The Importance of Terminology and sRGB Uncertainty - Notes - 0.4

The Importance of Terminology
and sRGB Uncertainty
Notes - 0.4
colour-science.org
1

Foreword
This presentation is the organised and formatted embodiment of the Colour
Science notes I have taken along the years. It is aimed at the VFX industry,
and is the work in-progress subset of a broader and generic Colour Science
presentation. Its creation wouldn’t have been possible without the works and
references cited in the Bibliography section.
Thomas Mansencal
2

The sRGB Uncertainty
• Understanding linear and sRGB color spaces : What does this mean?
sRGB is intrinsically linear!
• “We’ll start by learning how the sRGB and linear color spaces differ.”
• This is confusing for non experts because omitting an explicit emphasis of
the affected component of the sRGB colourspace.
4

What is Colour?
“Almost everyone knows what color is. After all, they have had ﬁrsthand
experience of it since shortly after birth. However, very few can precisely
describe their color experiences or even precisely deﬁne color.” [1]
1. Fairchild, M. D. (2013). Color Appearance Models (3rd ed., pp. 1–10831). Wiley. ISBN:B00DAYO8E2 5

What is Colour?
• Characteristic of visual perception that can be described by attributes of
hue, brightness (or lightness) and colourfulness (or saturation or chroma).
[1]
• Colour is perceived when light interacts with the human visual system
(HVS).
1. CIE. (n.d.). 17-198 colour (perceived). Retrieved June 26, 2014, from http://eilv.cie.co.at/term/198 6

Additive RGB Colourspace
• An additive RGB colourspace is deﬁned by specifying 3 mandatory
components:
• Primaries
• Whitepoint
• Conversion Functions (OECF and EOCF)
8

Additive RGB Colourspace
• An additive RGB colourspace is a colorimetric colour space having three
colour primaries (generally red, green and blue) such that CIE XYZ
tristimulus values can be determined from the RGB colour space values
by forming a weighted combination of the CIE XYZ tristimulus values for
the individual colour primaries, where the weights are proportional to the
radiometrically linear colour space values for the corresponding colour
primaries. [1]
• NOTE 2 Additive RGB colour spaces are deﬁned by specifying the CIE
chromaticity values for a set of additive RGB primaries and a colour
space white point, together with a colour component transfer
function.
1. ISO. (2004). INTERNATIONAL STANDARD ISO 22028-1 - Photography and graphic technology - Extended colour encodings for digital image storage,
manipulation and interchange, 2004. 9

Primaries
• The primaries chromaticity coordinates deﬁne the gamut of colours that
can be encoded by a given RGB colourspace.
• While commonly represented as triangles on a chromaticity diagram (such
as the CIE 1931 Chromaticity Diagram), RGB colourspace gamuts deﬁne
the boundaries of an actual solid within the CIE xyY colourspace.
11

Whitepoint
• The colourspace whitepoint is deﬁned as the colour stimulus to which
colour space values are normalized. [1]
• Any colour lying on the neutral axis normal to the xy plane and passing
through the whitepoint, no matter its luminance, will be achromatic.

Conversion Functions (Transfer Functions)
• A colour component conversion function is deﬁned as a single variable,
monotonic mathematical function applied individually to one or more
colour channels of a colour space. [1]
• They perform the mapping between the linear light components /
tristimulus values and a non-linear R'G'B' video signal.
• They are commonly used for faithful representation of images and
perceptual coding in relation with display non linear response and HVS
non linearity.

Opto-electronic conversion function
15

Opto-electronic conversion function
• The opto-electronic conversion function (OECF or OETF) maps (encodes)
estimated tristimulus values in a scene to a non-linear R'G'B' video
component signal value.
• Typical OECFs are usually expressed by a power function with an
exponent between 0.4 and 0.5.
16

Electro-optical conversion function
17

Electro-optical conversion function
• The electro-optical conversion function (EOCF or EOTF) maps (decodes) a
non-linear R'G'B' video component signal to a tristimulus value at the
display.
• Typical EOCFs are usually expressed by a power function with an
exponent between 2.2 and 2.6.
18

Misleading Terminology
Nuke’s Read node colorspace knob until Nuke 10 is only specifying an
electro-optical conversion function and will not perform gamut change.
20

Non Linearity of the Human Visual System
1. Davson, H. (1990). Physiology of the Eye (5th ed.). Elsevier Science Ltd. ISBN:978-0080379074 - colour-science.org 21

• Weber’s law states that the just-noticeable difference (JND) between two
stimuli is proportional to the magnitude of the stimuli: an increment is
judged relative to the previous amount.
• Fechner mathematically characterised Weber’s law showing that it follows
a logarithmic transformation: the perceived magnitude of a stimulus is
proportional to the logarithm of the physical stimulus intensity.
22

• Fechner’s scaling has been found to apply to the perception of brightness,
at moderate and high brightness, with perceived brightness being
proportional to the logarithm of the actual intensity.
• At lower levels of brightness, the de Vries-Rose law applies which states
that the perception of brightness is proportional to the square root of the
actual intensity.
23

