High-Dynamic Range (HDR) Demystified
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Transcript of High-Dynamic Range (HDR) Demystified
Scott Janus, Principal Engineer
D
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Presentation Goals
Explain the value of High Dynamic Range (HDR) and Wide Color Gamut (WCG)
Explain the technical challenges of deploying HDR + WCG
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What is HDR?
Practically speaking:
Content with a wider range of brightness and color
– Also an increased number of brightness and color levels
Experiencing HDR content requires new displays
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Let’s talk about color and brightness…
Really they are tightly interwoven, but let’s consider them individually for now
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Color is complicated
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Monitors Can Produce a Finite Range of Colors
Typically, red, green, and blue subpixels.
If you modulate each primary 0-100%, you can envision a three-dimensional color cube
All colors in this cube are the gamut of colors the device can reproduce
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Let’s simplify things…
Humans are bad at perceiving volumetric data, so compress the color volume to two dimensions
Chromaticity Diagram
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Wide Color
• Most PCs and HDTVs use 709 gamut
• Majority of next generation content uses Bt.2020 gamut
• 2015 UHDTVs are ~85% Bt.2020
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Bt 2020 content displayed unmodified
on 709 display
Bt 2020 contentgamut-mapped for
709 display
We must perform gamut mapping
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Advantages of Wide Color
We can reproduce lifelike colors we couldn’t before and create more immersive experiences
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Dynamic Range
Simple definition:
• Ratio of largest to smallest signal in a system
What is HDR (in this context)
Practically speaking:
Content with a wider range of brightness and color
Requires new monitors to experience HDR content
– Legacy content: 100 nit peak brightness, 709 gamut
– HDR content: 10,000 nit peak brightness, 2020 gamut
HDR is an ambiguous term
There are several industry specs defining various flavors of HDR
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(new) HDR: what it is not
“HDR Photography”
2005-era HDR rendering/gaming
Such as Half-Life2: Lost Coast demo
Both of these techniques generate images designed to be shown on an SDR monitor
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Let’s talk about light and humans…
Terminology
Luminance
Quantitative measurement of amount of light passing through an area
Can be unequivocally measured by instruments
Linearly proportional to # of photons
Brightness
Subjective human perception of luminance
Varies wildly based on ambient conditions and from human to human
Non-linearly proportional to # of photons
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Measuring Luminance
Nit (Candela/m2)
Approximately equal to the amount of light from a candle dispersed over a square meter
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Object Approximate Luminance (nits)
Sun 1,600,000,000
Arc lamp 150,000,000
Maximum visual tolerance 50,000
Cloud (sunny day) 35,000
2016 UHDTV 800-1000
Typical computer screen 100-300
White paper under lamp 50
Night sky (legacy) 0.001
Threshold of vision 0.000003
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nits
2,000 nits
500,000
nits
200,000
nits
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TL;DR
Standard Dynamic Range:
De facto brightness range of current content and displays
High Dynamic Range:
Substantially brighter next-generation content and displays
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Standard Dynamic Range
For the past 80 years, video has been graded to appear properly on a 100 nit display
However, no adjustments are made to comprehend displays of differing luminance
– 30 nit laptops in low power mode
– 600 nit HDTVs in Vivid mode
Similarly, no adjustments to the content are made when you surf the web and play games, switching from phones to tablets to PCs.
This is wrong, but has been good enough
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Disambiguation: HDR Photography
Multiple exposures from different times combined off-line to create a single SDR image which has different exposure levels for different areas of the picture
Not what we’re talking about today
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HDR Video
Single exposure captures a wide range of luminance at a
single instant in time
Intended to be displayed on an HDR display
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Things to ponder
With color, many real-world hues can be exactly reproduced on screens
Almost every real-world situation has objects at >100 nits
So the real world is HDR, and every SDR picture you take involves HDR->SDR conversion
A 100-nit object on the screen almost never corresponds to a 100-nit object in the real world
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Multiple ranges of HDR
0
nits
10,000
nits
ContainerCapture
15,000
nits
Movie DisplayContainerCapture Movie
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HDR Conversions
Capture Movie HDR Display
Mastering
Performed
by studio
creatives
Handled by
player
Handled
by
UHDTV
HDR->HDR
SDR Display
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HDR Content + SDR Display =Bad
Experience
SDR Content + SDR Display = SDR Experience
SDR Content + HDR Display = SDR Experience
HDR Content +HDR->SDR
Tone MappingSDR Display =
SDR
Experience
HDR Content + HDR Display = HDR Experience
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Real World
3,000
nits
300,000
nits
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Mastered for SDR
Coded
value=
0xFF
Coded
value=
0xFF
100
nits
100
nits
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Mastered for HDR
Coded
value=
0x23B Coded
value=
0x36C
200
nits
5,000
nits
Linear vs non-linear light
In the real world, luminance is determined by the number of photons
However, brightness (the human perception of luminance) is non-linearly proportional to number of photons
Code Words
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6
5
4
3
2
1
0
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Directly storing luminance is inefficient
To prevent banding using SDR, you would need to use 13-14 bits to code a contemporary 100-nit signal
Lum
inance (
linear
scale
)
Just noticeable difference
Luminance
Linear Light
Perceptually
non-uniform
Luma
Non-Linear Light
Perceptually
Uniform
0
1
0 1
To efficiently code video signals, we apply a non-linear transform function so that we can code perceptually uniform brightness intervals
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Problems
Although gamma is a reasonable approximation of human perception in the SDR (0-100 nit) range…
Gamma is not a good match for human perception in the 0-10,000 nit HDR
If you use gamma for a 10,000 nit signal, you need to use two extra bits/sample to eliminate banding
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SMPTE 2084: HDR Electro-Optical Transfer Function (EOTF)
0
1
0 1
Lu
min
an
ce
Perceptually Uniform Video Signal
Gamma
EOTF
Blending
On computers, we usually blend colors in non-linear space
Blended color= 0.5*(A+B)
This is fast and easy, but wrong because:
Brightness A is really Luminance A’=A2.2
Blended color ≠ 0.5*( A + B)
Correct answer:
Blended color = 0.5(A’+B’) -> (0.5*( A2.2 + B2.2)) 1/2.2
Synthetic Case: Blending red and green
Non-linear
Linear
WRONG
RIGHT!
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Scaling
The same math applies not just to blending, but spatial scaling operations as well
Scaling involves blending pixels together
Scaling in non-linear space will generate errors
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Advantages of HDR
HDR creates much more lifelike experiences
• “Like looking out a window”
• “Like you’re really there”
HDR done right is clearly distinguishable from existing content and displays
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HDR Usage Models
Movies
Gaming
Photography
VR
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Summary
HDR + WCG gives us powerful new tools to create visual experiences that are clearly visible to the average viewer
You can watch HDR+WCG movies right now
Making this all work requires lots of changes to the production pipeline and the products used to display it
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Call to Action
Make more HDR content and build more systems capable of playing HDR
Make sure you have a really compelling HDR experience before calling it HDR
Upcoming Intel products have cool HDR features…
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