Method and apparatus for correcting luminance and chrominance data in digital color images

• US 6,268,939 B1
• Filed: 01/08/1998
• Issued: 07/31/2001
• Est. Priority Date: 01/08/1998
• Status: Expired due to Term

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Klassen, R. Victor, Balasubramanian, Thyagarajan
• Primary Examiner(s): Coles, Edward
• Assistant Examiner(s): Pokrzywa, Joseph R.
• Application Number:
  US09/004,650
• Time in Patent Office: 1,300 Days
• Field of Search: 358/518, 358/500, 358/504, 358/505, 358/515, 358/516, 358/517, 358/519, 358/520, 358/525, 382/162, 382/167, 345/154, 345/150
• US Class Current: 358/518
• CPC Class Codes: H04N 1/60 Colour correction or control H04N1/54 takes precedence
• CPC Class Codes: H04N 1/6058 Reduction of colour to a range of reproducible colours, e.g. to ink- reproducible colour gamut

Abstract

The present invention is directed to a method and apparatus for improving the efficiency of color correcting subsampled luminance and chrominance based data. More specifically, the present invention is directed to a method of applying a full conversion between color spaces for one pixel in a selected pixel block. Chrominance values are assigned to the remaining pixels based upon their luminance values relative to the luminance value of the converted pixel. The invention may be used to convert between various types of color spaces, and may be used in the processing of both device dependent and device independent data.

Claims

1. A method of correcting data in an input device dependent digital color image generated at an input device, preparatory to reproduction at a plural channel output device, comprising the steps of:

a) providing input device dependent color image signals in an input device dependent color space said device dependent color image signals having been sub-sampled;

b) providing information about the color response characteristic of the input device and the output device;

c) using said information, mapping said input device dependent color image signals to output device dependent color image signals at said plurality of channels in an output device dependent color space, said output device dependent color space being an RGB space, with a complex mapping process which includes color correction, wherein said output device dependent color image signals correspond to said input device dependent color image signals said processing comprising;

i) deriving a set of input device dependent master signals from said input device dependent color image signals by designating a signal set at a given location within each block wherein said input device dependent color imaging signals are received at three channels, wherein a first channel L receives luminance signals at a luminance channel resolution, a second channel Cr receives red-green chrominance signals at a red-green chrominance channel resolution, and a third channel Cb receives blue-yellow chrominance signals at a blue-yellow chrominance resolution;

ii) mapping said input device dependent master signal set to an output device dependent master signal set in said output device dependent color space, said mapping comprising;

dividing the digital color image into blocks, each block including a plurality of device dependent signal sets, wherein each signal set includes a signal from each of said channels, and wherein each block includes one master signal set; and

applying a look up table conversion to transform said master signal set from said device dependent color space to said output device dependent color space; and

iii) defining output device dependent color image signals corresponding to a subset of said input device dependent color signals, said defining step further comprises creating output device dependent signals for each signal set in said block according to a luminance signal for each signal set relative to a luminance of said master signal set; and

wherein said creating step comprising;

setting output device dependent signal Rij=(Rm/Lm)Lij;

setting output device dependent signal Gij=(Gm/Lm)Lij; and

setting output device dependent signal Bij=(Bm/Lm)Lij for each signal set Pij in said block, wherein ii denotes a location of each set within said block, Lij is said luminance signal of signal set Pij, Rm, Gm, and Bm are master signal set output device dependent signals and Lm is said master signal set luminance signal; and

d) converting said output device dependent color image signals with a simple color conversion method to output device dependent coordinate signals which correspond to output device colorants.

2. A method of correcting data as claimed in claim 1 wherein said master signal set deriving step further comprises designating a pseudo-signal that has L channel, Cr channel, and Cb channel signals equal to the mean L channel, Cr channel, and Cb channel signals for all signal sets in said block.

3. A method of correcting data as claimed in claim 1 wherein said master signal set deriving step further comprises choosing a desired relative luminance signal.

4. A method of correcting data as claimed in claim 3 wherein said chosen luminance signal is a maximum signal of all luminance signals in said block.

5. A method of correcting data as claimed in claim 3 wherein said chosen luminance signal is nearest to an average for all luminance signals in said block.

