Color Management Resources

The Ins and Outs of HDR ― Gamma Curves

What is HDR? Gamma Curves Workflow Compatibility / FAQs

BT.2100 — a New International Standard for HDR

In July of 2016, the ITU-R BT.2100 international standard for HDR was established. As a result of this, the standards for content production and transmission have been determined, and it is expected that the adoption of HDR will be further accelerated.

As the table below shows, the five elements introduced on the previous page have clearly been gradually evolving since the establishment of the BT.709 standard for full HD. Though BT.2020 and BT.2100 are substantially very similar, they differ in the dynamic range that can be displayed.

International HDR Standards
  BT.709
Current Full HD Standard
BT.2020
4K/8K Standard
BT.2100
4K/8K HDR Standard
Resolution Full HD 4K, 8K HD, 4K, 8K
Bit Depth 8-bit 10 or 12 bits 10 or 12 bits
Frame Rate Up to 60p Up to 120p Up to 120p
Color Gamut BT.709 BT.2020 BT.2020
Brightness (Dynamic Range) SDR SDR HDR

 

Visible Difference (image for illustrative purposes)
SDR Image
SDR
HDR Image
HDR

Two Gamma Curves

In order to properly display HDR images, it's not enough to simply raise the level of brightness — it's crucial to display color and tones in a way that matches human eyesight. Color and tones are affected by an input-output characteristic called gamma that each input and output device has.

The BT.2100 standard provides two gamma curves as standards for different types of production work.

For Internet Streaming and Movies:
PQ (Perceptual quantization)

For Broadcast TV:
HLG (Hybrid Log-Gamma)

The PQ gamma curve is based on the characteristics of human visual perception, and is most suitable for the production of movies or streaming video content on the internet, where reproduction accuracy is key.

On the other hand, the HLG gamma curve is intended to allow for display on existing SDR TVs without looking out-of-place, and is most suitable for broadcast TV and live video feeds.

Advantages of the PQ and HLG Gamma Curves for HDR
  PQ (Perceptual Quantization) HLG (Hybrid Log-Gamma)
Target Internet video streaming, movies Broadcast TV, live video
Advantages New gamma curve based on human visual perception Compatible with SDR TVs
Brightness Value
  • Peak Brightness (max.): 10,000 cd/m2
  • Consistent, regardless of display device
  • Handles brightness in relative values
  • Varies by display device
Black Level 0.005 cd/m2 or lower 0.005 cd/m2 or lower
Proposed by Dolby BBC & NHK
Reference Standards SMPTE ST 2084、ITU-R BT.2100 ITU-R BT.2100
Reference Standards SMPTE ST 2084 & ITU-R BT.2100 Outstanding Good
Appearance on SDR TVs Poor Fair
Live Broadcasts Fair Outstanding

 

The graph below shows both the PQ and HLG gamma curves for HDR.

The peak brightness of the PQ gamma curve is defined as 10,000 cd/m2 (or higher). This means that the gamma curve is always the same up to the maximum brightness that can be reproduced by the display device. This allows consistent image reproduction with accurate gradation values up to the maximum brightness of the display device.

On the other hand, the HLG gamma curve's peak brightness matches to the maximum brightness of the display device. In other words, because the gamma curve varies depending on the maximum brightness of the display device, it allows for acceptable viewing of HDR content even on existing SDR displays, with less image degradation.

Comparison of PQ and HLG Gamma Curves for HDR
Comparison of PQ and HLG Gamma Curves for HDR

 

 

What is HDR? Gamma Curves Workflow Compatibility / FAQs

 

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