AVERAGING GREEN VALUES FOR GREEN PHOTOSITES IN ELECTRONIC CAMERAS
CROSS REFERENCE TO RELATED 5
Reference is made to commonly assigned U.S. Ser. Nos. 08/407,436 filed Mar. 17, 1995, now U.S. Pat. No. 5,506, 619, to James E. Adams, Jr. and John F. Hamilton, Jr. and 08/407,423 filed Mar. 17, 1995 to John F. Hamilton, Jr. and James E. Adams, Jr. filed concurrently herewith, the teachings of which are incorporated herein.
1. Field of the Invention
This invention relates to the field of electronic imaging 15 and more particularly to providing improved green pixel values.
2. 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.
The following shows how red (R), green (G) and blue (B) 3Q 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 to Bayer.
When a color image is captured using a CFA, it is necessary to reduce and limit the amount of noise in the green pixel values so that the scene detail in the final image is as visually noiseless as possible. The green pixel values are more important in this regard than the red and blue pixel 45 values as most of the spatial, or luminance, information in the image is represented by the green pixel values. For this reason, a green pixel value noise reduction operation is usually placed at or near the beginning of the image processing chain used by the electronic camera. 50
SUMMARY OF THE INVENTION
The object of this invention is to provide an improved apparatus for reducing noise in the green pixel values in a CFA.
This object is achieved in apparatus for processing a digitized image signal obtained from an image sensor having red, green, and blue color photosites aligned in rows and columns that generate at least three separate color values but 6Q only one color value for each photosite location, means for averaging existing green pixel values for green photosite locations comprising:
means for storing the digitized image signal;
means for defining green classifiers based upon adding the 65 Laplacian second-order values, and the gradient values to define a classifier;
means for modifying a measured green pixel value based upon adjacent pixel classifier values including:
(i) means for determining which classifier values represent a pixel neighborhood of slowly changing scene detail; and
(ii) means for replacing the measured green pixel values for a green photosite location with an average of surrounding predicted green pixel values as a function of the number of classifiers which are below a predetermined value.
The advantages of this invention are 1) is computationally efficient both in execution time and memory storage requirements; and 2) by use of the combination of the Laplacian second-order values and gradient values to produce a classifier, and use of averaging to produce a predictor, noise in green pixel values is substantially reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an electronic still camera employing interpolation processing according to the invention;
FIG. 2 is a block diagram of the logic of the interpolation and averaging processing techniques used in connection with the invention;
FIG. 3 is a detailed block diagram of the logic of the interpolation luminance section in FIG. 2;
FIG. 4 is a more detailed block diagram of the logic of the chrominance section in FIG. 2; and
FIGS. 5 and 6 show logic block diagrams for the interpolation processes in accordance with the invention.
FIG. 7 is a detailed block diagram of the logic of luminance averaging section in FIG. 2.
DETAILED DESCRIPTION OF THE
Since single-sensor electronic cameras employing color filter arrays are well known, the present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus and method in accordance with the present invention. Elements not specifically shown or described herein may be selected from those known in the art.
Referring initially to FIGS. 1 and 2, an electronic still camera is divided generally into an input section 2 and an interpolation and recording section 4. The input section 2 includes an exposure section 10 for directing image light from a subject (not shown) toward an image sensor 12. Although not shown, the exposure section 10 includes conventional optics for directing the image light through a diaphragm, which regulates the optical aperture, and a shutter, which regulates exposure time. The image sensor 12, which includes a two-dimensional array of photosites corresponding to picture elements of the image, is a conventional charge-coupled device (CCD) using either wellknown interline transfer or frame transfer techniques. The image sensor 12 is covered by a color filter array (CFA) 13, known as the Bayer array, which is described in U.S. Pat. No. 3,971,065 and herewith incorporated by reference. In the Bayer geometry each color covers a photosite, or picture element (pixel), of the sensor. In particular, chrominance colors (red and blue) are interspersed among a checkerboard pattern of luminance colors (green). The image sensor 12 is