• Stevens’s law supersedes Fechner's law and addresses its lack of
generality.
• The results of the physical-perceptual relationship of his experiments on a
logarithmic scale were characterised by straight lines with different slopes,
suggesting that the relationship between perceptual magnitude and
stimulus intensity follows a power law with varying exponent.
24

Lightness - CIE L*
• Because of the various HVS adaptation mechanisms, perceived
brightness has a non-linear relationship with the actual physical intensity of
the stimulus.
• It is commonly approximated by a cube root.
• Multiple approximations of lightness (or value in the Munsell Renotation
System) were proposed leading to the creation of CIE L* in 1976.
• CIE L* characterises the perceptual response to relative luminance.
28

Colour Imaging System
• A colour imaging system embodies any combination of technologies and
devices required to perform:
• Image capture
• Signal processing
• Image formation
30

Image Capture
• Image capture / acquisition of colour stimuli can be performed in a
number of different ways using for example:
• An electronic device (electronic video camera, DSLR)
• Photographic ﬁlm
32

Electronic Capture
• A movie camera may use a solid-state image sensor (CCD or CMOS) that
absorbs photons of light.
• As photons absorption occurs, electrons are collected into charge
packets.
• The image signal is produced by a sequential readout of the packets.
33

Electronic Capture
• Accurate image reproduction requires the capture device to be at least
trichromatic implying that colour stimuli spectral power distributions must
be separated into 3 colour signals.
• This separation can be achieved with:
• A beam splitter / colour filters combined to three sensors on high end
capture devices resulting in reduced noise and increased resolution.
• A single sensor covered with a mosaic of colour filters on systems
requiring a small form factor and lower price.
• Three sensor layers with different responses to wavelengths of light
stacked together similarly to photographic film (Foveon).
34

Photographic Film Capture
1. https://www.fujiﬁlmusa.com/shared/bin/AF3-150E_Sensia100_Data_Sheet_2003.pdf 35

Photographic Negative Film
• A photographic ﬁlm has red-, green-, and blue-light-sensitive layers
coated on a transparent base.
• The red and green layers are also sensitive to blue light, thus a yellow
ﬁltering layer is placed above them. It will be made colourless during
chemical processing.
• Light sensitivity is induced by silver halide grains with appropriate spectral
response scattered within each light sensitive layer. The sensitive layers
also contain an appropriate dye coupler.
36

Image Formation
1. https://commons.wikimedia.org/wiki/File:AdditiveColor.svg
2. https://commons.wikimedia.org/wiki/File:SubtractiveColor.svg 37

Image Formation
• The processed image signals control colour-forming elements of the
image formation medium / device.
• Two categories of image formation exist:
• Additive colour
• Subtractive colour
38

Additive Colour Formation
• CRT, LCD or plasma displays mix red, green and blue light through pixels
adjacency.
• DLP, digital cinema projectors perform superposition by using a beam
combiner.
39

Subtractive Colour Formation
• Photographic ﬁlm use cyan, magenta and yellow dyes to absorb red,
green and blue light.
• Similarly, most printing processes use CMY inks.
• Colour stimuli formed by subtractive colour are dependent (and affected)
by the viewing light source.
40

Picture Rendering
• The colour imaging system usually achieves representation of a scene in a
way that matches viewer expectation of the appearance of that scene
instead of attempting to reproduce physical colour stimuli quantities.
• A sunlight outdoor scene can have luminance of 50,000 cd.m-2 but may be
displayed on a consumer electronic display with white peak luminance of
320 cd.m-2.
41

Picture Rendering
• The different viewing conditions and image formation medium / device
capabilities impose that scene luminance must be mapped to image
formation medium / device luminance.
• A simple linear mapping from scene luminance to image formation
medium / device luminance is not satisfactory.
• Picture rendering adjusts the tone scale to achieve a perceptual uniform
mapping.
42

Non Triviality of Picture Rendering
1. Fairchild, M. D. (n.d.). The HDR Photographic Survey. Retrieved April 15, 2015, from http://rit-mcsl.org/fairchild/HDRPS/HDRthumbs.html 43

Effect of Lateral-Brightness Adaptation
Images seen with a dark surround appear to have less contrast than if
viewed with a dim, average or bright surround.
47

Effect of Lateral-Brightness Adaptation

Colour Encoding
• A colour encoding is a digital representation of colours for image
processing, storage, and interchange between systems.
• A colour encoding speciﬁcation (standardised input / output interface of a
colour imaging system) must deﬁne:
• A colour encoding method which determines the meaning of the
encoded data or what will be represented by the data.
• A colour encoding data metric characterising the colourspace and the
numerical units used to encode the data or how the the representation
will be numerically expressed.
49

