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Extended Color Gamut for Consumers - the Display side

This presentation can be downloaded from the HPA online website, or mailto: jeroen.stessen@philips.com (it includes notes and hidden sheets). Extended Color Gamut for Consumers - the Display side. Presented by: Jeroen Stessen. Philips Applied Technologies.

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Extended Color Gamut for Consumers - the Display side

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  1. This presentation can be downloaded from the HPA online website, or mailto: jeroen.stessen@philips.com (it includes notes and hidden sheets)

  2. Extended Color Gamut for Consumers - the Display side Presented by: Jeroen Stessen Philips Applied Technologies Digital Systems & Technology Eindhoven HPA Tech Retreat 2007 Rancho Mirage – Wednesday 2007-01-31

  3. Outline • What is a color gamut • Color gamuts for some displays • Color space conversion • Gamut mapping • Signal processing chain • Image results • Conclusions

  4. Outline source gamut display gamut the problem color space conversion gamut mapping source display image image processing the solution

  5. Outline source gamut display gamut color space conversion gamut mapping source display image image processing What is a color gamut

  6. What is a color gamut y  The horseshoe gamut covers all visible colors, the Pointer gamut covers all practical colors The background colors are incorrect, for orientation only The border of humanly visible colors (5 nm ) Pointer gamutof surface colors CIE-1931 (x,y) color space x 

  7. What is a color gamut y  The standard TV color gamut does not cover all surface colors, but we have been happy with it for 50 years, and this gamut is easy to reproduce ITU-Rec.709standard TV Pointer gamutof surface colors G R B x 

  8. What is a color gamut y  The digital cinema reference projector gamut is good ! Lacking only some cyan Digital Cinemareference (DLP) G Pointer gamutof surface colors ITU-Rec.709standard TV R B x 

  9. Y What is a color gamut y  The digital cinema XYZ signal gamutis more than everything Many codes are never used Digital CinemaXYZ signal Pointer gamutof surface colors ITU-Rec.709standard TV X x  Z

  10. What is a color gamut Gamut is 3D: For each surface color there is a theoretical maximum brightness,it is 100%only for awhite surface “bright gamut” white = 100% “wide gamut”

  11. What is a color gamut Gamut is 3D: Maximum luminance for each color point, compared with the Munsell color cascade of known surface colors(the Pointer gamut in 3D) white = 109% white = 100% xvYCC wide color gamut TV more than complete coverage ITU-Rec.709 standard TV incomplete coverage Illustrations byT. Matsumoto et al, Sony Corp. for SID

  12. Outline source gamut display gamut color space conversion gamut mapping source display image image processing Color gamuts for some displays

  13. Color gamuts for some displays y  This triangular color gamut is the de-facto standard for practically every TV or computer display in the world Also used by sRGB Pointer gamutof surface colors ITU-Rec.709standard TV x 

  14. Color gamuts for some displays y  The reference gamut for digital cinema is defined by a TI DLP projector It is clearly larger than standard TV, able to reproduce more colors Digital Cinemareference (DLP) ITU-Rec.709standard TV This is the current Digital Cinema gamut x 

  15. Color gamuts for some displays y  The NTSC gamut is nota standard for any TV, it is obsolete, but it is still much referred to: “the gamut of this display is nnn% of the NTSC area” “100% NTSC”display Digital Cinemareference (DLP) ITU-Rec.709standard TV x 

  16. Color gamuts for some displays y  An LCD with a “wide color gamut” CCFL backlight for“91% NTSC” color gamut Improved red and green primaries, but worse blue than Rec.709 “91% NTSC”LCD Digital Cinemareference (DLP) ITU-Rec.709standard TV x 

  17. Color gamuts for some displays y  An LCD with a novel LED backlight foran impressive color gamut The saturation for yellow is not as high as Digital Cinema LED-backlitLCD Digital Cinemareference (DLP) ITU-Rec.709standard TV x 

  18. Color gamuts for some displays y  With 3 RGB lasers we can make an almost perfect color gamut Novalux and COLOR are making the lasers today 532nm 3-primarylaser displays Digital Cinemareference (DLP) ITU-Rec.709standard TV 621nm 628nm 465nm This could be the next Digital Cinema gamut ! 447nm x 

