Hybrid Log Gamma

Description

Hybrid Log Gamma (HLG) is a high dynamic range (HDR) video transfer function standardized by the BBC and NHK and adopted into 3GPP specifications for media delivery. Unlike perceptual quantizer (PQ)-based HDR standards (like HDR10 or Dolby Vision) which are absolute and require metadata, HLG is a relative system designed for broadcast and live production. It defines a non-linear electro-optical transfer function (EOTF) that maps video signal values to display light. The 'hybrid' nature comes from its combination: it uses a gamma curve (similar to Standard Dynamic Range/SDR) for the lower part of the signal and a logarithmic curve for the higher luminance levels. This allows HLG signals to be displayed with an HDR effect on compatible displays while remaining viewable (without clipping) on legacy SDR displays, a feature known as backward compatibility.

In the 3GPP context, HLG is specified for use in multimedia broadcast and streaming services. It is supported in codecs like HEVC (H.265) and VVC (H.266) for efficient compression of HDR content. When a media server delivers an HLG-encoded video stream over a mobile network (e.g., via MBMS or unicast streaming), the client device interprets the HLG signal. On an HDR-capable display, the full dynamic range is reconstructed, providing brighter highlights, deeper blacks, and a wider color gamut. On an SDR display, the tone mapping is inherently simpler due to the gamma portion of the curve, resulting in a watchable, though not HDR, image without the need for complex metadata processing.

The technical specifications in 3GPP (e.g., TS 26.116 for media codec conformance, TS 26.265 for HEVC, TS 26.511 for 5G media streaming, TS 26.955 for MBMS) define how HLG is signaled, encapsulated, and processed within the media pipeline. This includes color space parameters (typically BT.2020), bit depth (often 10-bit), and the specific transfer function identifier. For broadcast services like LTE-based 5G Broadcast or FeMBMS, HLG provides a practical HDR solution because its relative nature and lack of mandatory metadata simplify live encoding and transmission, making it suitable for real-time events like sports.

Purpose & Motivation

HLG was developed to address the challenge of introducing High Dynamic Range video into broadcast and streaming ecosystems without disrupting the existing installed base of Standard Dynamic Range displays. Prior HDR approaches, primarily PQ-based, required static or dynamic metadata to describe the mastering display's characteristics for proper tone mapping. This added complexity for live production and posed a compatibility problem: an SDR display receiving a PQ signal would show a severely washed-out or dark image without sophisticated, device-specific tone mapping. HLG solves this by using a transfer function that is display-referred rather than scene-referred.

The motivation for its inclusion in 3GPP, starting in Release 16, was the industry push for enhanced multimedia experiences in 5G. 5G's high bandwidth and low latency enable high-quality video services, and HDR is a key component of that quality improvement. HLG's backward compatibility made it particularly attractive for mobile broadcast and multicast services (e.g., 5G Broadcast), where a single transmitted stream must serve a heterogeneous population of devices, from older smartphones with SDR screens to latest models with HDR capabilities. By adopting HLG, 3GPP allowed service providers to deliver a superior video experience to capable devices while maintaining service for all others, thus accelerating the adoption of HDR in mobile media without requiring a complete fleet turnover.