Image States
• The image state concept was deﬁned by Madden & Giorgianni.
• Some signal processing operations make the image transition to a different
colorimetric state.
• An image may exist in scene state which is not directly viewable on typical
image formation devices and must be transitioned to a new state, the
rendered state.
50

Image States
• A colour encoding specification defined in relation to scene quantities is
said to be scene-referred: it has a colorimetric link to a scene.
• A colour encoding specification defined in relation to digital display
characteristics is said to be display-referred (rendered state): it has a
colorimetric link to a digital display device.
51

Display-Referred Imaging
• Raw image processors used by photographers (Lightroom, Darktable,
DCRaw, etc…) perform picture rendering on the raw scene referred data
to deliver a display-referred image.
• Artists achieving direct content creation in 2d applications are generating
display-referred content.
• Images available on the Internet such as on Google Images or textures
vendors website are output- / display-referred.
• A photograph taken on a mobile phone and uploaded to a social network
is display-referred.
52

• Display-referred imagery created and exhibited on a display that matches
a standard reference (using sRGB speciﬁcation and viewing conditions)
will appear the same across similar display devices without any further
action required.
• A photograph processed on a consumer graphics desktop and output as
a sRGB JPG or PNG ﬁle will approximately look the same on other
consumer graphics desktop.
53

• Display-referred imagery has usually a restricted luminance dynamic
range and limited colour gamut thus some of the original captured scene-
referred data is lost upon encoding.
• This is unsuitable if the image is meant to be viewed on different image
formation devices with wider dynamic range.
54

Sony F35 - Out of Gamut Colours
1. http://www.oscars.org/science-technology/sci-tech-projects/aces 55

56

57

Scene-Referred Imaging
• Scene-referred representation of data contains enough information to
achieve the desired appearance of the scene on a variety of image
formation medium / device.
• Scene-referred imaging is the basis of physically-based rendering
allowing to reproduce realistic light interaction using plausible light
quantities. It makes possible realistic camera effects (motion-blur,
defocus).
58

• Measured scene linear-light quantities are usually normalised to a known
reference.
• Commonly middle grey is set at luminance = 0.18 which is the reﬂectance
of:
• Reference Kodak 18% Grey Card
• Background colour of a DSC Labs CamAlign ChromaDuMonde chart
• Reﬂectance of a X-Rite ColorChecker neutral 5 (.70 D) sample is ≈ 19%!
62

Energy Conservation
• Anti-aliasing or image ﬁltering operations should be energy preserving: the
total light emitted from the display should remain the same after the
processing operations.
• Resizing an image should not affect its luminance.
• Those operations must be performed on linear image data
64

Blur in Non-Linear Colourspace
67

Digital Image - Raster Graphics
• A digital image is a rectangular data structure (a 2 or 3-dimensional array)
of picture elements (pixels).
• A pixel colour is determined by a single code for achromatic images or
multiple codes for chromatic images (commonly three).
68

69

70

71

Quantization
• Quantization is the process of mapping a continuous signal (or large set of
input values) to a smaller set.
• Information between each quantizer steps is discarded and lost.
• Quantization error (signal distortion) decreases signal-to-noise ratio (SNR).
• Banding and contouring artefacts can be reduced by introducing a small
amount of noise (≈ 1 / 2 quantiser step) prior to the quantization. Dithering
decreases the SNR.
72

Quantization
Input Signal
1. Getty Images. (n.d.). Getty Images Test Image. Retrieved June 20, 2003, from https://www.drycreekphoto.com/tools/ 77

Quantization
4-Bit Linear Quantization

Quantization
4-Bit Perceptually Uniform Quantization

Perceptual Uniformity
• A colour imaging system is perceptually uniform if a small perturbation of a
component value is approximately equally perceptible across the range of
that value. [1]
• Most electronic colour imaging systems account for non linearity of the HVS
and its perceptual response to brightness when encoding RGB scene
relative luminance values (linear-light values) into R’G’B’ perceptually
uniform values. 
 
This is commonly achieved with a logarithmic transfer function (gamma,
L*).
• They leverage non linearity of the HVS to reduce the bandwidth and
number of bits needed per pixel by optimising digital codes allocation.
1. Poynton, C. (n.d.). Perceptual Uniformity. Retrieved March 5, 2016, from http://www.poynton.com/notes/Timo/Perceptual_uniformity.html 83

Perceptual Uniformity
• Cathode ray tubes (CRT) display electron gun characteristics imposed an
EOCF that is approximately the inverse of HVS perception of brightness.
• HVS perceptual response to brightness associated with the CRT power
function produces code values displayed in a perceptual uniform way.
• Modern display devices (LCD, plasma, DLP) replicate this behaviour by
imposing a 2.2, 2.4 or 2.6 power function (Gamma Correction) through
signal processing circuitry.
84

Perceptual Uniformity - Linear Ramp
86

Perceptual Uniformity - Linear Ramp
87

Perceptual Uniformity - Perceptually Uniform Ramp
88

Perceptual Coding
1. Poynton, C. (2012). Digital Video and HD, Second Edition: Algorithms and Interfaces (2nd ed.). Elsevier / Morgan Kaufmann. ISBN:978-0123919267 89