6. A method of correcting data as claimed in claim 1 wherein said creating step further comprises:

a) setting output device dependent signal Rij=a{circumflex over ( )}(loga Rm−

loga Lm+loga Lij);

b) setting output device dependent signal Gij=a{circumflex over ( )}(loga Gm−

loga Lm+loga Lij); and

c) setting output device dependent signal Bij=a{circumflex over ( )}(loga Bm−

loga Lm+loga Lij)

for each signal set Pij in said block, wherein ij denotes a location of each signal set within said block, Lij is said luminance signal of signal set Pij , Rm, Gm, and Bm are master signal set output device dependent signals and Lm is said master signal set luminance signal.

7. A method of correcting data as claimed in claim 1 wherein said creating step further comprises:

a) setting output device dependent signal Rij=Rm−

Lm+Lij

b) setting output device dependent signal Gij=Gm−

Lm+Lij; and

c) setting output device dependent signal Bij=Bm−

Lm+Lij

for each signal set Pij in said block, wherein ij denotes a location of each signal set within said block, Lij is said luminance signal of signal set Pij, Rm, Gm, and Bm are master signal set output device dependent signals and Lm is said master signal set luminance signal.

8. A method of correcting data as claimed in claim 1 wherein said output device dependent color space is an LCrCb space.

9. A method of correcting data as claimed in claim 8 wherein said creating step further comprises:

a) setting output device dependent signal L_ij′

=(L_m′

/L_m)L_ij;

b) setting output device dependent signal Cr_ij′

=(L_m′

/L_m)Cr_ij;

c) setting output device dependent signal Cb_ij′

=(L_m′

/L_m)Cb_ij;

for each signal set P_ij in said block, wherein ij denotes a location of each set within said block, L_ij is said device dependent luminance signal of signal set P_ij, L_m′

, is said master signal set output device dependent signal, and L_m is said master signal set device dependent luminance signal.

10. A method of correcting data as claimed in claim 1 wherein an output device dependent color space is a CMYK color space.

11. A method of correcting data as claimed in claim 10 wherein said defining step further comprises:

a) setting output device dependent coordinate signal C_ij=1−

((1−

C_m)/L_m) L_ij;

b) setting output device dependent coordinate signal M_ij=1−

((1−

M_m)/L_m)L_ij;

c) setting output device dependent coordinate signal Y_ij=1−

((1−

Y_m)/L_m)L_ij; and

d) setting output device dependent coordinate signal K_ij=K_m,

for each signal set P_ij in said block, wherein ij denotes a location of each signal set within said block, L_ij is said luminance signal of signal set P_ij, C_m, M_m, Y_m, and K_m are master signal set device dependent color image signals and L_m is a master signal set luminance signal.

12. A method of correcting data as claimed in claim 1 wherein said red-green channel resolution is less than said luminance channel resolution and said blue-yellow channel resolution is less than said red-green channel resolution.

13. A method of correcting data as claimed in claim 12 wherein said master signal deriving step further comprises:

a) dividing the digital color image into a plurality of blocks, and further dividing said blocks into a plurality of sub-blocks, wherein each block includes a plurality of signal sets that have a substantially constant blue-yellow resolution, and each sub-block includes a plurality of signal sets that have substantially constant red-green chrominance and blue-yellow resolutions;

b) selecting a master sub-block; and

c) choosing a signal set within said master sub-block as said master signal set.

14. A method of correcting data as claimed in claim 13 wherein said mapping step further comprises:

a) applying a look up table conversion to transform said master signal set from said device dependent color space to said output device dependent color space;

b) correcting said output device dependent red-green chrominance signals for a subset of said sub-blocks; and

c) correcting said output device dependent luminance signals for a subset of said signal sets.

15. A method of correcting data as claimed in claim 14 wherein said master sub-block selecting step further comprises designating a sub-block at a given location within each block.

16. A method of correcting data as claimed in claim 14 wherein said master sub-block selecting step further comprises creating a pseudo-sub-block that has L channel and Cr channel signals equal to the mean L channel and Cr channel signals for all signal sets in said block.

17. A method of correcting data as claimed in claim 14 wherein said master sub-block selecting step further comprises designating a desired relative red-green chrominance signal.

18. A method of correcting data as claimed in claim 17 wherein said designated red-green chrominance signal is a maximum magnitude of all chrominance red-green chrominance signals in said block.

