Image analysis – Image enhancement or restoration – Image filter
Reexamination Certificate
1998-09-30
2001-11-13
Lee, Thomas D. (Department: 2724)
Image analysis
Image enhancement or restoration
Image filter
C382S261000
Reexamination Certificate
active
06317521
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of digital image processing and, more particularly, to a method for preserving image detail when adjusting the contrast of a digital image.
BACKGROUND OF THE INVENTION
Existing methods for applying tone scales to digital color images include applying the tone scale to each of the color channels independently, applying the tone scale to the neutral channel and preserving the original pixel color difference signal, or applying the tone scale to just the low frequency image. These methods of tone scale application alter the sharpness and appearance of the color in processed images, and the latter can produce displeasing artifacts.
Traditional methods of applying a tone scale function to a digital color image modify the apparent sharpness of the image because the tone scale modifies the amplitudes of high frequency detail information. This phenomenon occurs when the tone scale function is applied to each of the color channels independently, or when the tone scale function is applied to the neutral channel and original pixel color difference signals are preserved.
In an effort to apply a tone scale function to a digital image without distorting the detail information, in U.S. Pat. No. 5,012,333 Lee et al. proposed separating the image into a high frequency and a low frequency image by using FIR filters. The tone scale function is then applied to only the low frequency image, and the high frequency image is added back to the tone scaled low frequency image.
Also, in U.S. Pat. No. 5,454,044 Nakajima suggests modifying the image contrast by the formula
Sproc=Sorg+f(Sus).
The low frequency image (Sus) is passed through function f() which is a monotonically decreasing function. This signal is added to the original (Sorg) to create the processed image Sproc.
Both of these methods preserve the high frequencies of the image, but this may lead to the creation of unsharp mask type artifacts (overshoot and undershoot) in the neighborhood of large edges (characteristic of occlusion boundaries or dark shadows).
Consequently, a need exists for overcoming the above-described drawbacks. More specifically, a need exists for permitting the application of a tone scale function to a digital image to adjust the macro contrast of the image, preserves the high frequency detail information, and prevents artifacts in the neighborhood of large edges.
SUMMARY OF THE INVENTION
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in a method for adjusting tone scale of a digital image channel, the method comprising: (a) receiving a digital image channel; (b) providing a tone scale function for adjusting the tone scale of the digital image channel; (c) spatial filtering the digital image with a spatial filter which varies with location in the digital image channel according to the control signal producing a space-variant spatial filtered version of the digital image channel; (d) using the tone scale function and the space-valiant spatial filtered version of the digital image channel to produce an enhanced digital image channel.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
ADVANTAGEOUS EFFECT OF THE INVENTION
The present invention has the advantage of preserving edge detail while altering the contrast of a digital image.
REFERENCES:
patent: 5012333 (1991-04-01), Lee et al.
patent: 5339365 (1994-08-01), Kawai et al.
patent: 5426517 (1995-06-01), Schwartz
patent: 5454044 (1995-09-01), Nakajima
patent: 5920646 (1994-08-01), Kamon
patent: 0 525 949 A2 (1993-02-01), None
Gallagher Andrew
Gindele Edward B.
Brinich Stephen
Eastman Kodak Company
Lee Thomas D.
Woods David M.
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