in which a majority of the elements in the CFA are neutral elements without color selectivity, and the remaining elements of the CFA color filtered ones clustering to form a repeating block pattern. 
         [0013]    A still further object of the invention is to provide a color image capturing apparatus comprising image formation means, in which the processing of gray scale images and color images is firstly separated, and then they are combined in a luminance-chrominance color space by replacing the luminance components of the color images with the gray-scale images. 
         [0014]    Preferred methods include application of interpolation schemes and algorithms to transform data into useful formats for subsequent processing, storage, transmission and rendering. 
         [0015]    Preferred devices include CCD, CID and CMOS image sensor arrays that have a filter grid layered over sensing elements and integrated electronic elements for reading and processing information captured by the sensors. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1 . The Bayer pattern, a prior art description of a CFA. 
           [0017]      FIG. 2 . One CFA embodiment of the present invention, in which 75% of elements are neutral (shown as blank cells). 
           [0018]      FIG. 3 . Schematic illustration that describes the process of the image formation to combine luminance and chrominance information. 
           [0019]      FIG. 4 . Other embodiments of the present invention that comprise different proportions of neutral elements and additive primary color filter elements (red, green, and blue). 
           [0020]      FIG. 5 . Some other embodiments of the present invention that comprise subtractive primary color filters, cyan, magenta, yellow, and green. 
           [0021]      FIG. 6 . Other embodiments of the present invention that every other color filter block shifts either horizontally, vertically, or obliquely to ensure there are color filter elements in as many rows and columns as possible. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The present invention is a device and method that addresses certain disadvantages of prior art for digital imaging devices that use a color filter array (CFA).  FIG. 1  shows the CFA pattern disclosed in U.S. Pat. No. 3,971,065 by Bayer. Unlike devices employing the Bayer pattern, the present invention collects and calculates real luminance and real chrominance information separately. Unlike the inventions disclosed in U.S. Pat. Nos. 5,323,233 and 5,914,749, the present invention is directed to sampling luminance information with a majority (&gt;50%) of image sensor elements, and to sampling chrominance information with a minority of image sensor elements. 
         [0023]    The benefit of providing neutral elements between color elements is several-fold. It enhances the signal to noise ratio that is addressed in Bawolek et al.&#39;s patent (U.S. Pat. No. 5,914,749) and Yamagami, et al.&#39;s patent (U.S. Pat. No. 5,323,233) so that imaging apparatus with high light sensitivity can be made by adopting the invention. The present invention can also yield higher image resolution than U.S. Pat. Nos. 5,323,233, 5,914,749, 6,476,865B1 and 6,714,243B1. 
         [0024]    Referring to  FIG. 2 , one CFA embodiment of the present invention is where neutral elements are inserted between color elements as represented by the blank cells. In this embodiment 75% of the elements of the CFA are neutral elements, which can be neutral density filters or completely transparent ones, and the remaining 25% of the elements are color filter elements. 
         [0025]    The imaging sensor elements beneath the neutral filter elements directly detect luminance, working like those in black-and-white cameras. From these pixels, full-frame gray scale images can be acquired by means of interpolation. A portion of the data matrix acquired with the embodiment is shown as follows, in which the four missing luminance values (X 1  to X 4 ) need to be calculated using known values from peripheral pixels. 
         [0000]      L11 L12 L13 L14 
         [0000]      L21 X1 X2 L24 
         [0000]      L31 X3 X4 L34 
         [0000]      L41 L42 L43 L44 
         [0026]    A simple interpolation method to estimate the luminance values at pixels of color filter elements is the linear interpolation. 
         [0000]        X 1=( L 21+( L 24− L 21)/3 +L 12+( L 42 −L 12)/3)/2 
         [0000]        X 2=( L 21+( L 24 −L 21)*2/3 +L 13+( L 43 −L 13)/3)/2 
         [0000]        X 3=( L 31+( L 34 −L 31)/3 +L 12+( L 42 −L 12)*2/3)/2 
         [0000]        X 4=( L 31+( L 34 −L 31)*2/3 +L 13+( L 43 −L 13)*2/3)/2 
         [0027]    In the CFA as shown in  FIG. 2 , red, green, and blue color filter elements are arranged in a repeating block fashion. Each block is the same as the repeating 2-by-2 pattern in Bayer&#39;s CFA, i.e. two green filters in diagonal cells, and one red and one blue filter in an opposing diagonal direction. From these color filter elements, color images can be obtained by means of demosaic interpolation. A simple approach can be to first piece color filter blocks together to form a regular Bayer pattern, and then to use existing demosaic algorithms to calculate full color images. Examples of such algorithms and interpolation means are disclosed by Adams Jr. et al. in U.S. Pat. No. 5,652,621 and in K. Hirakawa and T. W. Parks, “Adaptive homogeneity-directed demosaicing algorithm”, IEEE Transactions on Image Processing, 14(3), pp. 360-369, 2005 
         [0028]    The sizes of generated color images are normally smaller than those of the gray images mentioned above, but they can be easily resized to the same dimension. To combine the color images and the gray scale images, the color images, which are typically in RGB color space, it is preferable to transform the color images into a luminance-chrominance space, e.g. CIE 1976 Lab, YIQ, and HSB (hue-saturation-brightness). Luminance-chrominance color models are one type of model that specifically provide values of lightness to describe colors, unlike the tri-stimuli color models such as Red-Green-Blue (RGB) or Cyan-Magenta-Yellow (CMY). In the luminance-chrominance space, the luminance components (e.g. L component of Lab) of the color images are replaced by the gray scale images to result in new color images. Finally, they can be transformed to desired color spaces, e.g. RGB, and output. 