19. A method of correcting data as claimed in claim 17 wherein said designated red-green chrominance signal is nearest to an average for all red-green chrominance signals in said block.

20. A method of correcting data as claimed in claim 14 wherein said master signal set choosing step further comprises designating a signal set at a given location within each sub-block.

21. A method of correcting data as claimed in claim 14 wherein said master signal set choosing step further comprises designating a desired relative luminance signal.

22. A method of correcting data as claimed in claim 21 wherein said designated luminance signal is a maximum signal of all luminance signals in said sub-block.

23. A method of correcting data as claimed in claim 21 wherein said designated luminance signal is nearest to an average for all luminance signals in said sub-block.

24. A method of correcting data as claimed in claim 14 wherein said output device dependent color space is an RGB space.

25. A method of correcting data as claimed in claim 24 wherein said red-green chrominance correcting step further comprises:

a) setting L_ij′

=L_m′

(L_ij/L_m);

b) setting R_ij′

=(R_m/L_m)L_ij;

c) setting G_ij′

=(G_m/L_m)L_ij;

d) setting R_ij″

=R<sub>ij′

(Crkl</sub>+Cr)/2Cr_kl+G_ij′

(Cr_kl−

Cr)/2Cr_kl;

e) setting G_ij″

=R_ij′

(Cr_kl−

Cr)/2Cr_kl+G_ij′

(Cr_kl+Cr)/2Cr_kl, and

f) setting B_ij″

=B_ij′

where Cr_ij′

=R_ij′

−

G_ij′

, and B_ij′

=Cb_ij′

+(R_ij+G_ij′

)/2.

26. A method of correcting data as claimed in claim 24 wherein said red-green chrominance correcting step further comprises:

a) setting R_ij″

=a**(Ir+Iplus−

Idenom)+a**(Ig+Iminus−

Idenom); and

b) setting G_ij″

=a**(Ir−

Iplus+Idenom)+a**(Ig−

Iminus+Idenom) where Iplus=log_a[Cr_kl+Cr], Iminus=log_a[Cr_kl−

Cr], and Idenom=log_a[2Cr_kl].

27. A method of correcting data as claimed in claim 14 wherein said output device dependent color space is an LCrCb space.

28. A method of correcting data as claimed in claim 27 wherein said red-green chrominance correcting step further comprises setting Crkl′

=Crm′

(Crkl/Crm) for each sub-block, and said luminance correcting step further comprises setting Lij′

=Lm′

(Lij/Lm) for each signal set, wherein kl denotes a location of each sub-block within said block, Crkl′

is an output device dependent red-green chrominance signal of sub-block Skl, Crm′

is an output device dependent red-green chrominance master signal, Crkl is a device dependent red-green chrominance signal of sub-block Ski, and Crm is a device dependent red-green chrominance master signal, and wherein ij denotes a location of each signal set within said block, Lij′

is an output device dependent luminance signal of signal set Pij, Lm′

is an output device dependent master signal set luminance signal, Lij is a device dependent luminance signal of signal set Pij, Lm is a device dependent master signal set luminance signal.

29. A method of correcting data as claimed in claim 14 wherein an output device dependent color space is a CMYK color space.

30. A method of correcting data as claimed in claim 29 wherein said defining step further comprises:

a) setting output device dependent coordinate signal C_ij=1−

((1−

C_m)/L_m)L_ij;

b) setting output device dependent coordinate signal M_ij=1−

((1−

M_m)/L_m)L_ij;

c) setting output device dependent coordinate signal Y_ij=1−

((1−

Y_m)/L_m)L_ij; and

d) setting output device dependent coordinate signal K_ij=K_m,

for each signal set P_ij in said block, wherein ij denotes a location of each signal set within said block, L_ij is said luminance signal of signalset P_ij, C_m, M_m, Y_m, and K_m are master signal set device dependent color image signals and Lm is a master signal set luminance signal.

