Particular pattern of pixels for a color filter array which is used to derive luminance and chrominance values

A color filter array for an image sensor which has a plurality of pixels is disclosed. The color filter array includes a plurality of color pixel kernels, with each kernel having the plurality of pixels arranged in the following pattern A C D B wherein: PA1 A and B are companion colors; and PA1 C and D are companion colors.

FIELD OF THE INVENTION
 The present invention relates to color filter arrays for image sensors.
 BACKGROUND OF THE INVENTION
 In electronic color imaging, it is desirable to simultaneously capture
 image data in three color planes, usually red, green and blue. When the
 three color planes are combined, it is possible to create high-quality
 color images. Capturing these three sets of image data can be done in a
 number of ways. In electronic photography, this is sometimes accomplished
 by using a single two dimensional array of sensors that are covered by a
 pattern of red, green and blue filters. This type of sensor is known as a
 color filter array or CFA. Below is shown the red (R), green (G) and blue
 (B) pixels as are commonly arranged on a CFA sensor.
 When a color image is captured using a CFA, it is necessary to interpolate
 the red, green and blue values so that there is an estimate of all three
 color values for each sensor location. Once the interpolation is done,
 each picture element, or pixel, has three color values and can be
 processed by a variety of known image processing techniques depending on
 the needs of the system. Some examples of the reasons for processing are
 to do image sharpening, color correction or half toning.
 The diagram below shows how red green and blue pixels can be arranged in a
 particular color filter array pattern, hereinafter referred to as the
 Bayer color filter array. For a more detailed description see U.S. Pat.
 No. 3,971,065 issued Jul. 20, 1976 to Bayer.

G R
 B G
 In commonly assigned U.S. Pat. No. 5,596,367 issued Jan. 21, 1997,
 incorporated by reference, adaptive methods of calculating green pixel
 values at pixels where green is not directly measured is described. These
 methods rely on algorithmically selecting one or two ways to calculate the
 missing green values. After determining the missing green values, the red
 minus green color difference values are computed at red pixel sites and
 the blue minus green color difference values are computed at blue pixel
 sites. The missing values for each color difference are computed by simple
 linear interpolation. Because the red, green, and blue CFA filters absorb
 two thirds of the incident light, such CFA sensors tend to be
 photometrically slow. Especially in low light situations, it would be
 better to use subtractive CFA filters such as cyan, magenta, yellow, and
 green. It would also be desirable to have a CFA pattern that permits
 adaptive interpolation of the luminance value (analogous to green) and
 permits for adaptive interpolation of both color difference values as
 well.
 SUMMARY OF THE INVENTION
 It is an object of the present invention is to provide a color filter array
 pattern which provides signals which can be efficiently processed since
 the color filter array uses a 2.times.2 repeating pattern.
 It is a further object of the invention to have a CFA pattern which has at
 least two methods of luminance interpolation as well as two methods for
 each chrominance value to thereby provide improved interpolation for both
 luminance and chrominance.
 This object is achieved in a color filter array for an image sensor which
 has a plurality of pixels, the color filter array comprising:
 (a) a plurality of color pixel kernels, with each kernel having the
 plurality of pixels arranged in the following pattern

