Image-dependent color correction using black point and white point in a natural scene pictorial image

• US 5,371,615 A
• Filed: 10/22/1993
• Issued: 12/06/1994
• Est. Priority Date: 10/22/1993
• Status: Expired due to Term

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Eschbach, Reiner
• Primary Examiner(s): Coles, Sr., Edward L.
• Assistant Examiner(s): VU, KIM Y
• Application Number:
  US08/139,660
• Time in Patent Office: 410 Days
• Field of Search: 358/500, 358/515, 358/518, 358/529, 358/509, 358/516, 358/443, 358/296, 358/298, 358/461, 358/464, 355/326, 355/327, 348/673, 348/678, 348/251, 348/252, 348/615, 348/655, 348/28, 382/42
• US Class Current: 358/515
• CPC Class Codes: H04N 1/6027 Correction or control of colour gradation or colour contrast H04N1/6058 takes precedence
• CPC Class Codes: H04N 1/6077 Colour balance, e.g. colour cast correction
• CPC Class Codes: H04N 1/6086 by scene illuminant, i.e. conditions at the time of picture capture, e.g. flash, optical filter used, evening, cloud, daylight, artificial lighting, white point measurement, colour temperature

Abstract

A method for correcting color shifting produced by improper imaging, in a digital signal representation of a natural scene image including the steps of: determining the white point of an image; determining the black point in an image; calculating a color correction for the image as a function of the white point and the black point and applying the correction to each pixel in the digital signal representation of a natural scene image for printing.

Claims

1. A method for reproducing a natural scene image image from electronically encoded intensity signals, while correcting color shift therein produced by improper imaging including the steps of:

receiving an electronically encoded natural scene image S(m,n), providing a plurality of N separations S_sep1 . . . S_sepN each describing intensity for a given color primary forming the image, each separation formed by a plurality of input separation intensity signals S_sepN (m,n) defining the intensity for a discrete area therein, whereby color for a discrete area in the image is a combined output device response to separation intensity signals S_sepN (m,n) for the corresponding discrete area from each separation S_sep1 . . . S_sepN ;

identifying the combination of separation intensity signals S_sepN (m,n) in the image providing the color closest to white, and further identifying the largest signal S_sepN (m,n) in the combination as white signal W_max ;

identifying the combination of separation intensity signals S_sepN (m,n) in the image providing the color closest to black, and further identifying the smallest signal S_sepN (m,n) in the combination as black signal W_min ;

for each separation, modifying each intensity signal S_sepN (m,n) with a function;

##EQU2## Where Sep_in is a single input intensity signal in S_sepN (m,n) in a selected one of the N separations,

Sep_out is a single output intensity signal corresponding to Sep_in in S_sep (m,n) in the selected separation,

Sep_max is the separation signal in the combination of signals S_sepN (m,n) providing the color closest to white,

Sep_min is the separation signal in the combination of signals S_sepN (m,n) providing the color closest to black;

directing the modified signals to an output device to form a color shifted signals.

2. The method as described in claim 1, wherein there are three separations.

3. The method as described in claim 2, wherein the three separations correspond to the color primaries red, green and blue.

4. The method as defined in claim 1, wherein the color closest to white is the combination of signals S_sep (m,n) in the image having the largest minimum signal.

5. The method as defined in claim 1, wherein the color closest to black is the combination of signals S_sep (m,n) in the image having the smallest maximum signal.

6. The method as defined in claim 1, wherein the modifying step includes the following substeps:

calculating for each possible intensity signal in each separation a modified intensity signal;

storing the modified intensity signals in an addressable look-up table in memory;

receiving intensity signals S_sep (m,n) as addresses to the look-up table; and

directing modified intensity signals to an the output step.

7. The method as defined in claim 1, where the color white is identified from a set of combined signals S_sep (m,n) which do not include maximum system values.

8. The method as defined in claim 1, where the color black is identified from a set of combined signals S_sep (m,n) which do not include minimum system values.

9. An apparatus for reproducing a natural scene image image from electronically encoded intensity signals, while correcting color shift therein produced by improper imaging comprising:

means for receiving an electronically encoded natural scene image S(m,n), providing a plurality of N separations S_sep1 . . . S_sepN each describing intensity for a given color primary forming the image, each separation formed by a plurality of input separation intensity signals S_sepN (m,n) defining the intensity for a discrete area therein, whereby color for a discrete area in the image is a combined output device response to separation intensity signals S_sepN (m,n) for the corresponding discrete area from each separation S_sep1 . . . S_sepN ;

means for identifying the combination of separation intensity signals S_sepN (m,n) in the image providing the color closest to white, and further identifying the largest signal S_sepN (m,n) in the combination as white signal W_max ;

means for identifying the combination of separation intensity signals S_sepN (m,n) in the image providing the color closest to black, and further identifying the smallest signal S_sepN (m,n) in the combination as black signal W_min ;

means for modifying the intensity value of each signal in each separation N as a function of the separation signal S_sepN (m,n) in the color closest to white, the separation signal S_sepM (m,n) in the color closest to black, W_min and W_max ;

means for directing the color shifted signals to an output device.

