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

US 6,239,886 B1
Filed: 01/08/1998
Issued: 05/29/2001
Est. Priority Date: 01/08/1998
Status: Expired due to Term

First Claim

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1. A method of correcting device independent luminance and chrominance data in a digital color image, comprising the steps of:

a) providing device independent color image signals to a plurality of channels in a device independent color space, said device independent color image signals being sub-sampled and corresponding to color signals;

b) deriving a set of device independent master signals from said device independent color image signals, said device independent master signal set including a signal from each of said channels and wherein said device independent color imaging signals have been sub-sampled and are received at three channels, 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 said red-green chrominance channel resolution being less than said luminance channel resolution, and a third channel Cb receives blue-yellow chrominance signals at a blue-yellow chrominance channel resolution, said blue-yellow chrominance channel resolution being less than said red-green chrominance channel resolution, and wherein said master signal deriving step further comprises;

i.) 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;

ii.) selecting a master sub-block; and

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

c) mapping said device independent master signal set to an output device dependent master signal set in an output device dependent color space;

d) defining output device dependent color image signals corresponding to a subset of said device independent color signals wherein said output device dependent signals are created 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

e) converting said output device dependent color image signals to output device dependent coordinate signals which correspond to output device colorants.

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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.

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45 Claims

1. A method of correcting device independent luminance and chrominance data in a digital color image, comprising the steps of:
- a) providing device independent color image signals to a plurality of channels in a device independent color space, said device independent color image signals being sub-sampled and corresponding to color signals;
  
  b) deriving a set of device independent master signals from said device independent color image signals, said device independent master signal set including a signal from each of said channels and wherein said device independent color imaging signals have been sub-sampled and are received at three channels, 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 said red-green chrominance channel resolution being less than said luminance channel resolution, and a third channel Cb receives blue-yellow chrominance signals at a blue-yellow chrominance channel resolution, said blue-yellow chrominance channel resolution being less than said red-green chrominance channel resolution, and wherein said master signal deriving step further comprises;
  
  i.) 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;
  
  ii.) selecting a master sub-block; and
  
  iii.) choosing a signal set within said master sub-block as said master signal set;
  
  c) mapping said device independent master signal set to an output device dependent master signal set in an output device dependent color space;
  
  d) defining output device dependent color image signals corresponding to a subset of said device independent color signals wherein said output device dependent signals are created 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
  
  e) converting said output device dependent color image signals to output device dependent coordinate signals which correspond to output device colorants.
- View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33)
- - 2. A method of correcting data as claimed in claim 1 wherein said device independent color image signals have been produced by generating input device dependent color image signals in an input device dependent color space, followed by processing said input device dependent color image signals to produce device independent color image signals in a device independent color space.
  - 3. A method of correcting data as claimed in claim 1 wherein said mapping step further comprises:
4. A method of correcting data as claimed in claim 3 wherein said master signal set deriving step further comprises designating a signal set at a given location within each block.
5. A method of correcting data as claimed in claim 3 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.
6. A method of correcting data as claimed in claim 3 wherein said master signal set deriving step further comprises choosing a desired relative luminance signal.
7. A method of correcting data as claimed in claim 6 wherein said chosen luminance signal is a maximum signal of all luminance signals in said block.
8. A method of correcting data as claimed in claim 6 wherein said chosen luminance signal is nearest to an average for all luminance signals in said block.
9. A method of correcting data as claimed in claim 3 wherein said output device dependent color space is an RGB space.
10. A method of correcting data as claimed in claim 9 wherein said creating step further comprises:
- a) setting output device dependent signal R_ij=(R_m/L_m)L_ij;
  
  b) setting output device dependent signal G_ij=(G_m/L_m)L_ij; and
  
  c) setting output device dependent signal B_ij=(B_m/L_m)L_ijfor each signal set P_ijin said block, wherein ij denotes a location of each set within said block, L_ijis said luminance signal of signal set P_ij, R_m, G_m, and B_mare master signal set output device dependent signals and L_mis said master signal set luminance signal.
11. A method of correcting data as claimed in claim 9 wherein said creating step further comprises:
- a) setting output device dependent signal R_ij=a{circumflex over ( )}(log_aR_m−
  
  log_aL_m+log_aL_ij);
  
  b) setting output device dependent signal G_ij=a{circumflex over ( )}(log_aG_m−
  
  log_aL_m+log_aL_ij); and
  
  c) setting output device dependent signal B_ij=a{circumflex over ( )}(log_aB_m−
  
  log_aL_m+log_aL_ij) for each signal set P_ijin said block, wherein ij denotes a location of each signal set within said block, L_ijis said luminance signal of signal set P_ij, R_m, G_m, and B_mare master signal set output device dependent signals and L_mis said master signal set luminance signal.
12. A method of correcting data as claimed in claim 9 wherein said creating step further comprises:
- a) setting output device dependent signal R_ij=R_m−
  