A C
 D B
 wherein:
 A and B are companion colors; and
 C and D are companion colors.
 ADVANTAGES
 An advantage of the present invention is that the color filter array
 pattern uses kernels with only four pixels and this facilitates
 calculations and interpolation.
 A feature of this invention is that it permits non-linear interpolation of
 both luminance values and chrominance values, regardless of location
 within the CFA pattern.
 Another advantage of the present invention is that the same processing can
 be used irrespective of the location of the pixels in the CFA.
 Another advantage of the present invention is that it facilitates the use
 of the smallest kernel of CFA pixels can be used to provide a high quality
 estimate of luminance.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 is a plan view of a new four-color CFA kernel. The colors are chosen
 so that V=(A+B)/2=(C+D)/2, where V is luminance. The color pixels of each
 kernel which satisfy the above equation are arranged so that A and B are
 companion colors and C and D are companion colors as will be explained
 later. By use of the term "kernel" is meant a minimal repeating pattern of
 color pixels or elements of a CFA filter that is two-dimensionally
 replicated over the full extent of the color filter array. In a preferred
 embodiment set of colors is A=magenta (M), B=green (G), C=cyan (C) and
 D=yellow (Y). Expressing this set of colors in terms of red (R), green (G)
 and blue (B): Under these definitions, V=(R+2G+B)/4. This preferred
 embodiment is shown in FIG. 2 wherein the pattern has green, magenta,
 cyan, and yellow pixels.
 M=(R+B)/2 G=G
 C=(G+B)/2
 Y=(R+G)/2
 FIG. 3A is a cross-sectional view of an area image sensor 10 taken along
 the lines 3A--3A of FIG. 2. As shown, the image sensor 10 includes a
 silicon substrate 12 into which is doped pixel areas 14. In this
 particular embodiment, there are two dye-receiving layers 16 and 18
 respectively formed on the silicon substrate 12. In this arrangement, the
 dye pattern shown in FIG. 2 is exemplified.
 FIG. 3B is similar to FIG. 3A and includes the same elements except that it
 should be noted that a yellow portion is disposed directly over a cyan dye
 portion. These portions are aligned with a particular image sensor pixel.
 The combination of the yellow and cyan portions forms the green pixels
 shown in FIG. 2. Although the superimposed cyan and yellow colored
 portions are preferably the same cyan and yellow dyes shown in the
 dye-receiving layers 16, it will be understood by those skilled in the art
 that different cyan and yellow dyes can be used to form a green pixel. For
 an example of a color filter array which uses different color dyes, see
 commonly assigned U.S. Pat. No. 5,419,990 issued May 30, 1995 to Wake et
 al, the disclosure of which is incorporated herein.
 Due to the arrangement of color filter array pattern shown in FIG. 2, V can
 be interpolated in either of two diagonal directions at each location of
 the CFA, which permits adaptive CFA interpolation of luminance values. In
 FIG. 4, the same color pixel kernel shown as in FIG. 2; however, in FIG. 4
 each of the pixels carry a subscript number. The first number in the
 subscript corresponds to the row and the second subscript corresponds to
 the column. For clarity of illustration, FIG. 4 has been shows as a
 5.times.5, although the kernel, of course, is still 2.times.2. Most
 notably, at each location in the pattern, V can be estimated in the
 positive slope diagonal and negative slope diagonal directions. The value
 V.sub.33 can be calculated as below:
EQU V.sub.33 =(-M.sub.15 +3G.sub.24 +8M.sub.33 +3G.sub.42 -M.sub.51)/12
EQU V.sub.33 =(-M.sub.11 +3G.sub.22 +8M.sub.33 +3G.sub.44 -M.sub.55)/12
 Two chrominance channels are also defined for this pattern: C1=K1 (D-C) and
 C2=K2 (A-B). The values K1 and K2 are scaling constants. In the preferred
 embodiment of FIG. 2, K1=1/2, K2=1/2 and, as before, A=M, B=G, C=C and
 D=Y. As a result, once a luminance value has been calculated for each
 pixel location, C1 or C2 can be calculated directly. Missing values of C1
 and C2 can be adaptively interpolated because each location missing C1 or
 C2 is surrounded by four locations having defined values.
 For example, in FIG. 4, once V.sub.33 has been computed, the value for
 (C2).sub.33 now can be computed as (M.sub.33 -V.sub.33). At the pixel of
 M.sub.33, only the value of (C1).sub.33, which is derived from cyan and
 yellow, is missing. However, at the four adjacent pixels, the values
 (C1).sub.23, (C1).sub.32, (C1).sub.34, and (C1)43 are known.
 Therefore, the value (C1).sub.33 may be adequately computed using one of
 the following equations:
EQU (C1).sub.33 =((C1).sub.23 +(C1).sub.43 /2
EQU (C1).sub.33 =((C1).sub.32 +(C1).sub.34 /2
 Once there is a value of V, C1 and C2 at each pixel location, corresponding
 values of R, G and B can be obtained through a simple linear transform.
 In accordance with the above discussion, A and B are defined to be
 companion colors in that the sum of the colors is proportional to
 luminance and the difference of the colors is proportional to chrominance.
 Likewise, C and D are also companion colors.
 FIG. 5 shows an alternative embodiment of the present invention. It should
 be noted that the kernel pattern of FIG. 5 is similar to that shown in
 FIG. 2 with the exception that the cyan pixels are replaced by yellow
 pixels and the yellow pixels are replaced by cyan pixels. It should now be
 clear that these colors can be handled in a similar fashion using
 Equations 1.
 The invention has been described in detail with particular reference to
 certain preferred embodiments thereof, but it will be understood that
 variations and modifications can be effected within the spirit and scope
 of the invention.
 TS LIST
 10 image sensor
 12 silicon substrate
 14 pixel areas
 16 dye-receiving layers
 18 dye-receiving layers