Claims
- 1. A method of digital image processing comprising:receiving digital image data including a plurality of input pixel values selected from an input color gamut, each of said input pixel values defined in terms of plural gray values wherein each gray value is scaled into the range of 0≦gray value ≦1, respectively mapping each input pixel value to an output pixel value, said mapping including: (a) individually inverting said gray values for each input pixel value according to the equation: 1-gray value, (b) applying a gamma value (γ) individually to each inverted gray value according to the equation: (inverted gray value)γ, said gamma value being related to an amount by which a dynamic lightness range of the digital image data is to be compressed relative to an output color gamut of an image output device, and, (c) inverting each inverted gray value after application of said gamma function thereto according to the equation: 1-(inverted gray value)γso that said gray values defining said input pixel values are mapped to gamma-modified gray values defining said output pixel values, respectively.
- 2. The digital image processing method as set forth in claim 1 wherein, for every output pixel value out-of-gamut relative to said output gamut, said out-of gamut output pixel is mapped to said output gamut using a clipping operation.
- 3. The digital image processing method as set forth in claim 1 wherein steps (a)-(c) are performed in advance of receiving said digital image data for every input pixel value in said input gamut so that pre-computed gamma-modified gray values defining output pixel values respectively corresponding to each input pixel value in said input gamut are determined, said method further comprising:storing said pre-computed values in one-dimensional look-up tables; and, for each input pixel value of said received digital image data, accessing said look-up tables to obtain said pre-computed gamma-modified gray values.
- 4. The digital image processing method as set forth in claim 1 wherein said gamma value (γ) is selected according to the following equation:γ=log(1−Yx′)/log(1−Yx) wherein,Yx=(1−Ymin)*(1−(X/100))+Ymin Yx′=(1−Ymin′)*(1−(X/100))+Ymin′X=a compression percentage in the range of 92%-98% Ymin=a darkest expected input pixel luminance value in said image (0≦Ymin≦1) Ymin′=a darkest output luminance value in said output gamut (0≦Ymin′≦1).
- 5. The digital image processing method as set forth in claim 4 wherein said compression percentage (X) is selected as approximately 95% so that approximately 95% of the expected luminance dynamic range of the image is mapped to approximately 95% of the luminance dynamic range of the image output device.
- 6. The digital image processing method as set forth in claim 1 wherein said gamma value (γ) is selected according to the following equation:γ=log(1−Yx′)/log(1−Yx) wherein,Yx=(1−Ymin)*(1−(X/100))+Ymin Yx′=(1−Ymin′)*(1−(X/100))+Ymin′X=a compression percentage in the range of 92%-98% Ymin=a darkest actual input pixel luminance value in said (0 ≦Ymin≦1) Ymin′=a darkest output luminance value in said output gamut (0≦Ymin′≦1).
- 7. The digital image processing method as set forth in claim 6 wherein said compression percentage (X) is selected as approximately 95% so that approximately 95% of the actual luminance dynamic range of the image is mapped to approximately 95% of the luminance dynamic range of the image output device.
- 8. The digital image processing method as set forth in claim 1 wherein said mamma value is selected to be in the range of γ=1 to γ=2.5.
- 9. The digital image processing method as set forth in claim 1 wherein each of said input pixel values is defined in terms of red (R), green (G), and blue (B) gray values wherein said mapping steps (a)-(c) are carried out according to:gamma-modified R=1−(1−R)γgamma-modified G=1−(1−G)γgamma-modified B=1−(1−B)γ.
- 10. The digital image processing method as set forth in claim 2 wherein said out-of-gamut output pixel values are mapped directly into the output gamut by one of a nearest-point clipping and a centroid clipping mapping operation.
- 11. The digital image processing method as set forth in claim 1 wherein each of said input pixel values is defined in terms of a luminance value Y, a first chrominance value C1, and a second chrominance value C2, wherein said gamma value is applied only to the luminance value Y according to:gamma-modified Y=1−(1−Y)γ.
- 12. The digital image processing method as set forth in claim 11 wherein the input pixel value is defined in terms of CIELAB color space.
- 13. A digital image processing apparatus comprising:means for input of digital image data of an image in terms of a plurality of input pixel values selected from an input gamut and defined in terms of at least one gray value in or scaled into the range of 0 to 1; means for output of digital image data in terms of an output color gamut; and, an image processing unit for mapping each input pixel value to an output pixel value, said image processing unit comprising: means for inverting said at least one gray value to an inverted gray value; means for applying a gamma function to said inverted gray value according to: (inverted gray value)γ, wherein gamma (γ) is related to an amount by which a dynamic lightness range of the input image is to be compressed relative to the output color gamut; and, means for converting the inverted gray value, after application of said gamma function, to a non-inverted gray value.
