Method of improving a digital image having white zones

• US 20030026494A1
• Filed: 06/25/2001
• Published: 02/06/2003
• Est. Priority Date: 06/25/2001
• Status: Active Grant

• Current Assignee: National Aeronautics and Space Administration (Government of the United States of America), Scientific Technologies Incorporated (Omron Corporation)
• Original Assignee: Scientific Technologies Incorporated (Omron Corporation)
• Inventors: Rahman, Zia-Ur, Jobson, Daniel J., Woodell, Glenn A.
• Granted Patent: US 6,842,543 B2
• Time in Patent Office: Days
• US Class Current: 382/260
• CPC Class Codes: G06T 2207/10024 Color image
• CPC Class Codes: G06T 5/10 using non-spatial domain filtering
• CPC Class Codes: G06T 5/94 based on local image properties, e.g. for local contrast enhancement
• CPC Class Codes: H04N 9/646 for image enhancement, e.g. vertical detail restoration, cross-colour elimination, contour correction, chrominance trapping filters

Abstract

The present invention is a method of processing a digital image that is initially represented by digital data indexed to represent positions on a display. The digital data is indicative of an intensity value I_i(x,y) for each position (x,y) in each i-th spectral band. The intensity value for each position in each i-th spectral band is adjusted to generate an adjusted intensity value for each position in each i-th spectral band in accordance with $\sum _{n=1}^NW_n\left(\log I_i\left(x,y\right)-\log \left[I_i\left(x,y\right)*F_n\left(x,y\right)\right]\right),i=1,\dots ,S$

where W_n is a weighting factor, “*” is the convolution operator and S is the total number of unique spectral bands. For each n, the function F_n(x,y) is a unique surround function applied to each position (x,y) and N is the total number of unique surround functions. Each unique surround function is scaled to improve some aspect of the digital image, e.g., dynamic range compression, color constancy, and lightness rendition. The adjusted intensity value for each position in each i-th spectral band of the image is then filtered with a filter function to generate a filtered intensity value R_i(x,y). To prevent graying of white zones in the image, the maximum of the original intensity value I_i(x,y) and filtered intensity value R_i(x,y) is selected for display.

Claims

1. A method of processing a digital image, comprising the steps of:

providing digital data indexed to represent positions on a display, said digital data being indicative of an intensity value I_i(x,y) for each position (x,y) in each i-th spectral band;

adjusting said intensity value for said each position in each i-th spectral band to generate an adjusted intensity value for said each position in each i-th spectral band in accordance with

$\underset{n=1}{\overset{N}{\sum <br><br>}}<br><br>W_n<br><br>\left(\log <br><br>\text{\hspace{1em}<br><br>}<br><br>I_i<br><br>\left(x,y\right)-\log <br><br>\text{\hspace{1em}<br><br>}\left[I_i<br><br>\left(x,y\right)*F_n<br><br>\left(x,y\right)\right]\right),i=1,\mathrm{\dots <br><br>}<br><br>\text{\hspace{1em}<br><br>},S$

where S is the number of unique spectral bands included in said digital data and, for each n, W_n is a weighting factor and F_n(x,y) is a unique surround function applied to said each position (x,y) and N is the total number of unique surround functions;

filtering said adjusted intensity value for said each position of said image in each of said S spectral bands using a filter function wherein a filtered intensity value R_i(x,y) is defined; and

selecting a maximum intensity value V_i(x,y) from the group consisting of said intensity value I_i(x,y) and said filtered intensity value R_i(x,y).

2. A method according to claim 1 wherein each said unique surround function is a Gaussian function.

3. A method according to claim 2 wherein said Gaussian function is of the form

4. A method according to claim 1 further comprising the step of multiplying said filtered intensity value R_i(x,y) by

5. A method according to claim 1 wherein said each position (x,y) defines a pixel of said display.

6. A method according to claim 1 wherein, for each n, W_n=1/N.

7. A method according to claim 1 further comprising the step of displaying an improved image using said maximum intensity value V₁(x, y).

8. A method according to claim 4 further comprising the step of displaying an improved image using said maximum intensity value V_i(x,y).

9. A method according to claim 1 further comprising the steps of:

defining a classification of said image based on dynamic range of said image in each of said S spectral bands; and

selecting said filter function based on said classification of said image.

