High Dynamic Range

High Dynamic Range (HDR) is an imaging technique that provides a wider range of luminance and radiance of a scene. It captures a wide range of light, presenting a greater level of contrast between light and dark images on the screen. The HDR images generated are much more realistic and closer to what the human eye would normally see. Indeed, the human eyes are HDR and wide color gamut sensitive organs.

In virtual reality, High Dynamic Range is seen has an important development since it improves the images quality and, consequently, the user’s feeling of immersion. HDR has also been adopted by the video gaming industry.

High Dynamic Range Basics

Contrast is calculated by the difference between the brightest whites and darkest blacks of a display. This is measured in candelas per square meter (cd/m²) - a value called nits. Zero nits (completely black) is currently only possible on OLED displays, which completely turn off the pixels. Standard dynamic range (SDR) TVs produce 300 to 500 nits, while High Dynamic Range displays can reach thousands of nits.

The increased contrast provided by HDR gadgets gives an increased sense of sharpness, detail, clarity, color, and saturation (Figure 1) to a level that is not possible in current consumer displays or theaters. However, it is not only about parameters such as contrast, resolution, or color, but being able to combine these to get High Dynamic Range displays can improve picture detail so users have a more immersive experience. Bill Baggelaar, Senior Vice President of Technology for Colorworks and Post-Production Services at Sony Pictures, regarding the new technology said that “you can see things that you were never able to see before, things that were kind of hidden and you didn’t really notice. You also get a sense of immersion that I think goes beyond what we’ve been able to show with current display technology. So I think HDR has a really bright future.”

There are several formats for displaying High Dynamic Range, with the current main standards being the Dolby Vision format and the HDR10. While the first format, Dolby Vision, is proprietary, the second one is an open standard. For a while, Dolby Vision was essentially the single player in the HDR industry, meaning that they had a monopoly over it and that manufacturers who wanted to use the technology would have to pay fees to Dolby. Some manufacturers did not want to play by Dolby’s rules and started researching for alternatives. The companies realized that having a lot of different formats could lead to problems and so they agreed on an open standard - the HDR10.

HDR10 and Dolby

HDR10 is currently the most popular HDR format, although it is not as technologically advanced as the Dolby’s system. The HDR10 was codified by the Consumer Technology Association, and the spec presently uses 10-bit color depth. In addition to being an open standard (manufacturers can implement it free of charge), it is also more feasible for TV manufacturers to currently implement this format.

Technologically, Dolby Vision has an advantage over HDR10 and, in the future, it could become the most popular format with the gap between the two further increasing.

Dolby’s High Dynamic Range system supports 12-bit color depth and features higher theoretical brightness than the open standard. While HDR10 has a maximum of 1000 nits, Dolby Vision can reach 4000 nits, being able to transmit dynamic metadata that can vary by scene or frame. Finally, Dolby Vision allows the screen displaying a video to source data about the screen used to master the scene (i.e. the screen used during the editing phase at a studio). It automatically takes the differences between the two into account, resulting in an image that is automatically adapted to best fit the person’s display.

There are, however, other companies working on other High Dynamic Range solutions. Hybrid Log Gamma is a format developed by the BBC and NHK (a Japanese broadcaster), and it is suggested that the focus of the format may be live broadcasting, while it can still be used for pre-recorded content. This format does not use metadata.

Technicolor has partnered with Phillips to develop a new format compatible with SDR displays that the companies say “will simplify HDR deployments for distributors who will be able to send one signal to all of their customers, regardless of which TV they have.”

Virtual Reality High Dynamic Range

An essential feature of virtual reality is the sense of immersion. There are several factors that contribute to an increase of it, and perceptual realism of the displayed images is one of the most important.

One way to achieve perceptual realism is to get physical realism which is not an easy task since it raises problems in terms of real-time computation and physical modeling. According to Petit and Brémond (2010), “true photorealism implies very accurate 3D models. Next, one must collect the photometric and colorimetric inputs (light source description, spectral representation, material texture, BRDF, etc.) needed for the illumination computation. Then, the full simulation of the scene illumination (either with radiosity or ray tracing methods) in large virtual environments is far from real time. And finally, one cannot avoid the technical limits of current display gadgets, which are not designed to display physical High Dynamic Range (HDR) luminance values but Low Dynamic Range (LDR) 8-bit images.” However, the virtual reality field has seen major developments since the release of Petit and Brémond’s paper, including research on specific procedures to capture and broadcast High Dynamic Range imagery.

Intel is one of the companies investing in this area, collaborating with DVMobile to create a pipeline for capturing and broadcasting 360-degree HDR videos. The objective is to take virtual reality and immersion to the next stage of development.

Game consoles are also adopting High Dynamic Range with the PS4 Pro and Xbox One S. Sony has revealed an update of its PlayStation VR head-mounted display that offers HDR passthrough support.