The resolution of video formats has increased from standard definition to high definition and most recently to 4K. However, these formats still use the same color space. Additionally, they have a standard dynamic range (SDR) of between 6 and 10 stops. As TV screens grow bigger and brighter, the limitations of older technologies, originally developed for CRT televisions, are more apparent. High dynamic range (HDR) video addresses these issues. But what is HDR video?
In video terms, dynamic range relates to contrast. It’s the difference between the lightest and darkest parts of an image. Dynamic range is measured in stops. A stop is a change that doubles the amount of light. The dynamic range of the human eye is around 20 stops. That converts to a contrast ratio of 1,000,000 to 1.
HDR video gives you whiter whites & blacker blacks
HDR video overcomes the shortcomings of the older video formats, providing images with a wider range of color and brightness. It has a dynamic range of at least 13 stops, much higher than SDR video. This means that HDR video can produce whiter whites, blacker blacks and can distinguish more shades of gray in between. In practice, this means that HDR preserves more details in the darkness of shadows and in the brightest areas of the image.
Because of the additional information that it contains, HDR video requires at least a 10-bit color depth compared to 8-bit for SDR video. For more information on bit-depth, read Understanding Bit-depth and Color Rendition for Video.
Wide color gamut
Although technically, HDR only refers to the luminance (brightness) of a video image, the main HDR video formats also incorporate a wide color gamut. A color gamut (or color space) represents all the colors that a TV or projector can display. So, the bigger the color gamut, the more colors the device can display.
The industry-standard color gamut for high definition broadcast TV, video streaming and Blu-ray is called Rec. 709. However, Rec. 709 only covers around 36 percent of the range of the CIE 1931 color chart, which represents all the colors that the human eye can see (the visible spectrum). HDR video conforms to the Rec. 2020 wide color gamut which is much larger, covering nearly 76% of the CIE 1931 color chart.
All of this extra color and brightness information means that the data rates of HDR video are too high for Blu Ray playback. That means you will need to use 4kKUltra HD Blu Ray discs to work with the new format. Online streaming services, such as Amazon Video and Netflix, also offer HDR video content.
HDR video standards
As is often the case with new developments in technology, there are currently a number of different standards for HDR video. The two main formats are Dolby Vision and HDR10.
Dolby Vision is a proprietary HDR video format developed by Dolby. This standard uses dynamic metadata to adjust levels of brightness from scene to scene, and even frame to frame. This means that Dolby Vision preserves even more detail in the shadows of darker scenes or the highlights of lighter scenes.
In addition, Dolby Vision uses metadata to fine tune the video based upon the equipment used to display it. Light levels and color levels are adjusted based on values determined by Dolby and the manufacturer of a TV or projector. This means the image is optimized based on the specific capabilities of the device on which it is shown.
Dolby Vision is a licensed format and compatible equipment has to be certified by Dolby.
Unlike Dolby Vision, HDR10 uses static metadata. This means that light and color levels are absolute values for a video and do not vary from scene to scene.
The HDR10 format defines a range of specifications for content and displays to comply with. However, it does not adjust the video playback for the individual capabilities of a particular screen.
One advantage of HDR10 over Dolby Vision is that it is an open standard. This means that any company can use it without the need to pay a license fee.
Two other formats of which to be aware are HDR10+ and Hybrid Log-Gamma (HLG).
A variant of HDR10, Samsung developed HDR10+ to include dynamic metadata. HDR10+ adjusts the levels of light displayed for each scene or frame of a video but, like HDR10, does not take account of the specific characteristics of the screen.
Hybrid Log-Gamma (HLG)
Hybrid Log-Gamma was developed jointly by the BBC from the UK and NHK from Japan. Designed for broadcast use, HLG is backward compatible with standard dynamic range (SDR) displays. HLG doesn’t contain metadata. Instead, it employs a combination of a gamma curve for the brightness of SDR content and a logarithmic curve for higher levels of brightness when using an HDR display.
Capturing HDR video
High dynamic range photography has been popular for many years. This technique seeks to maximize the dynamic range captured when photographing a highly contrasted scene. It works by combining two (or more) photographs, exposed separately for the highlights and for the shadows. The resulting image has a higher effective dynamic range than any of the individual photos.
It works by combining two (or more) photographs, exposed separately for the highlights and for the shadows.
The program Magic Lantern for the Canon range of DSLRs utilized a similar method for capturing HDR video, recording alternate frames at different ISOs. However, the effectiveness of the technique is limited. It required a considerable degree of post-production processing and any movement in the frame would lead to excessive motion blurring.
RED developed HDRx as a proprietary HDR video solution for their EPIC and SCARLET cameras. It works by recording two exposures within the interval that a standard camera would record only one frame. The primary exposure is normal, using the standard aperture and shutter settings. The secondary exposure uses a much shorter exposure for highlight protection. Again, the two image streams are combined in post-production to achieve a higher dynamic range.
The sensor in Arri’s Alexa camera has a proprietary Dual Gain Architecture which simultaneously generates two separate read-outs, with different levels of amplification, from each pixel. These two signals are combined into a single high dynamic range image. This enhances low light performance and prevents clipped highlights.
In practice, most modern cinema cameras can reproduce 14 or more stops of dynamic range, exceeding the dynamic range of film, which is around 13 stops. This level of dynamic range is perfectly adequate when shooting for HDR video. Check out our guide to the best cinema cameras on the market today for more information.
Working with HDR
As HDR video is a 10-bit format, you will need to use a 10-bit HDR display when recording or editing to be able to accurately monitor your output. Working with 10-bit files and 4K video means you will also need a powerful computer with a dedicated graphics card to ensure smooth playback when previewing your footage for editing or color grading. For more guidance see An Editor’s Guide to Working with HDR Video.
The UHD Alliance
When it comes to buying a new television to watch HDR video, the picture isn’t as clear as you might hope — often literally! Many TVs on the market come with the “HDR Ready” label. However, that just means the TV can receive and display an HDR video signal. It doesn’t guarantee that the screen is capable of showing the full range of luminance and color information of the HDR video content.
The UHD Alliance includes a range of manufacturers, content providers and film production companies with Panasonic, Samsung, Sony, Netflix, Dolby and Twentieth Century Fox among their ranks. The UHD Alliance has developed the UHD Premium specification, which sets out minimum standards for a television to comply with, covering resolution, color bit-depth, color gamut and dynamic range. If you are looking to buy a new TV, you should look for one bearing the UHD Premium logo to ensure that you can view your video content in the highest quality.