Perceptual Coding

Perceptual Coding
• The luminance difference between L and L + ΔL is noticeable when ΔL is
about 1% of L.
• The 1.01 (101 / 100) ratio is known as the Weber contrast or fraction.
92

Perceptual Coding
• An ideal non-linear transfer function will allocate code values to minimise
the just-noticeable difference (JND).
• On a linear-light values scale, code 100 is the location where Weber
contrast reaches 1%.
• Weber contrast increases for codes below 100, raising the perceptible
difference between adjacent codes and possibly producing banding and
contouring artefacts.
• Weber contrast decreases for codes over 100, higher codes are getting
wasteful and could be discarded without affecting the perception.
93

Perceptual Coding
• High-quality image reproduction requires a contrast ratio >= 30:1 as
shown by the NTSC engineers in the 1950s.
• Using 8-bit linear-light coding, the contrast ratio that can be reproduced
without artefacts is only 2.55:1.
• Achieving a contrast ratio >= 30:1 with linear-light coding requires 12-bit
resulting in an artefacts free contrast ratio of 40.95:1 however most of
those codes cannot be visually discriminated.
94

Perceptual Coding
Maintaining a 1.01 Weber contrast over scene relative luminance range of
[0.01, 100], contrast ratio of 100:1, requires approximately 462 codes (≈ 9
bits). [1]
1. Poynton, C., & Funt, B. (2014). Perceptual uniformity in digital image representation and display. Color Research and Application, 39(1), 6–15. doi:10.1002/
col.21768 95
log100
log1.01
⇡ 462; 1.01462
⇡ 100
C =
log(CR)
log(WC)
where C is the number of codes, CR is the contrast ratio and WC is the desired
Weber contrast.

16-Bit Integer & Half Float
Perceptual coding is not required when using 16-bit integer (artefacts free
contrast ratio of 655.35:1) or half ﬂoat representations (Weber contrast of
0.1% [1], 2^10 = 1024 code values per stop)).
col.21768 96

8-Bit Colour Imaging System Dynamic Range
Dynamic range associated with code 1 on 8-bit colour imaging system is
closer to 200,000:1 (or 600,000:1) instead of the 255:1 (or 256:1) dynamic
range often alleged because of the incorrect assumption that linear-light
values are encoded.
97
1
255
!2.4
⇡ 0.0000016 ⇡
1
600000

Gamma
• Gamma (γ) is a numerical parameter giving the exponent of a power
function assumed to approximate the relationship between a signal
quantity (such as a video signal code) and light power. [1]
• Gamma Encoding (γE), characteristic of OECFs uses an exponent
approximately between 0.4 and 0.5.
• Gamma Decoding (γD), characteristic of EOCFs uses an exponent
approximately between 2.2 and 2.6.

Digital Colour Imaging System End-to-End Power Function
To overcome the loss in apparent contrast, the end-to-end power function of
a digital colour imaging system may have appropriate exponent values of 1,
1.25, and 1.5 for respectively bright, dim, and dark surrounds. [1]
1. Hunt, R. W. G. (2004). The Reproduction of Colour (6th ed.). Chichester, UK: Wiley. doi:10.1002/0470024275 101

Gamma Correction Misconceptions
• NTSC monochrome television was created in the 1940s and non linear
coding was a well understood element of good visual performance.
• Signiﬁcance of perceptual uniformity has been generally forgotten: video
engineers seem to see gamma correction as a mean to address CRT “non
linearity defect”.
• ‘‘If gamma correction was not already necessary for physical reasons at
the CRT, we would have to invent it for perceptual reasons.” [1]
col.21768 103

Digital Video & HD
• The luminance output of a CRT is proportional to input raised to the 5 / 2
power. A studio reference display CRT has a gamma ≈ 2.4.
• Gamma correction through the mean of an OECF is applied to pre-
compensate CRT display non linear power function and achieve perceptual
uniformity.
• In order to account for the different viewing conditions between original
scene and presentation, the correction under-compensate the actual CRT
display non linearity.
• This under-compensation yields an end-to-end power function with
exponent ≈ 1.2 which produces a pleasing television viewing experience in
dim surrounds.
104

Digital Video & HD
• Image Structure
• 1920 x 1080 progressive (24Hz, 30Hz), 16:9 aspect ratio
• 1920 x 1080 interlaced (30Hz), 16:9 aspect ratio
• 1280 x 720 progressive (24Hz, 30Hz, 60Hz), 16:9 aspect ratio
105

ITU-R BT.1886
• ITU-R BT.1886 deﬁnes the reference electro-optical transfer function for
CRT and LCD displays used in HDTV studio production.
• ITU-R BT.1886 adopts a power function with exponent γ = 0.5.
• The recommendation doesn’t standardise reference white and viewing
conditions.
• ITU-R BT.2035 deﬁnes a reference viewing environment for evaluation of
HDTV program material.
106

