High Dynamic Range (HDR) television has captured the imagination of the broadcast and movie industries. This paper presents an overview of the BBC’s “Hybrid Log-Gamma” solution, designed to meet the requirements of high dynamic range television.
The signal is “display independent” and requires no complex “mastering metadata”. In addition to providing high quality high dynamic range (HDR) pictures it also delivers a high quality “compatible” image to legacy standard dynamic range (SDR) screens and can be mixed, re-sized and compressed using standard tools and equipment.
The technical requirements for a high quality HDR television system are presented. Quantisation effects (or “banding”) are analysed theoretically and confirmed experimentally. It is shown that quantisation effects are comparable or below competing HDR solutions.
The psychovisual effects of presenting images on emissive displays in dim environments, “system gamma”, is shown experimentally and the analysis informs the design of this HDR system.
With improvements in technology, television with greater impact, more “presence”, deeper “immersion”, a “wow factor”, or, in short, better pictures, is now possible.
Ultra high definition, UHD, is not just about more pixels, it has the potential to deliver wider colour gamut (WCG), higher frame rates (HFR), and higher dynamic range (HDR). Of these perhaps high dynamic range offers the greatest improvement, and the costs of upgrading to HDR can be relatively low for both production and distribution.
High dynamic range offers unmistakeably better pictures, across the living room, even on a smaller displays, and even to those with less than perfect vision. Potentially HDR may be produced using mostly installed, legacy, standard dynamic range (SDR) infrastructure, and distributed over largely unchanged distribution networks.
No wonder that, even before the standards have been finalised, some movie studios are already talking about creating movies in HDR, Ultra HD for home viewing.
This paper describes the signal processing technology required for high dynamic range in the television production and distribution chains. It describes how one solution, the “hybrid log-gamma” approach, provides a “display independent” signal that can produce high quality images, which maintain the director’s artistic intent on a wide range of displays in diverse viewing environments.
So, for example, precisely the same signal may be viewed in a controlled production suite, a home cinema, an ordinary living room, or on a laptop or mobile device. Furthermore the signal may be displayed on a conventional standard dynamic range display to provide a high quality “compatible” image.
The log-gamma HDR signal may be mixed, re-sized, compressed, and generally “produced”, using conventional tools and equipment. The only specifically high dynamic range equipment needed is cameras and displays for quality monitoring (signal monitoring may continue to use SDR displays). No complex mastering metadata is required.
Conventional end user distribution techniques may be used (although a 10 bit signal is required). No layered or multichannel codecs are required. Only a single signal is required for both SDR and HDR displays and expensive multiple “grades” (for both HDR and SDR) are not necessary.
The paper continues by discussing the meaning of high dynamic range.
To understand HDR production and display we need to look at the television signal chain, which is discussed next. This then allows us to consider the design of the camera transfer characteristic (the opto-electronic transfer function (OETF)).
Next we discuss an important psychovisual aspect of HDR TV, the “system gamma”. Whilst this effect has long been known in television, movies, and the academic literature, it assumes an enhanced importance for HDR. Based on an understanding of system gamma we discuss the design of the electro-optic transfer function (EOTF) in the display, and how this can allow the display of high quality pictures on a diverse range of displays.
Once the EOTF is defined we can analyse the likely effect of quantisation and the performance, in terms of dynamic range, that may be expected from the system. This is compared to alternative HDR proposals and the theoretical analysis is compared to experimental results. The paper ends with some concluding remarks.
Dynamic range is the ratio between the whitest whites and blackest blacks in an image. For example printed images have a dynamic range of less than 100:1 (because it is difficult to make a black ink that reflects less than 1% of incident light). Dynamic range is often measured in “stops”, which is the logarithm (base 2) of the ratio.