Method of improving a digital image

• US 5,991,456 A
• Filed: 05/29/1996
• Issued: 11/23/1999
• Est. Priority Date: 05/29/1996
• Status: Expired due to Term

• Current Assignee: National Aeronautics and Space Administration (Government of the United States of America), Scientific Technologies Incorporated
• Original Assignee: Scientific Technologies Incorporated, United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Rahman, Zia-ur, Jobson, Daniel J., Woodell, Glenn A.
• Primary Examiner(s): LEE, TOMMY D
• Application Number:
  US08/654,840
• Time in Patent Office: 1,273 Days
• Field of Search: 382/254, 382/260, 382/274, 382/162, 382/279, 358/447, 358/448, 358/461, 358/530, 358/532
• US Class Current: 382/254
• CPC Class Codes: G06T 5/20 using local operators

Abstract

A method of improving a digital image is provided. The image 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 ##EQU1## where S is the number of unique spectral bands included in said digital data, W_n is a weighting factor and "*" denotes the convolution operator. Each surround function F_n (x,y) is uniquely scaled to improve an 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 is filtered with a common function and then presented to a display device. For color images, a novel color restoration step is added to give the image true-to-life color that closely matches human observation.

Claims

1. A method of improving 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 ##EQU9## 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 in each i-th spectral band with a common function; and

displaying an improved digital image based on said adjusted intensity value for each i-th spectral band so-filtered for said each position.

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 ##EQU10## satisfying the relationship ##EQU11## where ##EQU12## and, for each n, k_n is a normalization constant and c_n is a unique constant for each of said N unique surround functions.

4. A method according to claim 1 further comprising the step of multiplying, prior to said step of displaying, said adjusted intensity value for said each position in each i-th spectral band by ##EQU13## where B is a constant.

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 of improving 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;

convolving said digital data associated with each of said plurality of pixels with a function ##EQU14## to form a discrete convolution value for each of said plurality of pixels, said function satisfying the relationship ##EQU15## where ##EQU16## 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 function that is identical for said plurality of pixels; and

displaying an improved digital image based on said adjusted intensity value so-filtered for each of said plurality of pixels.

8. A method according to claim 7 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.

9. A method of improving 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 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;

convolving said digital data associated with each of said plurality of pixels in each i-th spectral band with a function ##EQU17## 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 ##EQU18## where ##EQU19## 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 function that is identical for said plurality of pixels in each said i-th spectral band; and

displaying an improved digital image based on said weighted sum so-filtered for each of said plurality of pixels in each said i-th spectral band.

10. A method according to claim 9 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.

11. A method according to claim 9 further comprising the step of multiplying, prior to said step of displaying, said weighted sum for said each of said plurality of pixels in each said i-th spectral band by ##EQU20## where B is a constant and S is a whole number greater than or equal to 2 and defines the total number of spectral bands included in said digital data.

12. A method of improving 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 ##EQU21## 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 ##EQU22## satisfying the relationship ##EQU23## where ##EQU24## 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 common function;

multiplying said adjusted intensity value for said each position in each i-th spectral band by ##EQU25## where B is a constant; and

displaying an improved digital image based on said adjusted intensity value for each i-th spectral band so-filtered and so-multiplied for said each position.

13. A method according to claim 12 wherein, for each n, W_n =1/N.

14. A method according to claim 12 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.