- 14. The digital image processing apparatus as set forth in claim 13 further comprising:means for deriving gamma (γ) based upon one of: (i) a darkest expected input pixel luminance value in said image; and, (ii) a darkest actual input pixel luminance value in said image.
- 15. The digital image processing apparatus as set forth in claim 14 wherein said means for deriving gamma (γ) derives gamma according to:γ=log(1−Yx′)/log(1−Yx) wherein,Yx=(1−Ymin)*(1−(X/100))+Ymin Yx′=(1−Ymin′)*(1−(X/100))+Ymin′X=a compression percentage in the range of 92%-98% Ymin=one of a darkest expected input pixel luminance value in said image expressed in terms of (0≦Ymin≦1) Ymin′=a darkest output luminance value in said output gamut expressed in terms of (0≦Ymin′≦1).
- 16. The digital image processing apparatus as set forth in claim 15 wherein said compression percentage is selected to be 95%.
- 17. The digital image processing apparatus as set forth in claim 13 further comprising:means for clipping output pixel values which are outside said output gamut to obtain associated in-gamut output pixel values.
- 18. The digital image processing apparatus as set forth in claim 17 wherein said clipping means maps out-of-gamut output pixel values into the output gamut by one of a nearest-point clipping and a centroid clipping mapping operation.
- 19. The digital image processing apparatus as set forth in claim 13 wherein each of said input pixel values is defined in terms of red (R), green (G), and blue (B) gray values each expressed in terms of a value in the range of 0-1, wherein said inversion means, said gamma function application means, and said converting means operate on said gray values according to:gamma-modified R=1−(1−R)γgamma-modified G=1−(1−G)γgamma-modified B=1−(1−B)γwherein gamma-modified R, gamma-modified G, and gamma-modified B define said output pixel value mapped from said input pixel value.
- 20. The digital image processing apparatus as set forth in claim 13 wherein each of said input pixel values is defined in terms of a luminance value Y, a first chrominance value C1, and a second chrominance value C2, wherein said gamma function application means operates only on said luminance value Y of each input pixel value according to:gamma-modified Y=1−(1−Y)γ, wherein said output pixel value is defined in terms of gamma-modified Y and said first and second chrominance values C1,C2.
- 21. A method of mapping an input pixel value defined in terms of a plurality of gray values, each of said gray values having a scaled value in the range of 0 to 1, from a value in an input gamut to an output value in an output gamut, said method comprising:inverting at least one of said gray values to obtain an inverted gray value; applying a gamma function to said inverted gray value to obtain a gamma-modified inverted gray value according to: gamma-modified inverted gray value=(inverted gray value)γ, wherein γ is determined according to: γ=log(1−Yx′)/log(1−Yx) Yx=(1−Ymin)*(1−(X/100))+Ymin Yx′=(1−Ymin′)*(1−(X/100))+Ymin′X=a compression percentage in the range of 92%-98% Ymin=one of: (i) a darkest expected input pixel luminance value in said image; and (ii) a darkest actual input pixel luminance value in said image, wherein Ymin is expressed in terms of 0≦Ymin≦1 Ymin′=a darkest output luminance value in said output gamut expressed in terms of 0≦Ymin′≦1; and, inverting said gamma-modified inverted gray value.
INCORPORATION BY REFERENCE
The disclosure of commonly assigned U.S. Pat. No. 5,414,538 to Eschbach issued May 9, 1995 and entitled “Image-Dependent Exposure Enhancement” is expressly incorporated by reference herein as are the following documents: (1) Sara, J. J., The Automated Reproduction of Pictures with Nonreproduicible Colors, Ph.D. Thesis, Massachusetts Institute of Technology, 1984; and, (2) Pariser, E. G., An Investigation of Color Gamut Reduction Techniques, IS&T 2nd Symposium on Electronic Publishing, 105-107 (1991).
US Referenced Citations (8)
Foreign Referenced Citations (1)
Number |
Date |
Country |
2000184225 |
Jun 2000 |
JP |
Non-Patent Literature Citations (2)
Entry |
Pariser, E.G., An Investigation of Color Gamut Reduction Techniques, IS&T 2nd Symposium on Electronic Publishing, pp. 105-107 (1991). |
Sara, J.J., The Automated Reproduction of Pictures with Nonreproducible Colors, 1984. |