Computer graphics processing and selective visual display system – Computer graphics processing
Reexamination Certificate
1998-12-10
2001-11-27
Vo, Cliff N. (Department: 2772)
Computer graphics processing and selective visual display system
Computer graphics processing
C345S419000, C382S254000
Reexamination Certificate
active
06323855
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to sharpening edges in digital images to provide high frequency edge enhancement.
BACKGROUND OF THE INVENTION
Currently, digital images are derived through various devices including digital cameras and the digital 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 microcontrast provides the sharpening effect. Such methods can be applied to black and white digital images as well as to colored digital images.
Referring to
FIG. 1A
, a one dimensional trace of an edge profile is shown in which 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. 2
,
3
A and
3
B 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.
When these conventional sharpening methods are applied to noisy images, high spatial frequency boost kernels (such as the 3×3 kernel shown in
FIG. 2
) tend to amplify the noise as well as image edge features. Excessive noise amplification may be reduced by using a mid-range spatial frequency boost kernel (such as the 5×5 kernels shown in FIGS.
3
A and
3
B), however, image edge features are primarily enhanced in the mid-range of spatial frequencies.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of edge sharpening which provides high spatial frequency enhancement of image edge features and reduces high frequency noise amplification.
These objects are achieved by a method of sharpening edge features in a digital image having pixels which provide high spatial frequency edge enhancement, 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; and
c) modifying the edge boost values of edge related pixels in accordance with an edge boost modification function selected to provide an edge enhanced digital image.
ADVANTAGES
Digital images are often in need of sharpening and conventional sharpening often amplifies unwanted high frequency noise. The present invention achieves high frequency edge enhancement. At the same time as the present invention performs the high frequency edge enhancement, it does not amplify undesirable high frequency noise thereby maintaining such noise at acceptable levels.
The present invention makes use of the fact that the magnitude of an edge feature is related to both the absolute size of the boost values and their spatial separation in the boost record.
This is accomplished by separating edge enhancement into two parts. The first part, edge feature detection, is done using a second order Laplacian kernel which is most sensitive to mid-range spatial frequencies. The second part, edge boost generation, is done using a high spatial frequency method, but applied only to those regions identified in the first step. In this way, the enhancement operation is applied to true image edge features, but not to high frequency noise which has only minor mid-range frequency content. The present invention may be used separately or in combination with other edge enhancement operations thus providing greater flexibility for artifact-free edge enhancement of digital images.
An important feature of the present invention is that mid-range spatial frequencies edge features are provided with high spatial frequency enhancement. The present invention advantageously sharpens edge features in a digital image having pixels while simultaneously providing high spatial frequency edge enhancement and reducing high frequency noise amplification.
REFERENCES:
patent: 6005983 (1999-12-01), Anderson et al.
patent: 6038348 (2000-03-01), Carley
Adams, Jr. James E.
Hamilton, Jr. John F.
Eastman Kodak Company
Owens Raymond L.
Vo Cliff N.
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