HDR Primer Part 3: Curves & Workflow

HDR Profiles - Pros & Cons. An obstacle to HDR adoption has been figuring out how legacy SDR TVs should display an HDR signal. There are about 1.6 Billion TV sets world-wide and 300 Million TV sets in the US.

One of the two common profiles for HDR is Hybrid Log Gamma (HLG). It’s more compatible with SDR TVs. It uses a traditional approach for TV brightness. The other HDR profile is PQ/Dolby Vision which was primarily developed for cinema. PQ’s curve can create a better HDR picture by using metadata to communicate scene parameters with HDR displays. As Standard Dynamic Range compatibility goes up, HDR picture improvement goes down. There’s no guarantee that a single approach to HDR will be agreed upon for all situations.

Figure 1. Display Brightness chart comparing OOTF Transfer Curves.

The Red curve represents today’s BT.790 EOTF (gamma curve) that limits the max display brightness to 100 nits.

The Blue curve is HLG and it’s more compatible with traditional TVs. Modern SDR TV displays can perform near-HDR, as shown by the shaded area. A high-quality LCD TV is capable of up to 450 nits brightness. Hybrid Log-Gamma uses a gamma curve that is like BT-709 up to 65% of the signal range.

The Green curve represents the Perceptual Quantization (PQ) curve. The difference is in the shape of the curve and how a traditional SDR display reacts to the PQ HDR curve.

Hitachi cameras provide HLG-600, HLG-1200 and PQ. HLG-1200 is the most compatible OETF because it produces a brighter and higher contrast picture on an SDR display. Below are comparison photos from our camera lab. 

HLG has better black and low-mid tones, improved contrast and detail with more saturated colors. The HD SDR display with HLG has slightly reduced contrast but is still acceptable.

The HDR and SDR pictures show significant differences when using the PQ curve. While PQ provides a higher level of HDR performance, the shape of the PQ curve produces an unacceptable picture when viewed in SDR. The PQ curve rises quickly, so contrast is low and because it reaches only about 76% of full white, the picture will appear washed out on an SDR display.

Workflow Issues

  • How do I start using High Dynamic Range?
  • What infrastructure is required for HDR?
  • What is the compatibility of HDR and SDR?
  • How do I handle Live HDR workflow camera shading?
  • Can I do simultaneous HDR and SDR?
  • What about broadcasting HDR?

Operational Practices For HDR

ITU R.BT.2408-0 recommended practices for HDR states: “The image system should provide a degree of compatibility with existing workflows and broadcasters’ legacy infrastructure, (ATSC 1.0 & 3.0), including graphics and video overlays. The HDR-TV image system should allow easy image and waveform monitoring throughout the broadcast chain allowing for different viewing environments while providing consistent image reproduction at each point.”

HDR To SDR Conversions: One of the most common and important issues is the conversion of HDR native acquisition to SDR for wide distribution. The converted SDR needs to look identical to a signal from a native SDR camera in overall image brightness, and the reproduction of tones and colors within a scene. SDR sources may also need to be converted for HDR productions.

Figure 2. The basic requirements for converting HDR to and from SDR.

Dual HDR And SDR Workflow

There are two basic ways to create HDR and SDR compatibility with dual-workflow operations.

A 3D-LUT (Look Up Table) converts HDR to SDR. In this case, 3D means RGB. A LUT is a fixed mathematical conversion. A better way is to create an SDR version from the HDR OETF in the camera chain. Hitachi camera CCUs apply an inverse gamma curve for the selected HDR profile. This process removes the HDR OETF curve. Due to the higher peak signal of HDR, a gain reduction of about 9 dB is applied to the SDR signal. Then the standard BT.709 gamma curve is applied. This produces an SDR picture just like it came from a native SDR camera.

Hybrid Log Gamma (HLG) was designed for live events. Camera video shaders should adjust the picture by viewing the converted SDR signal. If the SDR picture is shaded correctly, the HDR will be correct because the SDR range lies completely inside HDR. However, shading only for HDR could cause clipping in the SDR picture.

Television Broadcasting And HDR

The current ATSC Broadcast standard was established in 1996. That’s more than 10 years before the iPhone, iPad and Netflix. ATSC’s MPEG2 is 8 bit and cannot deliver 1080p or HDR pictures. Soon, ATSC 3.0 using new technology and H.265 HEVC compression will allow 4K, HDR, HFR, WCG and much more.

The ATSC 3.0 standard includes SL-HDR1 for compatible SDR and HDR broadcast delivery. It works by applying an Inverse OETF Function like the CCU function discussed earlier. It creates an SDR signal from the HDR input by separating the HDR information and encoding it in an area called the Supplemental Enhancement Information (SEI). An ATSC 3.0 TV with HDR capability will receive the SDR picture and add the HDR metadata. An SDR TV simply ignores the metadata and receives a normal SDR TV picture.

TV Manufacturers Support Of HDR Profiles.

As of this writing, all major TV manufacturers support HDR-10 (PQ). Most have announced support for HLG in new models. More than 80% of OTT is HDR-10 and HLG. Mobile devices receiving HDR are 100% HDR-10. Over time, we will eventually see most profiles in consumer TVs.

Figure 3. Currently, the consumer TV manufacturer support of HDR Profiles is widely varied.

UHD Alliance Standard For ‘Ultra HD Premium’.

Look for the “Ultra HD Premium” label on many 4K HDR TVs. This label says the TV must be 4K resolution and use the HDR-10 profile. Optionally, HLG, HDR10+, Dolby Vision or Advanced HDR can be included. The TV must be 10-bit color depth and display at least 90% of DCI-P3 color. DCI-P3 is a smaller color space than ST 2020. Today, no monitor or TV does 100% of ST 2020. LCD TVs peak brightness must be more than 1,000 nits & less than 0.05 nits black level. For OLED displays, more than 540 nits peak brightness & less than 0.0005 nits black level. This is about 20 stops. Any emissive display, like OLED and the discontinued Plasma have very impressive black levels that exceed what LCDs can do.

My hope is this 3-part series on High Dynamic Range has provided a base level of understanding needed to begin planning for HDR testing and implementation at your facility. Your productions will benefit from HDR’s wider exposure latitude, allowing more creativity. If you have questions about HDR please feel free to contact me.

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