Overview of HDR (High Dynamic Range) Technology

Bright lights coming to a screen near you

High Dynamic Range (HDR) presentation technology is rapidly evolving in the TV industry. Viewing of HDR will be far more compelling than Stereo 3D, Ultra-High Definition (UHD) 4K high-resolution, or UHD wide color gamut (WCG). By Christmas, several vendors will have HDR-capable TV sets available, most of which will use local dimming technology, enabling peak luminance above 1000 candela per square meter (the SI unit for brightness), while still meeting government-mandated power restrictions. Netflix, Amazon, YouTube are preparing HDR content and HDR streaming. Dolby, Philips, Technicolor, Samsung are offering HDR-to-SDR color conversion technologies. There are many standards to choose from for HDR display transfer functions (such as SMPTE ST 2084), larger display color gamuts (DCI P3 or Rec. 2020), and better media compression (HEVC/H.265). However, unifying broadcast standards defining complete systems (such as Recommendation ITU-R BT.709 that defined our current HDTV system 20 years ago) are still missing. And a massive amount of industry education and re-tooling will be needed to deliver fully on the promise of HDR. Many cameras are HDR-ready. Thanks to advancements in sensor technology, cameras from ARRI, Blackmagic Design, Canon, Sony, and others now support up to 16 stops of dynamic range in a single frame capture, needing no tricks with multiple exposures. Although several standardized file formats such as TIFF or OpenEXR are HDR-capable, today this content is often captured in proprietary formats such as raw or log. Also missing are low-power, high-speed, high-quality intra-frame codecs. VC-5 (SMPTE ST 2073), of Cineform fame, might be a candidate.

But I need to edit my video on my laptop

Today’s computer operating systems do not support the display of HDR. HDR content is reduced to the legacy Standard Dynamic Range (SDR) range of current displays, if not diverted to a separate HDR-capable display. Web pages, text documents, and emails are displayed at the display’s peak brightness. This would be awkward or downright unpleasant to view on an HDR display. This is likely to change soon as power-efficient HDR display technologies become ubiquitous. Effective HDR content editing on your laptop needs an HDR display with HDR windows. And HDR windows are doable with the proper OS support. In such a device, the screen buffer would operate in HDR mode, with pixel values stored either as 16-bit floats or in an overranged integer encoding such as 12-bit ST 2084. Your legacy apps (such as email) would continue to use SDR windows, where the content, in classic 8-bit gamma encoding, is linearized and mapped to a brightness range from black to diffuse white (1.0). The HDR windows would use an HDR encoding, (such as ST 2084) which would reach beyond diffuse white for more sparkle.

Be nice to your SDR friends

Even though HDR will be very exciting to watch, for many years, most viewing will be on current SDR displays. (Note that most US households still keep a Standard Def TV (SDTV) set as a secondary display in the garage or the kids’ room, for example.) Just as with HDTV programs today, to reach a large audience, HDR programs will need to be down-converted to HDTV and SDTV. This conversion is far more challenging than HDTV to SDTV, which was mainly a resolution change. For best results, the colorist will do a separate color grade for SDR. This grade can be delivered as metadata with the HDR content to guide the conversion in the receiver. This is already supported in the latest specifications for HEVC and Blu-ray.