ITU-R BT.1886
• HD Studio Mastering (Typical)
• Reference white is typically set at 100-120 cd.m-2
.
• Surround luminance is expected to be very dim at around 1% of reference white
luminance.
• Typical intra-image contrast ratio is 1000:1.
• HD Consumer (Typical)
• Reference white is typically set at 200 cd.m-2
.
• Surround luminance is expected to be dim at around 5% of reference white luminance.
107

ITU-R BT.2035
• ITU-R BT.2035 deﬁnes a reference viewing environment for evaluation of
HDTV program material.
• D.R.A.F.T
108

ITU-R BT.709 / Rec. 709
ITU-R BT.709 is the international standard deﬁning the parameter values for
HDTV.
109

BT.709 OECF
• BT.709 OECF deﬁnes a 0.45 exponent but its effective power function
exponent is γE ≈ 0.5.
• BT.709 OECF is a piece-wise function: in order to reduce noise in dark
region, a line segment limits the slope of the power function (slope of a
power function is inﬁnite at zero).
110

BT.709 - BT.1886 End-to-End γ
111

BT.709 Colourspace
• Primaries:  
 
[0.6400, 0.3300] 
[0.3000, 0.6000] 
[0.1500, 0.0600]
• Illuminant / Whitepoint: D65
• Pointer’s Gamut Coverage: 81.1674568
• Visible Spectrum Coverage: 36.6606209
113

UHDTV
The UHD Alliance (UHDA) developed three speciﬁcations to support the
next-generation premium home entertainment experience covering the
entertainment ecosystem in the following categories: [1]
• Devices
• Distribution
• Content
1. UHDA. (2016). UHD Alliance Deﬁnes Premium Home Entertainment Experience. Retrieved January 8, 2016, from http://www.uhdalliance.org/uhd-alliance-
press-releasejanuary-4-2016/ 114

UHDTV - Devices
An UHDA compliant device must meet or exceed the following speciﬁcations:
• Image Resolution: 3840×2160
• Color Bit Depth: 10-bit signal
• Color Palette (Wide Color Gamut)
• Signal Input: BT.2020 color representation
• Display Reproduction: More than 90% of P3 colours
• High Dynamic Range
• SMPTE ST2084 EOTF
• A combination of peak brightness and black level of either:
• More than 1000 nits peak brightness and less than 0.05 nits black level
• More than 540 nits peak brightness and less than 0.0005 nits black level
115

UHDTV - Distribution
An UHDA compliant distribution channel must support:
• Color Bit Depth: Minimum 10-bit signal
• Color: BT.2020 color representation
• High Dynamic Range: SMPTE ST2084 EOTF
116

UHDTV - Content Mastering
UHDA Content Master must meet the following requirements:
• Color Bit Depth: Minimum 10-bit signal
• Color: BT.2020 color representation
• High Dynamic Range: SMPTE ST2084 EOTF
Speciﬁcations of UHDA recommended mastering display:
• Display Reproduction: Minimum 100% of P3 colours
• Peak Brightness: More than 1000 nits
• Black Level: Less than 0.03 nits
117

ITU-R BT.2020 / Rec. 2020
ITU-R BT.2020 deﬁnes the parameter values for ultra-high deﬁnition
television systems for production and international programme exchange.
118

ITU-R BT.2020 / Rec. 2020
• Image Structure
• 7680 × 4320, 16:9 aspect ratio, 1:1
• 3840 × 2160, 16:9 aspect ratio, 1:1
• Frequency:120, 60, 60/1.001, 50, 30, 30/1.001, 25, 24, 24/1.001
• Progressive scan mode
119

BT.2020 OECF
BT.2020 OECF is the same than BT.709 OECF and is expected to be used in
conjunction with BT.1886 EOCF yielding an an end-to-end power function
with exponent ≈ 1.2.
120

BT.2020 - BT.1886 End-to-End γ
121

BT.2020 Colourspace
• Primaries:  
 
[0.708, 0.292] 
[0.170, 0.797] 
[0.131, 0.046]
123

SMPTE ST 2084
• SMPTE ST 2084 (PQ) is the international standard deﬁning the EOTF
characterizing high-dynamic-range reference displays used primarily for
mastering non-broadcast content.
• The perceptual quantizer has been modeled by Dolby Laboratories using
Barten (1999) contrast sensitivity function.
• Display peak luminance is expected to reach 10,000 cd.m-2 and use a 10
or 12-bit data representation.
124