  19. Outline source gamut display gamut color space conversion gamut mapping source display image image processing Color space conversion

  20. Color space conversion y  If you don’t do anything then you’ll get the wrong color gamut of the standard TV display • Example: G Source gamut: Digital Cinema Target gamut: standard TV R B x 

  21. Color space conversion y  With color space conversion we aim to getthe correctcolors of the digital cinema display back (if possible) • Example: G Source gamut: Digital Cinema Target gamut: standard TV R B x 

  22. Color space conversion • Block diagram: • Use the gamma functions from the transmission standard (709) • for an xvYCC source we must use the symmetrical gamma function • The CSC matrix is only a single 3x3 matrix • it has a dominant diagonal, because it converts RGBRGB • it must have sufficient accuracy, because of linear-light RGB data XYZ = absolute and linear-light color space source display R’ R X R R’ c c c c c c c c c c c c c c c c c c G’ G Y G G’ B’ B Z B B’ gamma color space conversion matrix inv. gamma

  23. Color space conversion • Calculation example: • simplification: a white point of D6500 is assumed on both sides source color space display color space absolute color space R X R 0.487 0.266 0.198 0.229 0.692 0.079 0.000 0.045 1.044 3.241 −1.537 −0.499 −0.969 1.876 0.042 0.056 −0.204 1.057 G Y G B Z B = R’ R R R’ 1.225 −0.225 0.000 −0.042 1.042 0.000 −0.020 −0.079 1.098 G’ G G G’ B’ B B B’ gamma color space conversion matrix inv. gamma

  24. Color space conversion • Interpretation: • Negative RGB values < 0 are illegal on the input of the display • because displaying negative light is physically impossible • in the given example, the top center coefficient of -0.225 creates the biggest problem: • A 100% Digital-Cinema Green source color (via xvYCC) wants to make a -23% red light output on a standard display • clipping negative signals to zero is one solution • the generic solution is called gamut mapping (more later) Rout 1.225 −0.225 0.000 Rin Gout = −0.042 1.042 0.000 * Gin Bout −0.020 −0.079 1.098 Bin

  25. Color space conversion • Interpretation: • Positive RGB values > 100% are also a problem on the display • they want to overdrive the display, causing ugly clipping artefacts • in the given example, the top left coefficient of 1.225 creates the biggest problem: • A 100% Digital-Cinema Red source color (via xvYCC) wants to make a 123% red light output on a standard display • a simple linear signal attenuation would prevent such problems • i.e. the entire signal can be attenuated to 91% (gamma domain) Rout 1.225 −0.225 0.000 Rin Gout = −0.042 1.042 0.000 * Gin Bout −0.020 −0.079 1.098 Bin

  26. Color space conversion • Conclusion: • Color space conversion can be mathematically correct • linear problem linear solution • it is just a 3x3 matrix in linear-light domain • it works perfectly • Conversion to a smaller color gamut can create illegal RGB signals < 0, negative light, underflow • impossible colors  gamut mapping (more later) • Conversion to a smaller color gamut can also create problematic RGB signals > 100%, overflow • only a problem of scale  attenuation

  27. Outline source gamut display gamut color space conversion gamut mapping source display image image processing Gamut mapping

  28. Gamut mapping y  First example A standard television display has a smaller color gamut than Digital Cinema What are the consequences ? Standardtelevisiondisplay G Digital Cinemareference R B x 

  29. Gamut mapping • Example: a Digital Cinema signal on a standard TV display • Starting from a Digital Cinema RGB signal, then from source to display we must apply this color space correction matrix: • Watch the negative coefficients in 4 places… Rout 1.225 −0.225 0.000 Rin Gout = −0.042 1.042 0.000 * Gin Bout −0.020 −0.079 1.098 Bin

  30. Gamut mapping y  Outside the display gamut one or more of the RGB drive signals go negative Display gamut Source gamut B<0 B>0 R<0 R>0 G>0 It is a problem on all 3 sides (R,G,B) G<0 x 

  31. Gamut mapping y  The green area needs only color space conversion ! B>0 R>0 G>0 This part can berendered correctly x 

  32. Gamut mapping y  The red area needs gamut mapping,to substitute somethingfor the unreachable colors This part isunreachable ! ! B<0 R<0 G<0 x 

