Why Wide Colour Gamut matters for UHD Martin Parsons Image Eyes

Transcription

1 Why Wide Colour Gamut matters for UHD Martin Parsons Image Eyes 1

2 Colour science Colour science looks at how the human visual system responds to the various wavelengths of light to create the sensation of colour Colour science also looks at how light interacts with different materials as well as how some materials are able to emit or respond to light Colour management Colour management is to provide consistent device independent colour from a given RGB stimuli No matter where in the production pipeline the RGB stimuli is viewed 2

3 The Electromagnetic Spectrum The Visible Spectrum 3

4 The Eye Cone cells are photoreceptors for medium to bright colour vision Three types of cone cell each sensitive to different ranges of wavelengths of light There is also a single type of low light receptor called a rod cell (monochromatic) It is the various responses of the three types of cone cell to light that give us colour vision and the sensation of colour Rods and Cones 100 million rods 6 million cones 4

5 Scotopic vision (below nits) Photopic vision (10 to 10 8 nits) 5

6 Scotopic vs. Photopic The light levels where both rods and cones are operational is called Mesopic vision ( nits) Rod and Cone cell distribution 6

7 Additive Colour Space Red, Green, Blue R+B = Magenta G+B = Cyan R+G = Yellow R+G+B = White No addition = Black CIE 1931 data Experiments carried out in 1920s Independently W. David Wright tested ten people John Guild tested seven people 2 2 degree observer 7

8 Colour matching functions for the CIE standard 2 observer Colour matching functions for the CIE standard 2 observer 8

9 CIE 1931 XYZ Colour Space The CIE's color matching functions, x(λ), y(λ), z(λ) are the numerical description of the chromatic response of the observer They can be thought of as the spectral sensitivity curves of three linear light detectors yielding the CIE tristimulus values X, Y and Z CIE 1931 x y Y Colour Space In 1931, the International Commission on Illumination (Commission Internationale de l Éclairage CIE) produced this diagram The diagram links the physical wavelengths of light to the physiological perceived colours in human vision The x and y values give co-ordinates of each colour. The luminance value, Y is the third axis coming out of the screen 9

10 CIE 1931 xy Y Colour Space Rec will have a larger colour volume than Rec. 709 Y HDR will have a larger colour volume than SDR y x 10

11 MacAdam ellipse The MacAdam ellipse is an elliptical region on the CIE chromaticity diagram which contains all the colours that are indistinguishable to the average human eye, from the colour at the centre of the ellipse The contour of the ellipse therefore represents the just noticeable differences of chromaticity CIE x y Y perceptually equal lines Perceptually uniform means that a change of the same amount in a colour value should produce the same visual change no matter where in the colour space the difference occurs As can be seen from the CIE xy diagram, the lines that link colours that have the same perceived difference vary greatly in length depending on where in the space the two colours are 11

12 CIE L*u*v* perceptually equal lines The CIE u v diagram shows how the length of these lines is much less variable when using the L u*v* colour space. One of the main uses of perceptually uniform colour spaces is in the calculation of colour differences. If the colour space is perpetual uniform then we can use standard vector mathematics to calculate distances and hence differences between colours. CIE 1976 L*u*v* Colourspace This colour space was created in 1976 by the CIE to be a more perceptually uniform colour space than CIE XYZ 12

13 Colour temperature Colour Temperature (showing Planckian Locus) Planck's lawdescribes the electromagnetic radiation emitted by a black body in thermal equilibrium at a definite temperature The Planckian locus is shown on the CIE xy chromaticity diagram along with the lines of constant correlated colour temperature (CCT) The colour temperature of a light source is determined by comparing its chromaticity with that of an ideal blackbody radiator 13

14 Daylight Illuminants The CIE has defined a number of standard illuminants that are called the D series of illuminants D65 corresponds roughly to a mid-day sun in Western Europe / Northern Europe, hence it is also called a daylight illuminant The D series contains the D50, D55, D65, D75 illuminants which are all named after the nearest colour temperature value when the chromaticity coordinates for each illuminant are plotted on the CIE (1931) xy chromaticity diagram The D illuminants are specified as a SPD (Spectral Power Distribution) 14

15 Daylight vs. Fluorescent SPDs ITU-R BT.709 colour gamut D65 white point x = y =

16 Film print colour gamut German cinema standard 5500K Kodak Cineon view recommendation 5400K US Cinema standard 5300K European cinema standard 5200K Average print white point x = 0.34 y = 0.35 DCI P3 colour gamut P3 white point x = y =