10. The apparatus as described in claim 9, wherein there are three separations.

11. The apparatus as described in claim 10, wherein the three separations correspond to the color primaries red, green and blue.

12. The apparatus as defined in claim 9, wherein the color closest to white is the combination of signals S_sep (m,n) in the image having the largest minimum signal.

13. The apparatus as defined in claim 9, wherein the color closest to black is the combination of signals S_sep (m,n) in the image having the smallest maximum signal.

14. The apparatus as defined in claim 9, wherein the modifying means includes:

means for calculating for each possible intensity signal in each separation a modified intensity signal;

means for storing the result in an addressable look-up table in memory;

means for receiving intensity signals S_sep (m,n) as addresses to the look-up table; and

means for directing modified intensity signals to an output step.

15. The apparatus as defined in claim 9, wherein the modifying means modifies each intensity signal S_sepN (m,n) forming the separation with a function:

##EQU3## Where Sep_in is a single input intensity signal in S_sepN (m,n) in a selected one of the N separations,

Sep_out is a single output intensity signal corresponding to Sep_in in S_sepN (m,n) in the selected separation,

Sep_max is the selected separation signal in the combination of signals S_sepN (m,n) in the image providing the color closest to white,

Sep_max is the selected separation signal in the combination of signals S_sepN (m,n) in the image providing the color closest to black.

16. A color reproduction system system, including color correction, for reproduction of natural scene original images in which the color is undesirably shifted including:

a color image scanner, producing from a scanned natural scene original document in which the color is undesirably shifted, a set of electronic signals in a format providing a plurality of N separations describing intensity for N color primaries within the image, each separation formed by a plurality of separation intensity signals S_sepN (m,n) for discrete areas therein, whereby color for a discrete area in the image is a combined output response of separation intensity signals S_sepN (m,n) for the corresponding area m,n in the image from each of the N separations;

a separation maximum detector receiving as an input the separation intensity signals S_sepN (m,n) for the image and producing as an output a set of N separation intensity signals S_sep1max . . . S_sepNmax representing an unsaturated color in the image closest to white and a maximum separation intensity signal W_max thereof;

a separation minimum detector receiving as an input the separation intensity signals S_sepN (m,n) for the image and producing as an output a set of N separation intensity signals S_sep1min . . . S_sepNmin representing a color in the image closest to black and a minimum separation intensity signal W_min thereof;

a shift correction calculation circuit, calculating for each possible intensity signal in each separation 1 . . . N a modified intensity signal as a function of the separation signal S_sepN (m,n) in the color closest to white, the separation signal S_sepM (m,n) in the color closest to black, W_min and W_max ;

a device memory receiving said set of corrected separation intensity signals in a table for each separation, addressable by a separation intensity value from the color scanner;

an output reproduction device, operative responsive to the corrected signals in each of N separation to produce a color response.

17. The device as defined in claim 16, wherein there are three separations.

18. The device as described in claim 17, wherein the three separations correspond to the color primaries red, green and blue.

19. The apparatus as defined in claim 16, wherein the color closest to white is the combination of signals S_sepN (m,n) in the image having the largest minimum signal.

20. The apparatus as defined in claim 16, wherein the color closest to black is the combination of signals S_sepN (m,n) in the image having the smallest maximum signal.

21. The apparatus as defined in claim 16, wherein the calculating includes:

a calculating circuit deriving a modification for each possible intensity signal in each separation a modified intensity signal;

an addressable memory storing the result in a look-up table;

a storage control arrangement for storing calculated values to the addressable memory and directing modified intensity signals to an output step.

22. A color reproduction system including color correction for reproduction of electronically encoded natural scene original images in which the color is undesirably shifted including:

a color image scanner, producing from a scanned original image S(m,n) in which the color is undesirably shifted, a set of electronic signals in a format providing separations describing intensity for red, green and blue colors, each separation formed by a plurality of intensity signals for discrete areas in the image, whereby color for a discrete area in the image is a combined output response of a triplet of intensity signals for a discrete area (m,n) from each separation R, G, B;

a maximum detector receiving as an input the separation intensity signals from each separation and identifying as an image white point a triplet of intensity signals containing the color closest to white and one intensity signal W_max of the triplet which is largest;

a minimum detector receiving as an input the separation intensity signals from each separation and identifying as an image black point a triplet min containing the color closest to black and one intensity signal W_min of the triplet which is largest;

a color shift calculation circuit determining for each separation r, g, b a corrected intensity values in accordance with the function;

##EQU4## Where (R,G,B)_in are single input intensity signals in a selected separations,

(R,G,B)_out are single output intensity signals corresponding to (R,G,B)_in in a selected separation,

(R,G,B)_w is the corresponding separation signal in the color closest to white,

(R,G,B)_B is the corresponding separation signal in the color closest to black.

an output reproduction device, operative responsive to the corrected signals (R,G,B)_out (m,n)in each of the separations to produce a color response S'"'"'(n,m).