Multimedia & Desktop Graphics
sRGB IEC 61966-2-1:1999 speciﬁcation is deﬁned for multimedia
applications, desktop graphics considering a brighter surround than the one
of a studio reference display.
126

sRGB IEC 61966-2-1:1999
• sRGB adopts ITU-R BT.709 RGB colourspace gamut but a different set of
OECF / EOCF.
• sRGB reference white is speciﬁed at 80 cd.m-2 in accordance to CRTs.
• Surround luminance is expected to be average at around 20% of
reference white luminance.
• Modern LCD displays commonly peak at 320 cd.m-2.
127

sRGB OECF
• sRGB OECF doesn’t account for picture rendering: the end-to-end gamma
is ≈ 1.0 when associated with sRGB EOCF (γE ≈ 2.2), thus it is not suitable
for image capture.
• sRGB is deﬁned as a display-referred colour encoding.
128

BT.709 - sRGB End-to-End γ
129

sRGB Colourspace
• Primaries (Rec. 709):  
 
[0.6400, 0.3300] 
[0.3000, 0.6000] 
[0.1500, 0.0600]
132

Digital Cinema
• Picture rendering was traditionally imposed by a camera negative film
gamma ≈ 0.5-0.6, an inter-positive film having a unity gamma and a
release print film stock with gamma ≈ 2.8-3.2, resulting in an end-to-end
gamma ≈ 1.4-1.8, suitable for dark film projection surrounds.
• DCI / SMPTE standard reference digital cinema projector apply a 2.6
gamma to the X’Y’Z’ DCDM (Digital Cinema Distribution Master) non linear
components.
133

Digital Cinema
• The X’Y’Z’ DCDM is encoded with JPEG-2000 compression.
• The X’Y’Z’ DCDM image file format is mapped into TIFF. Colour channels
are represented by 12-bit unsigned integer code values. These 12 bits are
placed into the most significant bits of 16-bit words, with the remaining 4
bits filled with zeroes.
• Image Structure
• 4096 x 2160, 24Hz, 1:1
• 2048 x 1080, 24Hz, 1:1
• 2048 x 1080, 48Hz, 1:1
134

Digital Cinema
• Digital cinema standards are display-referred: colour appearance of the
digital intermediate is fully baked into the X’Y’Z’ DCDM.
• Digital cinema reference white is speciﬁed at 48 cd.m-2.
• Surround luminance is expected to be dark (0% of reference white
luminance).
• DCI-P3 is the wide gamut RGB colourspace in which digital cinema
material is mastered.
135

DCI-P3 Colourspace
• Primaries:  
 
[0.680, 0.320] 
[0.265, 0.690] 
[0.150, 0.060]
• Illuminant / Whitepoint: 0.314, 0.351
137

Digital Capture for Digital Cinema
• Motion picture camera vendors commonly encode their scene-referred
data using a log encoding function ('ALEXA Log C', 'C-Log', 'Panalog', 'S-
Log', ‘V-Log', etc…) tailored to account camera specific dynamic range
and noise characteristics.
• They also define dedicated gamuts accounting for the specific spectral
responses of their respective camera.
138

Camera Vendors Log Encoding Functions
139

Digital Capture for Digital Cinema
• Those log encoding functions draw inspiration into Cineon Digital Film
System developed by Eastman Kodak Company.
• Cineon is a logarithmic encoding of the colour film negative optical
density.
• “Film has traditionally been represented by a characteristic curve which
plots density vs log exposure. This is a log/log representation. In defining
the calibration for the Cineon digital film system, Eastman Kodak Co.
talked to many experts in the film industry to determine the best data
metric to use for digitizing film. The consensus was to use the familier
density metric and to store the film as logarithmic data.” [1]
1. Kodak. (1995). Conversion of 10-bit Log Film Data To 8-bit Linear or Video Data for The Cineon Digital Film System. 141

Cineon Digital LAD
1. KODAK Digital LAD Test Image - Eastman Kodak Company 142

sRGB Digital LAD
1. KODAK Digital LAD Test Image - Eastman Kodak Company 143

Visual Effects Colour Pipeline
1. Selan, J. (2012). Cinematic color. ACM SIGGRAPH 2012 Posters on - SIGGRAPH ’12, 1–54. doi:10.1145/2343483.2343492 - colour-science.org 144

• Visual effects vendors generate scene-referred imagery that is seamlessly
integrated onto client plates while not altering their image state.
• This fundamental principle is at the heart of visual effects as shots with
visual effects must be intercutted with shots without visual effects (or
coming from other vendors).
• The digital intermediate (DI) expects a delivery that is a high ﬁdelity
representation of the original capture.
145

• The visual effects colour pipeline is a complex colour imaging system built
on individual chained colour imaging systems.
• Colour encoding specifications must be defined (and identifiable to be
accounted for) for every input / output signal processing operations.
146

Working Colour Encoding Specification
• A modern paradigm is to define a working colour encoding specification
(for example based on ACEScg, DCI-P3, or Rec. 2020 gamuts and
representing scene-referred linear-light quantities) and convert all the input
imagery with their respective colour encoding specifications to that
working specification.
• Plates are usually converted to the working colour encoding specification
by using an invertible decoding 1D LUT specific to their originating
gamma / log encoding and then to the working gamut by mean of a 3x3
matrix (or a 3D LUT).
147

Working Colour Encoding Specification
Some facilities perform the compositing stage within the client delivery
gamut: it can be beneficial when the working colour encoding specification
doesn’t encompass the captured plates gamut (avoiding complicated to
handle negative values). 
 