         [0029]      FIG. 3  schematically illustrates the process of the image formation described above, where panel L′ represents the gray scale images acquired from neutral elements, multiple panels RGB represent the color images acquired from color filter elements, multiple panels Lab represent the color images in luminance-chrominance spaces, and multiple panels L′ab represent the output color images. This process can be applied to all the embodiments of the present invention as well as the embodiments of other inventions comprising neutral elements. 
         [0030]      FIG. 4  illustrates some other embodiments of the present invention that comprise different proportions of neutral elements. Neutral elements make up 55% (5/9) of the CFA in  FIG. 4   a,  67% (6/9) in  FIG. 4   b , and 92% (33/36) in  FIG. 4   c . The remaining elements in the CFAs shown in  FIG. 4  are red, green and blue color filters whose spectrum characteristics are the same as those used in the Bayer CFA. 
         [0031]    Because the luminance and chrominance are sampled and calculated separately, the sampling rate of luminance between 55% and 92% can be easily designed to suit different applications by configuring the repeating period of color filter block. For example, the repeating period is 3 pixels in  FIG. 4   b,  4 pixels in  FIG. 2 , and 6 pixels in  FIG. 4   c  both horizontally and vertically. To achieve different chrominance sampling rates for horizontal and vertical directions, the repeating periods in the two directions can be different. The higher the luminance sampling rate is, the higher light sensitivity, but the worse the color distortion. CFA with more than 92% of luminance sampling rate will result in too coarse a sampling of chrominance to be useful. 
         [0032]    Not only by repeating the period, the sampling rate of luminance can be varied also by configuring whether each color filter block includes three or four color filter elements. For instance, in CFAs shown in  FIG. 2  and  FIG. 4   a , each color filter block includes four color filter elements: two green filters in diagonal cells, and one red and one blue filters in an opposing diagonal direction. In CFAs shown in  FIG. 4   b  and  FIG. 4   c , each color filter block includes three color filter elements. For these CFAs, a simple demosaicing method can be employed; namely, a nearest neighbor algorithm. This method computes the values for each “color” pixel using the three values from the surrounding pixels in the same one block. 
         [0033]      FIG. 5  illustrates some other embodiments of the present invention that comprise subtractive primary color filters, cyan (C), magenta (M), and yellow (Y). The configuration of neutral elements of these CFAs is the same as that for CFAs comprising RGB filters, but the color filters are mainly CMY instead.  FIG. 5  shows two CFAs in which the luminance sampling rates are 67% (6/9) as in  FIG. 5   a  and 75% (12/16) as in  FIG. 5   b , respectively. As in the RGB type of aforementioned CFAs, the CMY type of CFAs of the present invention can be configured with different luminance sampling rates ranging from 56% to 92%. CFAs comprised of CMY filters have been used in some digital cameras, for example, the Kodak DCS620X. To improve color fidelity, a green filter element can be used in each color filter block, as shown in  FIG. 5   b.    
         [0034]      FIG. 6  illustrates some other embodiments of the present invention that every other color filter block (circled by dashed lines) shifts either horizontally, vertically, or obliquely a certain amount to ensure there are color filter elements in as many rows and columns as possible. For instance, in the CFA shown in  FIG. 2 , every third and fourth rows and every third and fourth columns are all neutral elements. Obtaining chrominance information in these rows and columns may be favorable.  FIG. 6   a  illustrates a variation of the CFA shown in  FIG. 2 , in which the color filter block in every other fifth and sixth rows shift two elements to the right so that there are color filter elements in every column of the CFA. Similarly,  FIG. 6   b  illustrates a variation of the CFA shown in  FIG. 5   a , in which the color filter block in every other fourth and fifth columns shift one element down so that there are color filter elements in every row and column of the CFA.  FIG. 6   c , illustrates a variation of the CFA shown in  FIG. 2 , in which the filter blocks circled by dashed line shift obliquely to south-west. The shift configuration can be applied to all the embodiments of the present invention. 
         [0035]    A number of embodiments of the invention have been described. Nevertheless, it shall be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.