17 ITU-R BT.2020 colour gamut D65 white point x = y = Colour Gamut Comparison Standard% Format of CIE 1931 HDTV (Rec.709) 35.9% Digital Cinema (P3) 53.6% UHDTV (Rec.2020) 75.8% 17

18 Comparison of CIE xy white points Application x y ITU-R BT.709 (HD video) ITU-R BT.2020 (UHD video) DCI P3 (Digital film) Film Print (Analogue film) 0.34(average) 0.35(average) Comparison of CIE xy colour gamuts Application HDVideo (Rec. 709) UHD Video (Rec. 2020) DCI P3 (Digital Film) White x White y Red x Red y Green x Green y Blue x Blue y

19 HDTV vs. SDTV colour gamut NTSC was once a wide colour gamut NTSC 1953 P-22 (1979 original SMPTE-C) SMPTE-C (1994, RP145 spec.) P-22 phosphors are used on consumer televisions Spec s are fairly loose Chosen to be as bright as possible SMPTE-C phosphors have a more linear light response Spec s are tighter 19

20 Rec.709 vs. P3 vs. Rec.2020 Rec.709 simulation 20

21 P3 (D65 white point) simulation Rec.2020 simulation 21

22 Moving between colour gamuts Moving between colour gamuts CROP 22

23 Moving between colour gamuts SCALE Moving between colour gamuts ROLL OFF 23

24 Moving between colour gamuts $ 2 $ 1 $ RE-GRADE SMPTE Monthly Educational Webcast: UHD Color Conversion Challenges Discover if color conversion methods mandated by the current television standards produce a good colour match when converting colors from HD to the UHD color space Lars Borg ofadobe explains common colour conversion errors and how display-referred colorimetry may be used for effective colour matching Thursday, 28 January 2016 at 18:00 GMT (SMPTE members No charge / Non-members $49) Clue 24

25 ACES ACES is an acronym for the Academy Colour Encoding Space The ACES colour space is a working colour space only It is NOT: a capture colour space a display colour space ACES colour gamut 25

26 ACES colour space The ACES colour space is: high-dynamic range (HDR) floating point in scene-referred space linear (to light) very wide colour gamut ACES Before we get onto how it all works it is fundamental to know... In the ACES workflow, the intent is for artistic changes to be made by manipulating the ACES data It s where the magic happens! 26

27 ACES: Scene-referred Scene-referred is how the image is recorded i.e. the number of light photons captured by the camera sensor It's for the maths to work on not how we can see it properly ACES: Display-referred Display-referred is how the image will look to you on any display device: projector monitor the web ipad phone... 27

28 ACES: Workflow Helmholtz-Kohlrausch effect The International Electrotechnical Commission defines the Helmholtz-Kohlrausch effect as: A change in brightness of perceived colour produced by increasing the purity of a colour stimulus while keeping its luminance constant within the range of photopic vision. 28

29 Helmholtz-Kohlrausch effect The Helmholtz-Kohlrausch (H-K) effect describes a situation whereby if two colour stimuli sources with the same luminance are compared, the perceived brightness induced by the colour stimuli of higher purity will be greater than that of lower purity. Digital Projection Insight 4K Dual LED Projector The 1931 Colour Plot shows the three colour gamuts along with target tolerance boxes for the DCI Digital Cinema primaries (nonshaded boxes) and suggested target tolerance boxes for BT2020 (shaded) 29

30 Colour Reproduction in Electronic Imaging Systems: Photography, Television, Cinematography By Michael S. Tooms NEW! Use Promo Code VBG91for 20% Off! Buy now at Key Features: Addresses important theory and common misconceptions in colour science and reproduction, from the perception and characteristics of colour to the practicalities of its rendering in the fields of television, photography and cinematography Offers a clear treatment of the CIE chromaticity charts and their related calculations, supporting discussion on system primaries, their colour gamuts and the derivation of their contingent red, green and blue camera spectral sensitivities Reviews the next state-of-the-art developments in colour reproduction beyond current solutions, from Ultra-High Definition Television for the 2020s to laser projectors with unprecedented colour range for the digital cinema Includes a companion website hosting a workbook consisting of invaluable macro-enabled data worksheets; JPEG files containing images referred to in the book, including colour bars and grey scale charts to establish perceived contrast range under different 59 environmental conditions; and, guides to both the workbook and JPEG files ISBN: $ Cloth 648 Pages January 2016 Thank you! Martin Parsons Image Eyes Ltd 30