Note: ARRI Alexa cameras are notorious to have a very wide gamut.
148

View Transform
• The scene-referred data is visualised using a dedicated view transform
(1D LUT or 3D LUT) that commonly model the typical characteristic curve
of a print ﬁlm (print ﬁlm emulation, S-Curve, sigmoid function combined
with a log curve, etc…).
• The view transform is never baked into the DI delivered imagery.
149

Duiker Print Film Emulation View Transform
151

Compositing
• Plates are neutralised using an invertible process to overcome lighting
changes across a sequence.
• This permits reusability of light rigs at the rendering stage and establish a
better consistency across shots during the compositing stage.
• The neutralisation is reversed on compositing output.
152

Texturing & Matte Painting
• D.R.A.F.T
153

Digital Intermediate & Mastering
1. http://www.parkroad.co.nz/wp-content/uploads/2015/10/Clare_Mahana_DI.jpg 154

• Digital intermediate is a display-referred finishing process originally
involving motion picture digitisation, colour manipulation (colour timing /
grading, contrast adjustment, etc…) and recording back to film again to
create a master internegative.
• The viewing environment replicates the final exhibition viewing
environment, and is adapted accordingly to each type of exhibition image
formation device (digital cinema, typical home theater, etc…).
• Calibration tolerances to the standards (DCI / SMPTE) are very strict.
155

• The DI process is commonly split into an initial pass that neutralises per
shot variation and a secondary pass that deﬁnes the colour artistic intent /
look of the ﬁlm.
• The DI house may provide a Colour Decision List (CDL) or 3D LUT per
shot to visual effects vendors to give them an overview of the look being
developed.
156

• DI often creates masters for multiple image formation medium / devices.
• Artistic grading is performed on the “gold standard” image formation
device (usually the digital cinema projector) with approval of the director.
• Trim passes are executed for the other image formation devices and will
include speciﬁc corrections for the respective devices characteristics and
viewing conditions.
157

Academy Color Encoding System
1. http://www.orbitnet.com/ampas/ACES_1.html 158

Academy Color Encoding System
• ACES is a colour management and image interchange system designed
for production, mastering and long-term archiving of motion pictures. [1]
• It enables consistent, high-quality colour management from production to
distribution.
• It provides digital image encoding and speciﬁcations preserving original
imagery latitude and colour range while establishing a common standard
so deliverables can be efﬁciently and predictably created and preserved.
1. The Academy of Motion Picture Arts and Sciences. (n.d.). ACES. Retrieved March 22, 2016, from http://www.oscars.org/science-technology/sci-tech-
projects/aces 159

ACES Components - Input
• Reference Input Capture Device (RICD) 
The RICD, an ideal capturing device, records all the colour (and dynamic
range) of a given scene. It provides a documented, unambiguous and
ﬁxed relationship between scene colours and encoded RGB values.
• Input Device Transform (IDT) 
An image captured by a physical or virtual camera is transformed by the
IDT into ACES RGB relative exposure values that the RICD would have
recorded if used in-place.
160

ACES Components - Output
• Reference Rendering Transform (RRT) 
ACES images are an intermediate representation and cannot be used for
final image evaluation. The RRT is an idealised replacement for print-film
emulations (S-Curve) with an extremely wide gamut and high dynamic
range (32 stops).
• Output Device Transform (ODT) 
The ODT performs rendering of the RRT wide gamut and dynamic range
on a given physical display, accounting for its specific characteristics
(gamut, dynamic range, and EOCF) and viewing conditions.
161

ACES Components - Negative Film
• Academy Printing Density (APD) 
Reference printing density for calibrating film scanners and film recorders.
• Academy Density Exchange (ADX) 
Densitometric encoding (similar to Cineon) used for capturing data from
film scanners.
162

ACES Encodings
• ACES2065-1 (ACES Primaries 0, AP0) 
The ACES common colour encoding colourspace used for exchange of full ﬁdelity images
and archiving.
• ACEScg (ACES Primaries 1, AP1) 
A linearly encoded colourspace for CG rendering and compositing, using the improved set
of primaries that encompass Rec. 2020 and DCI-P3 gamuts.
• ACEScc (ACES Primaries 1, AP1) 
A logarithmically encoded colourspace for use in colour grading applications, using the AP1
primaries.
• ACES proxy (ACES Primaries 1, AP1) 
A lightweight encoding using the AP1 primaries, for transmission over HD-SDI (or other
production transmission schemes), onset look management. Not intended to be stored or
used in production imagery or for ﬁnal colour grading / mastering. 
163

ACES Primaries 0, AP0
• Primaries:  
 
[0.73470, 0.26530] 
[0.00000, 1.00000] 
[0.00010, -0.07700]
164

ACES Primaries 1, AP1
• Primaries:  
 
[0.713, 0.293] 
[0.165, 0.830] 
[0.128, 0.044]
165

Colour Grading - GoG
1. https://vimeo.com/116019668 167

Colour Grading - GoG
168
y = ax + b (1)
y = (ax + b + c(1 x))1/
(2)
Yo = (gain ⇥ Yi + offset + lift ⇥ (1 Yi))(1/gamma)
(3)
where Yi is the input luminance and Yo the output luminance.
Note 1: (1) is slope-intercept form of a linear equation.
Note 2: On a television the contrast and brightness controls are respectively
mapped to the gain and o↵set variables.

