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

7

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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