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Martin Parsons

Image Eyes

Why Wide Colour Gamut

matters for UHD

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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

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The Electromagnetic Spectrum

The Visible Spectrum

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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

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Scotopic vision (below 0.001 nits)

Photopic vision (10 to 108 nits)

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Scotopic vs. Photopic

The light levels where both rods and cones are operational is called Mesopic vision (0.001 - 3 nits)

Rod and Cone cell distribution

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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

2 degree observer2°

Experiments carriedout in 1920s

IndependentlyW. David Wrighttested ten people

John Guild testedseven people

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Colour matching functions for the CIE standard 2° observer

Colour matching functions for the CIE standard 2° observer

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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

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CIE 1931 x y Y Colour Space

y

x

YRec. 2020 will have a larger

colour volume than Rec. 709

HDR will have a larger

colour volume than SDR

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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

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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

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Colour temperature

Colour Temperature (showing Planckian Locus)

Planck's law describes 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 black-body radiator

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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)

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Daylight vs. Fluorescent SPDs

ITU-R BT.709 colour gamut

D65 white point

x = 0.313y = 0.329

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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 = 0.34y = 0.35

DCI P3 colour gamut

P3 white point

x = 0.314y = 0.351

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ITU-R BT.2020 colour gamut

D65 white point

x = 0.313y = 0.329

Colour Gamut Comparison

Standard %

Format of CIE 1931

HDTV (Rec.709) 35.9%

Digital Cinema (P3) 53.6%

UHDTV (Rec.2020) 75.8%

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Comparison of CIE xy white points

Application x y

ITU-R BT.709 (HD video) 0.3127 0.3290

ITU-R BT.2020 (UHD video) 0.3127 0.3290

DCI P3 (Digital film) 0.3140 0.3510

Film Print (Analogue film) 0.34 (average) 0.35 (average)

Comparison of CIE xy colour gamuts

Application White

x

White

y

Red

x

Red

y

Green

x

Green

y

Blue

x

Blue

y

HD Video

(Rec. 709)0.3127 0.3290 0.640 0.330 0.300 0.600 0.150 0.060

UHD Video

(Rec. 2020)0.3127 0.3290 0.708 0.292 0.170 0.797 0.131 0.046

DCI P3

(Digital Film)0.314 0.351 0.680 0.320 0.265 0.690 0.150 0.060

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HDTV vs. SDTV colour gamut

NTSC was once a wide colour gamutNTSC 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

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Rec.709 vs. P3 vs. Rec.2020

Rec.709 simulation

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P3 (D65 white point) simulation

Rec.2020 simulation

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Moving between colour gamuts

Moving between colour gamuts

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CROP

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Moving between colour gamuts

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SCALE

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Moving between colour gamuts

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ROLL OFF

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Moving between colour gamuts

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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 of Adobe 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 –

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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

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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 !

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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 ...

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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.”

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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.

The 1931 Colour Plot shows

the three colour gamuts

along with target tolerance

boxes for the DCI Digital

Cinema primaries (non-

shaded boxes) and

suggested target tolerance

boxes for BT2020 (shaded)

Digital Projection

Insight 4K Dual LED Projector

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Colour Reproduction in Electronic Imaging Systems: Photography, Television, Cinematography

NEW! Use Promo Code VBG91 for 20% Off!

Buy now at www.wiley.comKey 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 environmental conditions; and, guides to both the workbook and JPEG files

By Michael S. Tooms

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ISBN: 978-1-119-02176-6 $120.00 | € 101.20 | £ 74.95Cloth | 648 PagesJanuary 2016

Thank you !

Martin ParsonsImage Eyes Ltdwww.image-eyes.com