  33. Gamut mapping y  Gamut mapping is an age-old artistic process, the choice of substitute colors is subjective ! Shift towards thedisplay gamut ? ? B>0 R>0 ? G>0 Note: this is usually not about natural colors x 

  34. Gamut mapping y  Gamut mapping summary Gamut mapping(subjective choice) Color spaceconversion(objective math) x 

  35. Gamut mapping y  Second example, a LED-backlit LCD panel The display’s wide color gamut differs from the digital cinema wide color gamut What are the consequences ? LED-backlitLCD G Digital Cinemareference R B x 

  36. Gamut mapping • Example: a Digital Cinema signal on an LCD with LED-backlight • Starting from a Digital Cinema RGB signal, then from source to display we must apply this color space correction matrix: • Watch the negative coefficients on the bottom row (= Bout) … • A wider display gamut makes our problems easier Rout 0.810 0.168 0.022 Rin Gout = 0.041 0.958 0.001 * Gin Bout−0.017 −0.049 1.066 Bin

  37. Gamut mapping y  Outside the display gamut only the Blue drive signals goes negative Display gamut Source gamut B<0 B>0 R>0 G>0 x 

  38. Gamut mapping y  The green area needs only color space conversion ! B>0 This part can berendered correctly x 

  39. Gamut mapping y  The red area needs gamut mapping,to substitute somethingfor the unreachable colors This part isunreachable ! ! B<0 x 

  40. Gamut mapping y  Gamut mapping is an age-old artistic process, the choice of substitute colors is subjective ! Shift towards thedisplay gamut ? ? B>0 ? x 

  41. Gamut mapping y  The yellow area is not asked for, but there is a temptationto fill it up by “gamut”“extension” This part is not asked for ? ? R<0 R>0 G>0 ? LED red isso beautiful… G<0 x 

  42. Gamut mapping y  Gamut mapping summary Gamut mapping(subjective choice) Nothing (or somegamut extension) Color spaceconversion(objective math) x 

  43. Outline source gamut display gamut color space conversion gamut mapping source display image image processing Signal processing chain

  44. Signal processing chain • A legacy chain with only Rec.709 compliant components source gamut Rec.709 gamut Rec.709 display Camera Standard TVgamut XYZ or wide RGB XYZ sensor CSCmatrix gamut map 1 ... the only Rec.709 Y’CbCr processing & channel Rec.709 display inverse ... native displaygamut display gamut Rec.709 gamut =

  45. Signal processing chain • An xvYCC wide gamut chain with a wide gamut display source gamut xvYCC gamut DCI display Camera Digital Cinemagamut XYZ or wide RGB XYZ sensor CSCmatrix gamut map 1 CSCmatrix ... dominant xvYCC + Y’CbCr processing & channel not dominant wide gamut display gamut map 2 CSCmatrix inverse ... native displaygamut display gamut xvYCC gamut =

  46. Signal processing chain • A mixed chain with a legacy Rec.709 display (compatibility) source gamut xvYCC gamut DCI display Camera Digital Cinemagamut XYZ or wide RGB XYZ sensor CSCmatrix gamut map 1 CSCmatrix ... not dominant xvYCC + Y’CbCr processing & channel dominant Rec.709 display gamut CLIP inverse ... native displaygamut display gamut xvYCC gamut ? <

  47. Signal processing chain • What’s newin a wide gamut chain with a wide gamut display: larger source gamut digital cinema reference display DCI display Camera Digital Cinemagamut XYZ or wide RGB XYZ sensor CSCmatrix gamut map 1 CSCmatrix ... xvYCC + Y’CbCr processing & channel wide gamut display gamut map 2 CSCmatrix inverse ... receiver-side gamut mapping xvYCC signal & processing

  48. Outline source gamut display gamut color space conversion gamut mapping source display image image processing Image results

  49. Image results • Block diagram for simulation of gamut mapping algorithms: source gamut display gamut reference “DC” 0a, 0b display gamut “DC” CSCmatrix inversematrix “DC” 1, 2, 3a, 3b image display map a gamut bottleneck result

  50. Image results • Result 0a: ORIGINAL IMAGE (assume that this is the full DC gamut)

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