31. An apparatus for correcting data in a device dependent digital color image generated at an input device, preparatory to reproduction at a plural channel output device, comprising:

a) means for providing input device dependent color image signals in an input device dependent color space said device dependent color image signals having been sub-sampled;

b) means for mapping said input device dependent color image signals to output device dependent color image signals at a plurality of channels in an output device dependent color space, said output device dependent color space being an RGB space, with a complex color mapping process which includes color correction said processing means further comprises;

i) means for deriving a set of input device dependent master signals from said input device dependent color image signals by designating a signal set at a given location within each block wherein said input device dependent color imaging signals are received at three channels, wherein a first channel L receives luminance signals at a luminance channel resolution, a second channel Cr receives red-green chrominance signals at a red-green chrominance channel resolution, and a third channel Cb receives blue-yellow chrominance signals at a blue-yellow chrominance resolution;

ii) means for mapping said input device dependent master signal set to an output device dependent master signal set in said output device dependent color space, said means for mapping comprising;

means dividing the digital color image into blocks, each block including a plurality of device dependent signal sets, wherein each signal set includes a signal from each of said channels, and wherein each block includes one master signal set; and

means for applying a look up table conversion to transform said master signal set from said device dependent color space to said output device dependent color space; and

iii) means for defining output device dependent color image signals corresponding to a subset of said input device dependent color signals, said means for defining comprising means for creating output device dependent signals for each signal set in said block according to a luminance signal for each signal set relative to a luminance of said master signal set; and

wherein said means for creating comprises;

means for setting output device dependent signal Rij=(Rm/Lm)Lij;

means for setting output device dependent signal Gij=(Gm/Lm)Lij; and

means for setting output device dependent signal Bij=(Bm/Lm)Lij for each signal set Pij in said block, wherein ij denotes a location of each set within said block, Lij is said luminance signal of signal set Pij, Rm, Gm, and Bm are master signal set output device dependent signals and Lm is said master signal set luminance signal; and

c) means for providing information about the color response characteristic of the input device and the output device, and

d) means, using said information provided, for converting said output device dependent color image signals to output device dependent coordinate signals which correspond to output device colorants with a simple color space conversion method.

32. An apparatus for correcting data as claimed in claim 31 wherein said creating means further comprises:

a) means for setting output device dependent signal R_ij=a {circumflex over ( )}(log_a Rm−

log_a L_m+log_a L_ij);

b) means for setting output device dependent signal G_ij=a {circumflex over ( )}(log_a G_m−

log_a L_m+log_a L_ij); and

c) means for setting output device dependent signal B_ij=a {circumflex over ( )}(log_a B_m−

log_a L_m+log_a L_ij)

for each signal set P_ij in said block, wherein ij denotes a location of each signal set within said block, L_ij is said luminance signal of signal set P_ij, R_m, G_m, and B_m are master signal set output device dependent signals and L_m is said master signal set luminance signal.

33. An apparatus for correcting data as claimed in claim 31 wherein said creating means further comprises:

a) means for setting output device dependent signal R_ij=R_m−

L_m+L_ij

b) means for setting output device dependent signal G_ij=G_m−

L_m+L_ij; and

c) means for setting output device dependent signal B_ij=B_m−

L_m+L_ij

for each signal set P_ij in said block, wherein ij denotes a location of each signal set within said block, L_ij is said luminance signal of signal set P_ij, R_m, G_m, and B_m are master signal set output device dependent signals and L_m is said master signal set luminance signal.

34. An apparatus for correcting data as claimed in claim 31 wherein said creating means further comprises:

a) means for setting output device dependent signal L_ij′

=(L_m′

/L_m)L_ij;

b) means for setting output device dependent signal Cr_ij′

=(L_m′

/L_m)Cr_ij;

c) means for setting output device dependent signal Cb_ij′

=(L_m′

/L_m)Cb_ij;

for each signal set P_ij in said block, wherein ij denotes a location of each set within said block, L_ij is said device independent luminance signal of signal set P_ij, L_m′

, is said master signal set output device dependent signal, and L_m is said master signal set device independent luminance signal.

35. An apparatus for correcting data as claimed in claim 31 wherein said defining means further comprises:

a) means for setting output device dependent coordinate signal C_ij=1−

((1−

C_m)/L_m)L_ij;

b) means for setting output device dependent coordinate signal M_ij=1−

((1−

M_m)/L_m)L_ij;

c) means for setting output device dependent coordinate signal Y_ij=1−

((1−

Y_m)/L_m)L_ij; and

d) means for setting output device dependent coordinate signal K_ij=K_m,

for each signal set P_ij in said block, wherein ij denotes a location of each signal set within said block, L_ij is said luminance signal of signal set P_ij, C_m, M_m, Y_m, and K_m are master signal set device dependent color image signals and L_m is a master signal set luminance signal.

