Image analysis – Image enhancement or restoration – Edge or contour enhancement
Reexamination Certificate
1998-10-30
2001-07-10
Lee, Thomas D. (Department: 2724)
Image analysis
Image enhancement or restoration
Edge or contour enhancement
C382S260000
Reexamination Certificate
active
06259822
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
Reference is made to commonly assigned U.S. patent application Ser. No. 09/156,480 filed Sep. 18, 1998, entitled “Edge Enhancement Using Modified Edge Boost Function” to Hamilton et al., the disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTION
This invention relates to edge enhancing digital images such that the visibility of false edge contours is reduced.
BACKGROUND OF THE INVENTION
Currently, digital images are derived through various devices including digital cameras and scanners for digitally scanning of film images. Many times the sharpness of an image is degraded by optical elements or by irregularities in the image sensor. For these reasons, it is often desirable to sharpen an image after it has been converted to a digital form. Conventional sharpening methods, such as unsharp masking, achieve the appearance of edge sharpening by locally lightening the lighter portion of an edge region and locally darkening the darker portion of an edge region. The resulting increase in contrast provides the sharpening effect. Such methods can be applied to black and white digital images as well as to colored digital images.
Shown in
FIG. 1A
is a one-dimensional edge profile, which is a graph of image pixel code values plotted as a function of their position on a line running perpendicularly across an edge feature in a digital image. Higher code values correspond to lighter shades and lower code values to darker shades. In
FIG. 1B
, the same edge profile has been further blurred in accordance with the prior art technique of unsharp masking. The curve in
FIG. 1B
is subtracted from the curve in FIG.
1
A and the resulting curve shown in
FIG. 1C
in which the amplitudes P and N, for positive and negative boost respectively, are approximately the same size. The difference curve of
FIG. 1C
is added to the original curve in FIG.
1
A and this final curve, shown in
FIG. 1D
, depicts the profile of the sharpened edge. Although unsharp masking was originally a film technique, it also has a digital version. Shown in
FIGS. 2A-C
are examples of boost kernels which, when applied to a digital image, directly produce boost values analogous to those shown in FIG.
1
C and sharpened edges analogous to those shown in FIG.
1
D and shows the true edge contour, labeled (T), and the two false contours, labeled (F). The light and dark ridges are labeled (L) and (D) respectively.
When these conventional sharpening methods are applied too aggressively, image artifacts will appear. One recurring artifact is a false secondary edge contour which runs parallel to the true edge. This artifact can often be seen on both the lighter and darker sides of an edge that has been enhanced. The cause of the false contours for a single color channel is shown in
FIG. 3
using a one dimensional edge profile. The enhanced edge profile follows the profile of FIG.
1
D and shows the true edge contour, labeled (T), and the two false contours, labeled (F). The light and dark ridges are labeled (L) and (D), respectively.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of edge enhancing digital images which reduces the visibility of false contour image artifacts caused by conventional sharpening methods.
This object is achieved by a method of edge enhancing a digital image having pixels which reduces the visibility of false edge contours, comprising the steps of:
a) acquiring a digital image;
b) computing an edge boost record having positive and negative boost values for different portions of the edge regions in a digital image;
c) modifying the edge boost record by applying an adaptive blur kernel to the pixels of such edge boost record; and
d) applying the modified edge boost record to the digital image to provide an edge enhanced digital image which has reduced visibility of false edge contours.
ADVANTAGES
Digital images are often in need of sharpening and conventional sharpening often produces false contours near edge regions. The present invention reduces the visibility of such false contours by lowering their contrast while maintaining the enhanced contrast of the true edge. This is accomplished by applying an adaptive blue kernel to the edge boost record which reduces the contrast of only the false contour regions. In this way, the enhancement of the true edge remains unchanged, but the false contours are rendered either invisible or with reduced visibility. An additional benefit is that the influence of image noise is reduced in the edge boost record. This provides a greater degree of artifact-free edge enhancement for digital images.
REFERENCES:
patent: 4503461 (1985-03-01), Nishimura
patent: 5528698 (1996-06-01), Kamei et al.
patent: 5551428 (1996-09-01), Godlewski et al.
patent: 5966134 (1999-10-01), Arias
Adams, Jr. James E.
Hamilton, Jr. John F.
Brinich Stephen
Eastman Kodak Company
Lee Thomas D.
Owens Raymond L.
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