10. A method according to claim 9 wherein said step of defining comprises the step of using image statistics associated with said image in each of said S spectral bands.

11. A method according to claim 10 wherein said image statistics include brightness and contrast of said image in each of said S spectral bands.

12. A method according to claim 4 further comprising the steps of:

defining a classification of said image based on dynamic range of said image in each of said S spectral bands; and

selecting said filter function based on said classification of said image.

13. A method according to claim 12 wherein said step of defining comprises the step of using image statistics associated with said image in each of said S spectral bands.

14. A method according to claim 13 wherein said image statistics include brightness and contrast of said image in each of said S spectral bands.

15. A method of processing a digital image, comprising the steps of:

providing digital data indexed to represent the positions of a plurality of pixels of a J-row by K-column display, said digital data being indicative of an intensity value I(x,y) for each of said plurality of pixels where x is an index of a position in the J-th row of said display and y is an index of a position in the K-th column of said display wherein a J×

K image is defined;

convolving said digital data associated with each of said plurality of pixels with a function

${\mathrm{<br><br>}}^{\frac{-r^2}{c^2}}$

to form a discrete convolution value for each of said plurality of pixels, said function satisfying the relationship

$k<br><br>\int <br><br>\int <br><br>{\mathrm{<br><br>}}^{\frac{-r^2}{c^2}}<br><br><br><br>x<br><br><br><br>y=1$

where

r={square root}{square root over (x²+y²)}

k is a normalization constant and c is a constant;

converting, for each of said plurality of pixels, said discrete convolution value into the logarithm domain;

converting, for each of said plurality of pixels, said intensity value into the logarithm domain;

subtracting, for each of said plurality of pixels, said discrete convolution value so-converted into the logarithm domain from said intensity value so-converted into the logarithm domain, wherein an adjusted intensity value is generated for each of said plurality of pixels;

filtering said adjusted intensity value for each of said plurality of pixels with a filter function wherein a filtered intensity value R(x,y) is defined; and

selecting, for each of said plurality of pixels, a maximum intensity value V(x,y) from the group consisting of said intensity value I(x,y) and said filtered intensity value R(x,y).

16. A method according to claim 15 wherein the value of said constant c is selected to be in the range of approximately 0.01 to approximately 0.5 of the larger of J and K.

17. A method according to claim 15 further comprising the step of displaying an improved image using said maximum intensity value V(x,y).

18. A method according to claim 15 wherein said step of filtering includes the step of selecting said filter function based on dynamic range of said J×

K image.

19. A method of processing a digital image, comprising the steps of:

providing digital data indexed to represent the positions of a plurality of pixels of an J-row by K-column display, said digital data being indicative of an intensity value I_i(x,y) for each i-th spectral band of S spectral bands for each of said plurality of pixels where x is an index of a position in the J-th row of said display and y is an index of a position in the K-th column of said display wherein a (J×

K)_i image is defined for each of said S spectral bands and a J×

K image is defined across all of said S spectral bands;

convolving said digital data associated with each of said plurality of pixels in each i-th spectral band with a function

${\mathrm{<br><br>}}^{\frac{-r^2}{c_n^2}}$

for n=2 to N to form N convolution values for each of said plurality of pixels in each said i-th spectral band, said function satisfying the relationship

$k_n<br><br>\int <br><br>\int <br><br>{\mathrm{<br><br>}}^{\frac{-r^2}{c_n^2}}<br><br><br><br>x<br><br><br><br>y=1$

where

r={square root}{square root over (x²+y²)}

and, for each n, k_n is a normalization constant and c_n is a unique constant;

converting, for each of said plurality of pixels in each said i-th spectral band, each of said N convolution values into the logarithm domain;

converting, for each of said plurality of pixels in each said i-th spectral band, said intensity value into the logarithm domain;

subtracting, for each of said plurality of pixels in each said i-th spectral band, each of said N convolution values so-converted into the logarithm domain from said intensity value so-converted into the logarithm domain, wherein an adjusted intensity value is generated for each of said plurality of pixels in each said i-th spectral band based on each of said N convolution values;

forming a weighted sum for each of said plurality of pixels in each said i-th spectral band using said adjusted intensity values;

filtering said weighted sum for each of said plurality of pixels in each said i-th spectral band with a filter function wherein a filtered intensity value R_i(x,y) is defined; and

selecting a maximum intensity value V_i(x,y) from the group consisting of said intensity value I_i(x,y) and said filtered intensity value R_i(x,y).