36. An apparatus for correcting data as claimed in claim 31 wherein said master signal deriving means further comprises:

a) means for dividing the digital color image into a plurality of blocks, and further dividing said blocks into a plurality of sub-blocks, wherein each block includes a plurality of signal sets that have a substantially constant blue-yellow resolution, and each sub-block includes a plurality of signal sets that have substantially constant red-green chrominance and blue-yellow resolutions;

b) means for selecting a master sub-block; and

c) means for choosing a signal set within said master sub-block as said master signal set.

37. An apparatus for correcting data as claimed in claim 36 wherein said mapping means further comprises:

a) means for applying a look up table conversion to transform said master signal set from said device independent color space to said output device dependent color space;

b) correcting said output device dependent red-green chrominance signals for a subset of said sub-blocks; and

c) correcting said output device dependent luminance signals for a subset of said signal sets.

38. An apparatus for correcting data as claimed in claim 37 wherein said red-green chrominance correcting means further comprises:

a) means for setting L_ij′

=L_m′

(L_ij/L_m);

b) means for setting R_ij′

=(R_m/L_m)L_ij;

c) means for setting G_ij′

=(G_m/L_m)L_ij;

d) means for setting R_ij″

=R_ij′

(Cr_kl+Cr)/2Cr_kl+G_ij′

(Cr_kl−

Cr)/2Cr_kl;

e) means for setting G_ij″

=R_ij′

(Cr_kl−

Cr)/2Cr_kl+G_ij′

(Cr_kl+Cr)/2Cr_kl, and

f) means for setting B_ij″

=B_ij′

where Cr_ij′

=R_ij′

−

G_ij′

, and B_ij′

=Cb_ij′

+(R_ij′

+G_ij′

)/2.

39. An apparatus for correcting data as claimed in claim 37 wherein said red-green chrominance correcting means further comprises:

a) means for setting R_ij″

=a**(Ir+Iplus−

Idenom)+a**(Ig+Iminus−

Idenom); and

b) means for setting G_ij″

=a**(Ir−

Iplus+Idenom)+a**(Ig−

Iminus+Idenom) where Iplus=log_a[Cr_kl+Cr], Iminus=log_a[Cr_kl−

Cr], and Idenom=log_a[2Cr_kl].

40. An apparatus for correcting data as claimed in claim 37 wherein said red-green chrominance correcting means further comprises means for setting Cr_kl′

=Cr_m′

(Cr_kl/Cr_m) for each sub-block, and said luminance correcting means further comprises means for setting L_ij′

=L_m′

(L_ij/L_m) for each signal set, wherein kl denotes a location of each sub-block within said block, Cr_kl′

is an output device dependent red-green chrominance signal of sub-block Ski, Cr_m′

is an output device dependent red-green chrominance master signal, Cr_kl is a device independent red-green chrominance signal of sub-block S_kl, and Cr_m is a device independent red-green chrominance master signal, and wherein ij denotes a location of each signal set within said block, L_ij′

is an output device dependent luminance signal of signal set P_ij, L_m′

is an output device dependent master signal set luminance signal, L_ij is a device independent luminance signal of signal set P_ij, L_m is a device independent master signal set luminance signal.

41. An apparatus for correcting data as claimed in claim 37 wherein said defining means further comprises:

a) means for setting output device dependent coordinate signal C_ij=1−

((1−

C_m)/L_m)L_ij;

b) means for setting output device dependent coordinate signal M_ij=1−

((1−

M_m)/L_m)L_ij;

c) means for setting output device dependent coordinate signal Y_ij=1−

((1−

Y_m)/L_m)L_ij; and

d) means for setting output device dependent coordinate signal K_ij=K_m,

for each signal set P_ij in said block, wherein ij denotes a location of each signal set within said block, L_ij is said luminance signal of signal set P_ij, C_m, M_m, Y_m, and K_m are master signal set device dependent color image signals and L_m is a master signal set luminance signal.