1D Lut & 3D Lut
• A 1D Lut is a single variable indexed one dimensional table.  
 
Expensive runtime computation are replaced with a simpler array indexing
operation / look up.
• A 3D Lut is a three variable indexed three dimensional table (3D lattice)
where each variable (lattice axis) represent a colour component.  
 
Output colour values for input variable points not exactly matching output
lattice points are interpolated.
182

Bibliography
• Fairchild, M. D. (2013). Color Appearance Models (3rd ed.). Wiley.
ISBN:B00DAYO8E2
• Wyszecki, G., & Stiles, W. S. (2000). Color Science: Concepts and
Methods, Quantitative Data and Formulae. Wiley. ISBN:978-0471399186
• Poynton, C. (2012). Digital Video and HD, Second Edition: Algorithms and
Interfaces (2nd ed.). Elsevier / Morgan Kaufmann. ISBN:978-0123919267
• Madden, T. E., & Giorgianni, E. J. (2007). Digital Color Management (Vol.
20). doi:10.1002/9780470994375
• Dutré, P., Bekaert, P., & Bala, K. (2006). Advanced Global Illumination, 2,
384. ISBN:1439864950
183

Bibliography
• ISO. (2004). INTERNATIONAL STANDARD ISO 22028-1 - Photography and graphic technology -
Extended colour encodings for digital image storage, manipulation and interchange, 2004.
• International Telecommunication Union. (2011). Recommendation ITU-R BT.1886 - Reference electro-
optical transfer function for ﬂat panel displays used in HDTV studio production BT Series Broadcasting
service.
• International Telecommunication Union. (2015). Recommendation ITU-R BT.709-6 - Parameter values
for the HDTV standards for production and international programme exchange BT Series Broadcasting
service (Vol. 5). Retrieved from https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.709-6-201506-I!!
PDF-E.pdf
• International Telecommunication Union. (2013). Recommendation ITU-R BT.2035 - A reference viewing
environment for evaluation of HDTV program material or completed programmes BT Series
Broadcasting service.
• International Telecommunication Union. (2015). Recommendation ITU-R BT.2020 - Parameter values for
ultra-high deﬁnition television systems for production and international programme exchange (Vol. 1).
Retrieved from https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.2020-2-201510-I!!PDF-E.pdf
184

Bibliography
• Reinhard, E. (2009). A Reassessment of the Simultaneous Dynamic Range of the Human Visual
System, 17–24.
• Poynton, C., & Funt, B. (2014). Perceptual uniformity in digital image representation and display.
Color Research and Application, 39(1), 6–15. doi:10.1002/col.21768
• Selan, J. (2012). Cinematic color. ACM SIGGRAPH 2012 Posters on - SIGGRAPH ’12, 1–54. doi:
10.1145/2343483.2343492
• Kodak. (2002). KODAK: Student Filmmaker’s Handbook. Retrieved from http://ultra.sdk.free.fr/misc/
TechniquePhoto/Kodak Student Handbook.pdf
• Gilchrist, A. (2008). Perceptual organization in lightness. Vasa, 1–25. Retrieved from http://
www.gestaltrevision.be/pdfs/oxford/Gilchrist-Perceptual_organization_in_lightness.pdf
• Nilsson, M. (2015). BT Media and Broadcast - Ultra High Deﬁnition Video Formats and
Standardisation. Retrieved from http://www.mediaandbroadcast.bt.com/wp-content/uploads/
D2936-UHDTV-ﬁnal.pdf
185

Bibliography
• Brendel, H. (2005). ARRI COMPANION TO DI - Chapter 2. Motion Picture
Film. Retrieved March 12, 2016, from http://dicomp.arri.de/digital/
digital_systems/DIcompanion/ch02.html
• Pritchard, B. R. (n.d.). Why Colour Negative is Orange. Retrieved March
19, 2016, from http://www.brianpritchard.com/
why_colour_negative_is_orange.htm
• https://github.com/colour-science/colour-ipython
• Wikipedia. (n.d.).
186

The Importance of Terminology and sRGB Uncertainty - Notes - 0.4

More Related Content

What's hot

Similar to The Importance of Terminology and sRGB Uncertainty - Notes - 0.4

Recently uploaded

The Importance of Terminology and sRGB Uncertainty - Notes - 0.4