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Rethinking Color for Next-Generation TV

Rethinking Color for Next-Generation TV. Craig Todd, Dolby ct@dolby.com Masayuki Sugawara, NHK, sugawara.m-fq@nhk.or.jp Seo Young Choi, Samsung, seoyoung.choi@samsung.com. Parameters in draft UHDTV signal format. Annex 4 to Document ITU-R WP6C/564-E, 17 October 2011. Perception Limit.

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Rethinking Color for Next-Generation TV

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  1. Rethinking Color for Next-Generation TV Craig Todd, Dolby ct@dolby.com Masayuki Sugawara, NHK, sugawara.m-fq@nhk.or.jp Seo Young Choi, Samsung, seoyoung.choi@samsung.com

  2. Parameters in draft UHDTV signal format Annex 4 to Document ITU-R WP6C/564-E, 17 October 2011

  3. Perception Limit

  4. UHDTV Color Primaries Picture information can be linearly indicated by the tristimulus values of RGB in the range of 0-1. The colorimetric values of the picture information can be determined based on the reference RGB primaries and the reference white. The color primaries are on the spectral locus.

  5. Color gamut comparison in the CIE u′ v′ chromaticity diagram

  6. Motivation for color difference encoding • Convert R, G, B to Y′, R′-Y′, B′-Y′ • Human perception is less sensitive to resolution in color difference signals vs luma signals • High frequency cut-off at around 60 cpd in luma,but at around 10-11 cpd in chrominance (R′-Y′, B′-Y′) signals( cpd - cycle per 1 degree of visual angle) • We can sub-sample R′-Y′, B′-Y′ to save bandwidth • In storage and interfacing • In compression • Sub-sample R′-Y′, B′-Y′ by a factor of two in each dimension • Resulting number of samples is ½ that of original RGB

  7. Why luminance and chrominance signals are used ?(A hypothesized diagrammatic representation to describe color perception mechanism) • Zone Theory (G. E. Muller, "Ueber die Farbenempfindungen," Bd. 1 and 2. Psychol. Physiol. Sinnesorg., 1930, Erganzungsbd. 17, 1-430, 1930) LMS cones in retina absorb light reaching eye • One achromatic and two opponent color signals in nerve fibers • De-correlating cone absorptions • Providing efficient neural representation of color • Taking advantage of low spatial acuity of the opponent colors for image storage and transmission Sending these three signals to the lateral geniculate nucleus in brain Transferring to the visual cortex in brain for interpretation (http://www.nature.com/nrn/journal/v4/n7/fig_tab/nrn1138_F1.html)

  8. The background for down-sampling chrominance signals Barten's luminance contrast sensitivity as a function of spatial frequency for different angular display sizes Mullen's chromatic contrast sensitivity as a function of spatial frequency for red-green and yellow-blue channels Red-Green Yellow-Blue (P. G. J. Barten, "Evaluation of Subjective Image Quality with the Square-Root Integral Method", Journal of the Optical Society of America 7: 2024) (K. T. Mullen, "The Contrast Sensitivity of Human Color Vision to Red-Green and Blue-Yellow Chromatic Gratings” Journal of Physiology (London) 359: 381) • High frequency cut-off at around 10-11 cpd for the chromatic contrast sensitivity function but at around 60 cpd for the luminance contrast sensitivity function (※ cpd - cycle per 1 visual angle)

  9. The conventional way to create lumaNon-constant luminance Non-Linear Linear R’ R Y’ 0.2627R’+ 0.6780G’+ 0.0593B’ G’ G (γ=0.45) B’ B Y’ is not a pure luma signal; it contains some chroma information because it was made from non-linear RGB signals. This is called “non-constant luminance”, or NCL

  10. The conventional way to create Y’, Cb’, Cr’ Non-constant luminance Non-Linear Linear G’ Y’ 0.2627R’+ 0.6780G’+ 0.0593B’ R Y’ R’ R’- Y’ R’ Cb’ G (γ=0.45) B’ B’ B’- Y’ B Cr’ The Cb’ and Cr’ signals contain some luminance information. If they are sub-sampled, there will be some loss in luminance resolution

  11. Constant Luminance Y’ Y Y’ R R’ 0.2627R+ 0.6780G+ 0.0593B R’- Y’ R Cb’ G (γ=0.45) B’ B B’- Y’ Cr’ B Linear Y’ is now a pure luma signal; it contains no chroma information because it was made from linear RGB signals. This is called “constant luminance”, or “CL”. The color difference signals contain less luminance information and can be down sampled with much less effect on the luminance resolution. The Cb’, Cr’ signals can be created using slightly different schemes.

  12. Benefits of Constant Luminance Encoding • Luminance resolution is less affected by down-sampling the chroma signals • detail and edge information is more accurately maintained • Lower correlation between Cr’, Cb’ and Y’ • Better compression efficiency • Chroma information survives compression better

  13. How much correlated in Y’-Cb’, Y’-Cr’, and Cb’-Cr’? For high luminance colors, correlation is significantly high between Y’ and Cb’ in the NCL Y’-Cb’ For most luminance values, correlation is significantly high between Y’ and Cr’ in the NCL → non-orthogonal! Y’-Cr’ Using uniformly sampled 4096 colors from the UHDTV gamut For high luminance colors, correlation is high between Cb’ and Cr’ in the NCL Cb’-Cr’

  14. The impact of the maintenance of lightness on image sharpness Non-Constant Luminance Signals Constant Luminance Signals • The lightness values are almost maintained in the right-side four dark pixels after 4:1:0 sub-sampling • Original sharp boundary (white box) Apparent black strip line (green box) • Significant lightness discrepancies in the right-side four dark pixels after 4:1:0 sub- sampling • Blurred boundary (white box) Smeared black stripe line (green box) 4:1:0 4:2:0 Original 4:1:0 4:2:0 Original

  15. Relative compression efficiency using HEVC (V. 4) Test sequence having high colorfulness PSNR-lightness (L*) PSNR-color difference (ΔE*)

  16. Downside of Constant Luminance RGB, R’G’B’, NCL Y’Cr’Cb’ • It is a non-linear color space, i.e. color mixing is not along straight lines + = CL Y’Cr’Cb’ Cyan + Red = Grey ? = not quite Grey ?

  17. Cross-fade between two pictures (addition of pictures in different domains) R, G, B R’, G’, B’ orNCL Y’, Cb’, Cr’

  18. Cross-fade between two pictures R’G’B’ orNCL Y’, Cb’, Cr’ CL Y’, Cb’, Cr’

  19. [Mixing two images in RGB domain]

  20. [Mixing two images in nonlinear R’G’B’ domain]

  21. [Mixing two images in NCL-Y’Cb’Cr’ domain]

  22. [Mixing two images in CL-Y’Cb’Cr’ domain]

  23. Advantages of non-constant luminance • Same results for color mixing between R’G’B’ and Y’Cr’Cb’ • We have been doing NCL for 60 years • Tried and true • No-risk involved, no surprises lurking • Why change something that works?

  24. Conclusion • Open issues re UHDTV signal specification: • Terminology - do we call both 4k and 8k “Ultra HDTV”? Or should we use different words for each? • EOTF – don’t make the same mistake as Rec.709 • Color encoding – any reason to change? What is a suitable threshold of benefit that a new method should reach to justify a change? • If the UHDTV signal specification matters to you, get involved in the discussions and help drive this to the right conclusion. ct@dolby.com

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