  L_m+L_ijb) setting output device dependent signal G_ij=G_m−
  
  L_m+L_ij; and
  
  c) setting output device dependent signal B_ij=B_m−
  
  L_m+L_ijfor each signal set P_ijin said block, wherein ij denotes a location of each signal set within said block, L_ijis said luminance signal of signal set P_ij, R_m, G_m, and B_mare master signal set output device dependent signals and L_mis said master signal set luminance signal.
13. A method of correcting data as claimed in claim 1 wherein said output device dependent color space is an LCrCb space.
14. A method of correcting data as claimed in claim 13 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_ijin said block, wherein ij denotes a location of each set within said block, L_ijis said device independent luminance signal of signal set P_ij, L_m′
  
  , is said master signal set output device dependent signal, and L_mis said master signal set device independent luminance signal.
15. A method of correcting data as claimed in claim 3 wherein an output device dependent color space is a CMYK color space.
16. A method of correcting data as claimed in claim 15 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_ijin said block, wherein ij denotes a location of each signal set within said block L_ijis said luminance signal of signal set P_ij, C_m, M_m, Y_m, and K_mare master signal set device dependent color image signals and L_mis a master signal set luminance signal.
17. A method of correcting data as claimed in claim 1 wherein said mapping step further comprises:
- a) 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.
18. A method of correcting data as claimed in claim 17 wherein said master sub-block selecting step further comprises designating a sub-block at a given location within each block.
19. A method of correcting data as claimed in claim 17 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.
20. A method of correcting data as claimed in claim 17 wherein said master sub-block selecting step further comprises designating a desired relative red-green chrominance signal.
21. A method of correcting data as claimed in claim 20 wherein said designated red-green chrominance signal is a maximum magnitude of all chrominance red-green chrominance signals in said block.
22. A method of correcting data as claimed in claim 20 wherein said designated red-green chrominance signal is nearest to an average for all red-green chrominance signals in said block.
23. A method of correcting data as claimed in claim 17 wherein said master signal set choosing step further comprises designating a signal set at a given location within each sub-block.
24. A method of correcting data as claimed in claim 17 wherein said master signal set choosing step further comprises designating a desired relative luminance signal.
25. A method of correcting data as claimed in claim 24 wherein said designated luminance signal is a maximum signal of all luminance signals in said sub-block.
26. A method of correcting data as claimed in claim 24 wherein said designated luminance signal is nearest to an average for all luminance signals in said sub-block.
27. A method of correcting data as claimed in claim 17 wherein said output device dependent color space is an RGB space.
28. A method of correcting data as claimed in claim 27 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_ij′
  
  (Cr_kl+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.
29. A method of correcting data as claimed in claim 27 wherein said red-green chrominance correcting step further comprises:
- a) setting R_ij″
  
  =a**(lr+lplus−
  
  ldenom)+a**(lg+lminus −
  
  ldenom); and
  
  b) setting G_ij″
  
  =a**(lr−
  
  lplus+ldenom)+a**(lg−
  
  lminus+ldenom) where lplus=log_a[Cr_kl+Cr], lminus=log_a[Cr_kl−
  
  Cr], and ldenom=log_a[2Cr_kl].
30. A method of correcting data as claimed in claim 17 wherein said output device dependent color space is an LCrCb space.
31. A method of correcting data as claimed in claim 30 wherein said red-green chrominance correcting step further comprises setting Cr_kl′
- =Cr_m′
  
  (Cr_kl/Cr_m) for each sub-block, and said luminance correcting step further comprises 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 S_kl, Cr_m′
  
  is an output device dependent red-green chrominance master signal, Cr_klis a device independent red-green chrominance signal of sub-block S_kl, and Cr_mis 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_ijis a device independent luminance signal of signal set P_ij, L_mis a device independent master signal set luminance signal.
32. A method of correcting data as claimed in claim 17 wherein an output device dependent color space is a CMYK color space.
33. A method of correcting data as claimed in claim 32 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−
  
  ((1C_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_ijin said block, wherein ii denotes a location of each signal set within said block, L_ijis said luminance signal of signal set P_ij, C_m, M_m, Y_m, and K_mare master signal set device dependent color image signals and L_mis a master signal set luminance signal.

