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ABSTRACT

High dynamic range (HDR) video can offer consumers a much improved viewing experience compared to current broadcast video.

The dynamic range of current television images, referred to as standard dynamic range (SDR), is governed by cathode ray tube physics first documented about eighty years ago.

The standards include the Electro-Optical Transfer Function (EOTF) and the Opto-Electrical Transfer Function (OETF), as defined in Recommendations ITU-R BT.1886 and ITU-R BT.709, respectively.

Alternative transfer functions have been defined to support the transmission and rendering of HDR video signals.

These transfer functions aim to provide perceptually uniform mapping of video signals to the higher luminance range of future HDR displays while maintaining video signal bit depths used across current broadcast infrastructures.

In consequence, these new transfer functions exhibit much higher non-linearity compared to the transfer functions used in today’s SDR systems.

This could lead to several implications, such as an increase in bitrates required to transmit HDR services; changes to the existing broadcast infrastructure, including graphics equipment and vision mixers; and the compatibility of HDR services with existing SDR displays.

This paper studies the impact of such transfer functions on the efficiency of the video compression used for content exchange as well as delivery to the final user.

Results, in terms of compression efficiency and subjective picture quality, using single-layer High Efficiency Video Coding (HEVC, also known as H.265 and MPEG-H Part 2) video compression algorithms are presented.

This seeks to answer the question of what bitrates will be required to provide HDR services using existing video compression technology.

INTRODUCTION

4K Ultra High Definition (UHD) TV displays were introduced in 2012, with the promise of fundamentally changing television through having four times the spatial resolution of High Definition TV (HDTV), with 3840x2160 pixels.

Since perception of spatial resolution is strongly linked to screen size and viewing distance, research suggests increasing resolution alone will have limited consumer impact on today’s TV sizes viewed at today’s viewing distances. Therefore, other enhancements are increasingly under study to improve the viewing experience.

These include standardizing on progressively-scanned 50 and 60 frames per second (fps), or possibly higher frame rates, to improve motion representation; and a wider colour gamut, which allows the representation of colours to be closer to that of the human visual system (HVS).

Over the past year, however, one aspect has arguably stood out above all others as having the largest impact on advancing the viewing experience or TV realism and that is High Dynamic Range (HDR).

HIGH DYNAMIC RANGE: IMPACT ON THE TV VIEWING EXPERIENCE

The Human Visual System (HVS) has a very wide dynamic range, being able to discern luminance levels ranging from bright sunlight at 105 cd/m2 (candelas per square meter or “nits”) to starlight at 10-4 cd/m2.

It is highly complex, adaptive and not fully understood in terms of television viewing.

Unlike increasing resolution, which consumer research by CableLabs [1] and EBU [2] has shown to have limited viewer perception of an increase in picture quality at today’s screen sizes and viewing distances, increasing the dynamic range that a viewer can see is equally applicable to a wide range of screen sizes and viewing distances and appears to have strong consumer appeal.

For example, the benefit of HDR in high definition (HD) services is clearly evident.

The production standard for consumer video, however, has not been changed since the physics of cathode ray tubes (CRTs) were first documented in the 1930s, including setting the peak white level to 100 cd/m2.

Although modern video cameras can capture a very wide dynamic range and the very latest HD and UHD TVs claim maximum peak output in the range 400-1,200 cd/m2, TV production standards have not been updated as of yet.

Reduced dynamic range translates to the inability to see both lowlights (e.g., details in deep shadows) and highlights (e.g., clouds in a bright sunny day) simultaneously; one or the other will be “lost”.

The impact of reduced dynamic range particularly is noticeable for specular reflections, such as sunlight reflecting off of the surface of water or metal; with HDR, such light usually causes a physiological response in the viewer (“feeling” the light, including squinting of the eyes, for example).

Demonstrations of the benefit of HDR over the past two years have convinced standards development organizations to study how to specify this new dimension of immersion into the TV viewing experience.

Significant benefits have been shown for not only UHD but also HD resolutions, resulting in many believing that HDR is arguably the most important new development for TV.

For more detailed explanations of proper TV viewing distance, motion artefacts, the need for higher sample bit depths, Wide Colour Gamut (WCG), and HDR, refer to [3].