High dynamic range is a video and image technology that improves the way light intensity variations are represented. It overcomes the limits of standard dynamic range (SDR) video and image technologies. HDR offers the possibility to represent substantially brighter highlights, darker shadows, more details in both sides, and more colorful colors than what was previously possible.
HDR does not increase display's capabilities, rather it allows to make better use of displays that have high brightness, contrast, and color capabilities. Not all HDR displays have the same capabilities, and HDR contents will thus look different depending on the display used.
HDR emerged first for videos in 2014. HDR10, HDR10+, Dolby Vision, and HLG are common HDR formats. Some still picture formats do also support HDR.
Other technologies before HDR improved the image quality by increasing the pixel quantity (resolution and frame rate). HDR improves the pixel quality.
While CRT is not used anymore and modern displays are often much higher capacity, SDR format is still based and limited to CRT's characteristics. HDR overcomes those limits.
The maximum luminance level that can be represented is limited to around 100 nits for SDR while it is at least 1000 nits for HDR up to 10,000 nits (depends on the format used). HDR also allows for lower black levels and more saturated colors (i.e. more colorful colors). The most common SDR format is limited to Rec.709/sRGB gamut while common HDR formats use Rec. 2100 color primaries which is a Wide Color Gamut (WCG).
Those are the technical limits of HDR formats. HDR contents are often limited to a peak brightness of 1000 or 4000 nits and DCI-P3 colors, even if they are stored in a higher-capable format. Display's capabilities vary and no current display is able to reproduce all the maximal range of brightness and colors that can be stored in HDR formats.
HDR video involves capture, production, content/encoding, and display.
The increased maximum brightness capability can be used to increase the brightness of small areas without increasing the overall image's brightness, resulting in for example bright reflections off shin objects, bright stars in a dark night scene, bright and colorful fire or sunset, bright and colorful light-emissive objects. Content creators can choose the way they use the increased capabilities. They also can choose to restrain themselves to the limits of SDR even if the content is delivered in an HDR format.
For the creative intents to be preserved, video formats require contents to be displayed and viewed following the standards.
HDR formats that use dynamic metadata (such as Dolby Vision and HDR10+) can be viewed on any compatible display. The metadata allows for the image to be adjusted to the display accordingly to the creative intents. Other HDR formats (such as HDR10 and HLG) require the consumer display to have at least the same capability as the mastering display. The creative intents are not ensured to be preserved on lower capable displays.
For optimal quality, contents are required to be viewed in a relatively dark environment. Dolby Vision IQ and HDR10+ Adaptive adjust the content according to the ambient light.
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The HDR capture technique used for years in photography increase the dynamic range captured by cameras. The photos would then need to be tone mapped to SDR. Now they could be saved in HDR format (such as HDR10) that can be used to display the photos in a higher range of brightness.
Previous high dynamic range formats such as raw and logarithmic format were only intended to be used for storage. Before reaching the consumer, they would need to be converted to SDR (i.e. gamma curve). Now they can also be converted into an HDR format (such as HDR10) and are displayed with a higher range of brightness and color.
Since 2014, multiple HDR formats have emerged including HDR10, HDR10+, Dolby Vision and HLG. Some formats are royalty-free, others require a license and some are more capable than others.
Dolby Vision and HDR10+ include dynamic metadata while HDR10 and HLG don't. Those are used to improve image quality on limited displays that are not capable to reproduce an HDR video in the way it has been created and delivered. Dynamic metadata allows content creators to control and choose the way the image is adjusted. When low-capable displays are used and dynamic metadata are not available, the result will vary upon the display's choices and artistic intents might not be preserved.