34. An apparatus for correcting device independent luminance and chrominance data in a digital color image, comprising:
- a) means for providing device independent color image signals to a plurality of channels in a device independent color space, said device independent color image signals corresponding to color signals;
  
  b) means for deriving a set of device independent master signals from said device independent color image signals, said device independent master signal set including a signal from each of said channels and wherein said device independent color image signals have been sub-sampled and are received at three channels, 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 channel resolution wherein said red-green channel resolution is less than said luminance channel resolution, and a third channel Cb receives blue-yellow chrominance signals at a blue-yellow channel resolution said blue-yellow channel resolution is less than said red-green channel resolution and wherein said master signal deriving means further comprises;
  
  i.) 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;
  
  ii.) means for selecting a master sub-block; and
  
  iii.) means for choosing a signal set within said master sub-block as said master signal set;
  
  c) means for mapping said device independent master signal set to an output device dependent master signal set in an output device dependent color space wherein said mapping means further comprises;
  
  i.) means for dividing the digital color image into blocks, each block including a plurality of device independent signal sets, wherein each signal set includes a signal from each of said channels, and wherein each block includes one master signal set; and
  
  ii.) 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;
  
  d) means for defining output device dependent color image signals corresponding to a subset of said device independent color signals wherein said defining means further comprises 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
  
  e) means for converting said output device dependent color image signals to output device dependent coordinate signals which correspond to output device colorants.
- View Dependent Claims (35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45)
- - 35. An apparatus for correcting data as claimed in claim 34 wherein said providing means further comprises generating input device dependent color image signals in an input device dependent color space, followed by processing said input device dependent color image signals to produce device independent color image signals in a device independent color space.
  - 36. An apparatus for correcting data as claimed in claim 34 wherein said creating means further comprises:
37. An apparatus for correcting data as claimed in claim 34 wherein said creating means further comprises:
- a) means for setting output device dependent signal R_ij=a {circumflex over ( )}(log_aR_m−
  
  log_aL_{m +log}_aL_ij);
  
  b) means for setting output device dependent signal G_ij=a {circumflex over ( )}(log_aG_m−
  
  log_aL_m+log_aL_ij); and
  
  c) means for setting output device dependent signal B_ij=a {circumflex over ( )}(log_aB_m−
  
  log_aL_m+log_aL_ij) for each signal set P_ijin said block, wherein ij denotes a location of each signal set within said block, L_ijis said luminance signal of signal set P_ij, R_m, G_m, and B_mare master signal set output device dependent signals and L_mis said master signal set luminance signal.
38. An apparatus for correcting data as claimed in claim 34 wherein said creating means further comprises:
- a) means for setting output device dependent signal R_ij=R_m−
  
  L_m+L_ijb) 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_ijfor each signal set P_ijin said block, wherein ij denotes a location of each signal set within said block, L_ijis said luminance signal of signal set P_ij, R_m, G_m, and B_mare master signal set output device dependent signals and L_mis said master signal set luminance signal.
39. An apparatus for correcting data as claimed in claim 34 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 i}n said block, wherein ij denotes a location of each set within said block, L_ijis said device independent luminance signal of signal set P_ij, L_m′
  
  , is said master signal set output device dependent signal, and L_mis said master signal set device independent luminance signal.
40. An apparatus for correcting data as claimed in claim 34 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_ijin said block, wherein ij denotes a location of each signal set within said block, L_ijis said luminance signal of signal set P_ij, C_m, M_m, Y_m, and K_mare master signal set device dependent color image signals and L_mis a master signal set luminance signal.
41. An apparatus for correcting data as claimed in claim 34 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.
42. An apparatus for correcting data as claimed in claim 41 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.
43. An apparatus for correcting data as claimed in claim 41 wherein said red-green chrominance correcting means further comprises:
- a) means for setting R_ij″
  
  =a**(lr+lplus−
  
  ldenom)+a**(lg+lminus −
  
  ldenom); and
  
  b) means for setting G_ij″
  
  =a**(lr−
  
  lplus+ldenom)+a**(lg−
  
  lminus+ldenom) where lplus=log_a[Cr_kl+Cr], lminus=log_a[Cr_kl−
  
  Cr], and ldenom=log_a[2Cr_kl].
44. An apparatus for correcting data as claimed in claim 41 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 S_kl, 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_mis 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_ijis a device independent luminance signal of signal set P_ij, L_mis a device independent master signal set luminance signal.
45. An apparatus for correcting data as claimed in claim 41 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_ijin said block, wherein ij denotes a location of each signal set within said block, L_ijis 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_mis a master signal set luminance signal.

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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

US09/004,295
Time in Patent Office

1,237 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 1/608 within the L, C1, C2 colour...

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

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

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