Television – Image signal processing circuitry specific to television – Transition or edge sharpeners
Reexamination Certificate
1999-10-19
2003-01-14
Miller, John (Department: 2614)
Television
Image signal processing circuitry specific to television
Transition or edge sharpeners
C348S631000, C348S672000, C382S168000
Reexamination Certificate
active
06507372
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to image enhancement, and more particularly, to a method and apparatus for enhancing the quality of an image by compensating a color signal in response to a luminance variation or adjustment due to mean matching histogram equalization and quantized mean matching histogram equalization.
The basic operation of histogram equalization is to transform a given input image on the basis of its histogram, wherein the histogram represents the density of a gray level distribution of an input image. The histogram of the gray level distribution provides an overall depiction of the appearance of an image. A gray level properly controlled according to a sample distribution of an image enhances the appearance or contrast of the image.
Among various methods for contrast enhancement, histogram equalization is widely known and has been disclosed in the following documents: [1] J. S. Lim, “Two-Dimensional Signal and Image Processing,” Prentice Hall, Englewood Cliffs, N.J., 1990; and [2] R. C. Gonzalez and P. Wints, “Digital Image Processing,” Addison-Wesley, Reading, Mass., 1977.
The well-known characteristics of histogram equalization act as a defect in some practical cases. For example, since the output density of histogram equalization is constant, the mean brightness of an output image approaches a middle gray level. As a result, the mean brightness of an output image in histogram equalization is exactly the middle gray level regardless of the mean brightness of the input image. This characteristic is not desirable for certain applications such as in photographed night scenes. Because the histogram equalization provides an output at the middle region of the gray levels, a scene photographed at night appears extremely bright after histogram equalization.
Additionally, if color compensation is not performed on a color signal according to a luminance variation produced by histogram equalization, a pure color signal is distorted.
For instance, in a color system that is composed of signals Y, R-Y and B-Y, it is assumed that the signal Y is transformed into a signal Y′ (=Y+&Dgr;y) by a predetermined luminance processing. The adjusted signals Y′, R-Y and B-Y are varied into values R, G and B without color compensation, and the resultant color signals are expressed by Equations (1) through (3):
R
′
=
(
R
-
Y
)
+
Y
′
=
R
+
Δ
y
(
1
)
G
′
=
(
G
-
Y
)
+
Y
′
=
G
+
Δ
y
(
2
)
B
′
=
(
B
-
y
)
+
Y
′
=
B
+
Δ
y
·
(
3
)
When Y is transformed into Y′, an exemplary pure red signal (R,
0
,
0
) is mapped to a signal (R+&Dgr;y, &Dgr;y, &Dgr;y) if there is no color compensation. Thus, the resultant color signal is no longer a pure red signal. Similarly, if there is no color compensation, all of the other pure color signals are also distorted.
Meanwhile, a contrast correction method for extracting Y from input color signals R, G and B, extracting a correction signal from the extracted Y, and adding the correction signal to the respective input signals R, G and B as shown in Equations (1) through (3), has been disclosed in an U.S. Pat. No. 5,345,277. Even in the above-described patent document, a pure color signal is distorted.
SUMMARY OF THE INVENTION
To solve the above problems, it is an object of the present invention to provide a method for enhancing the image quality by varying a color signal based on the adjusted luminance, while enhancing image contrast by controlling a cumulative density function of an input luminance image used as a transform function to map a mean level of the luminance image to itself.
It is another object of the present invention to provide a method for enhancing the image quality by varying a color signal based on an adjusted luminance, while enhancing image contrast by controlling an interpolated cumulative density function obtained by interpolating a quantized cumulative density function of an input luminance image, used as a transform function to map a mean level of the input luminance image to itself during histogram equalization.
It is still another object of the present invention to provide a circuit for enhancing the image quality by varying a color signal based on an adjusted luminance, while enhancing image contrast by controlling a cumulative density function of an input luminance image used as a transform function to map a mean level of the luminance image to itself.
It is yet another object of the present invention to provide a circuit for enhancing the image quality by varying a color signal based on an adjusted luminance, while enhancing image contrast by controlling an interpolated cumulative density function obtained by interpolating a quantized cumulative density function of an input luminance image, used as a transform function to map a mean level of the input luminance image to itself during histogram equalization.
To accomplish the first object, there is provided an image enhancement method comprising the steps of: extracting a luminance signal from input color signals; performing histogram equalization using a cumulative density function of the extracted luminance signal as a transform function, and outputting an adjusted luminance signal by controlling the transform function to map a mean level of the extracted luminance signal to itself; and varying the input color signals based on the adjusted luminance signal, and outputting compensated color signals.
To accomplish the second object, in the adjusted luminance signal outputting step, a quantized cumulative density function of the extracted luminance signal is histogram-equalized using an interpolated cumulative density function as a transform function, and the adjusted luminance signal is output by controlling the transform function to map a mean level of the extracted luminance signal to itself.
To accomplish the third object, in an image enhancement circuit, an extractor extracts a luminance signal from input color signals, a histogram equalizer performs histogram equalization using a cumulative density function of the extracted luminance signal as a transform function, and outputs an adjusted luminance signal by controlling the transform function to map a mean level of the extracted luminance signal to itself, and a color compensator outputs compensated color signals by varying the input color signals based on the adjusted luminance signal.
To accomplish the fourth object, the histogram equalizer performs histogram equalization using an interpolated cumulative density function obtained by interpolating a quantized cumulative density function of the extracted luminance signal as a transform function, and outputs an adjusted luminance signal by controlling the transform function to map a mean level of the extracted luminance signal to itself.
REFERENCES:
patent: 5345277 (1994-09-01), Blankevoort et al.
patent: 5857033 (1999-01-01), Kim
patent: 5862254 (1999-01-01), Kim et al.
patent: 5937090 (1999-08-01), Kim
patent: 5963665 (1999-10-01), Kim et al.
patent: 6018588 (2000-01-01), Kim
patent: 6049626 (2000-04-01), Kim
patent: 6078686 (2000-06-01), Kim
Miller John
Samsung Electronics Co,. Ltd.
Yenke Brian
LandOfFree
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