ABSTRACT

High Dynamic Range (HDR) video constitutes a new type of video with brighter brights and darker darks compared to conventional video.

Recent developments in display technology have made it possible to deliver a more immersive viewing experience through being able to reproduce HDR video.

This new video type has caused experts to investigate whether existing compression tools can operate efficiently or whether new tools need to be introduced.

In MPEG and VCEG the current state is somewhere in-between: existing tools work well with HDR but adjusting their settings to specifically optimize for HDR video, makes it possible to reduce the bit-rate and improve visual quality.

This paper will present background information around compression of HDR video and the work on HDR that has been, and still is being, performed in MPEG and VCEG.

INTRODUCTION

Digital video compression has transformed the way in which television has been consumed and delivered to consumers for more than 25 years.

Video coding standards such as MPEG-2 (Part 2) and AVC (MPEG-4 Part 10 / H.264) have played a fundamental role in enabling worldwide interoperability of compressed video.

The most recent video coding standard is the High Efficiency Video Coding (HEVC) standard (also known as MPEG-H Part 2 and H.265), which was published in its first version in 2013. The development of HEVC has been performed jointly by the Moving Picture Experts Group (MPEG), formally known as ISO/IEC JTC1/SC29/WG11, and the Video Coding Experts Group (VCEG), formally known as ITU-T Q.6/SG16 in the Joint Collaborative Team on Video Coding (JCT-VC). In recent years, several extensions have been added to HEVC including for example SHVC for scalable video coding and MV-HEVC for multi-view video coding.

Common to all of these video coding standards and extensions is that a vast amount of technical work is being conducted by a large number of experts in the field in order to firmly investigate the properties of the technology being included in the standards. Typically, the work of a new standard or extension is preceded by a Call for Evidence (CfE) and/or a Call for Proposals (CfP). These calls encourage submissions relevant to a specific area and further development work is commonly initiated with such inputs as a starting point.

In 2015, MPEG issued a CfE on High Dynamic Range (HDR) and Wide Colour Gamut (WCG) video following the several exploration activities on HDR that had been performed by MPEG starting in 2013. One of the key questions to determine was if technology could be identified which would motivate the creation of a new extension of HEVC specifically for HDR, but in early 2016 it was determined that no such technology had been identified.

However, during the course of the investigations, several findings and differences were identified between how to convert and compress HDR video compared to SDR (Standard Dynamic Range) video. These findings are being documented in a technical report that is expected to be completed in October 2016 and approved in January 2017 as a new part of ISO/IEC 23008 called MPEG-H Part 14. This paper will provide more details of this work and the methods described in MPEG-H Part 14.

BACKGROUND OF HIGH DYNAMIC RANGE VIDEO

Conventional video signals can be referred to as Standard Dynamic Range (SDR) video in order to emphasize the difference from High Dynamic Range (HDR) video. SDR video is created with the target of being displayed on screens ranging from roughly 0.01 cd/m2 to 100 cd/m2 corresponding to a 10000:1 relationship between the brightest and the darkest pixels. The reason for this limitation is very simple: up until quite recently consumer display technology had not been able to support a wider dynamic range.

However, recently the peak luminance level of consumer displays has been increasing at the same time as the black level has remained constant or even been reduced. There are already TVs in the market supporting peak luminance levels of up to 1000 cd/m2 and professional monitors reaches up to 10,000 cd/m2. These displays are based on LCD technology with LED backlight, a method that makes it possible to substantially increase the amount of light being emitted by the display while still preserving good black levels.

OLED displays have also been able to reach higher luminance levels (although not as high as LCD with LED backlight) and have the advantage of being able to reach black levels lower than 0.0005 cd/m2.

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