20. A method according to claim 19 wherein the value for each said unique constant c_n is selected to be in the range of approximately 0.01 to approximately 0.5 of the larger of J and K.

21. A method according to claim 19 further comprising the step of multiplying said filtered intensity value R_i(x,y) by

22. A method according to claim 19 further comprising the step of displaying an improved image using said maximum intensity value V_i(x,y).

23. A method according to claim 21 further comprising the step of displaying an improved image using said maximum intensity value V_i(x,y).

24. A method according to claim 19 further comprising the steps of:

defining a classification of said J×

K image based on dynamic range of each said (J×

K)_i image; and

selecting said filter function based on said classification of said J×

K image.

25. A method according to claim 24 wherein said step of defining comprises the step of using image statistics associated with each said (J×

K)_i image.

26. A method according to claim 25 wherein said image statistics include brightness and contrast of each said (J×

K)_i image.

27. A method according to claim 21 further comprising the steps of:

defining a classification of said J×

K image based on dynamic range of each said (J×

K)_i image; and

selecting said filter function based on said classification of said J×

K image.

28. A method according to claim 27 wherein said step of defining comprises the step of using image statistics associated with each said (J×

K)_i image.

29. A method according to claim 28 wherein said image statistics include brightness and contrast of each said (J×

K)_i image.

30. A method of processing a digital image, comprising the steps of:

providing digital data indexed to represent positions of an image having S spectral bands for simultaneous output on a display, said digital data being indicative of an intensity value I_i(x,y) for each position (x,y) in each i-th spectral band;

adjusting said intensity value for said each position in each i-th spectral band to generate an adjusted intensity value for said each position in each i-th spectral band in accordance with

$\underset{n=1}{\overset{N}{\sum <br><br>}}<br><br>W_n<br><br>\left(\log <br><br>\text{\hspace{1em}<br><br>}<br><br>I_i<br><br>\left(x,y\right)-\log <br><br>\text{\hspace{1em}<br><br>}\left[I_i<br><br>\left(x,y\right)*F_n<br><br>\left(x,y\right)\right]\right),i=1,\mathrm{\dots <br><br>}<br><br>\text{\hspace{1em}<br><br>},S$

where S is a whole number greater than or equal to 2 and defines the total number of spectral bands included in said digital data and, for each n, W_n is a weighting factor and F_n(x,y) is a unique surround function of the form

${\mathrm{<br><br>}}^{\frac{-r^2}{c_n^2}}$

satisfying the relationship

$k_n<br><br>\int <br><br>\int <br><br>{\mathrm{<br><br>}}^{\frac{-r^2}{c_n^2}}<br><br><br><br>x<br><br><br><br>y=1$

where

r={square root}{square root over (x²+y²)}

and, for each n, k_n is a normalization constant and c_n is a unique constant where N is the total number of unique surround functions;

filtering said adjusted intensity value for said each position in each i-th spectral band with a filter function wherein a filtered intensity value R_i(x,y) is defined;

multiplying said filtered intensity value R_i(x,y) by

$\log \left[\frac{{\mathrm{BI}}_i<br><br>\left(x,y\right)}{\underset{i=1}{\overset{S}{\sum <br><br>}}<br><br>I_i<br><br>\left(x,y\right)}\right]$

to define a color-restored intensity value R′

_i(x,y), where B is a constant; and

selecting a maximum intensity value V_i(x,y) from the group consisting of said intensity value I_i(x,y) and said color-restored intensity value R′

_i(x,y).

31. A method according to claim 30 wherein, for each n, W_n=1/N.

32. A method according to claim 30 wherein the value for each said unique constant c_n is selected to be in the range of approximately 0.01 to approximately 0.5 of the larger of J and K.

33. A method according to claim 30 further comprising the step of displaying an improved image using said maximum intensity value V_i(x,y).

34. A method according to claim 30 further comprising the steps of:

defining a classification of said image based on dynamic range of said image in each of said S spectral bands; and

selecting said filter function based on said classification of said image.

35. A method according to claim 34 wherein said step of defining comprises the step of using image statistics associated with said image in each of said S spectral bands.

36. A method according to claim 35 wherein said image statistics include brightness and contrast of said image in each of said S spectral bands.