Image analysis – Image enhancement or restoration – Image filter
Reexamination Certificate
1999-03-25
2001-01-16
Lee, Thomas D. (Department: 2724)
Image analysis
Image enhancement or restoration
Image filter
C382S266000
Reexamination Certificate
active
06175656
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to filtering video signals to sharpen their appearance when displayed.
BACKGROUND OF THE INVENTION
Various techniques have been proposed to sharpen the appearance of video when displayed. The need for an effective and robust sharpening filter will be particularly apparent as higher definition video begins to become a household standard and users wish to view older, lower definition recordings on higher definition monitors.
One known technique for enhancing the sharpness of video is to apply a high-frequency boosting spatial filter to the video signals to thus enhance the edges in the video. However, this technique has proven less than satisfactory for the reason that linear filters having a high-boost characteristic also exhibit ringing in their step response. This ringing will manifest as regions of luminance or color variation near to the edges of an object when placed against a background of substantially different luminance or color.
Various nonlinear filters have also been proposed for video sharpening, which for example compare the luminance values within a region and determine statistical properties Such as the minimum, maximum, average and median luminance values, from which a new value for pixels within the region can be derived. However, despite the efforts that have been applied to developing a nonlinear sharpening filter, known filters continue to display unwanted artifacts that make those filters less than completely desirable for all applications.
SUMMARY OF THE INVENTION
The present invention provides a nonlinear filter which overcomes these difficulties, and provides an output of increased sharpness with reduced artifacts as compared to prior art filters.
In one aspect, a method of filtering according to the invention proceeds by identifying a “pivot” value at a pixel based on the value of pixels neighboring that pixel. Then, a new value for the pixel is computed by adding to the pivot value, an amount based on the difference between the original pixel value and the pivot value.
In the specific embodiment described below, the pixel values are luminance values. The pivot value is computed as the average of the original values for a number, e.g., 2
n
, neighboring pixels, where n is, for example, equal to 3. Alternatively, the pivot value may be computed as the average of the maximum and minimum original pixel values among the 2
n
neighboring pixels.
To compute the new value for a pixel, the difference between the original pixel value and the pivot value is multiplied by a factor k, and then added to the pivot value. For computational simplicity, the factor k is selected to be a power of two. Filtering for k=2, k=4 and k=16 are illustrated.
The new value may be limited to the maximum and minimum original pixel values among the 2
n
original pixel values.
The invention also features filtering hardware for carrying out the filtering methods described above.
The above and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof.
REFERENCES:
patent: 5224177 (1993-06-01), Doi et al.
patent: 5598184 (1997-01-01), Barkans
patent: 5701074 (1997-12-01), Zhu
patent: 5828500 (1998-10-01), Kida et al.
patent: 5869822 (1999-02-01), Meadows, II et al.
Brinich Stephen
Lee Thomas D.
Sony Corporation
Wood,Herron&Evans, L.L.P.
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