HLG Hybrid Log-Gamma — broadcast-compatible HDR

What HLG is and how it differs from PQ

HLG is a transfer function jointly developed by the BBC and NHK for high dynamic range television. It received formal approval as ARIB STD-B67 from the Association of Radio Industries and Businesses, and it is one of the two HDR transfer functions defined in ITU-R BT.2100, the other being PQ.

The more interesting distinction is what kind of HDR system HLG is. PQ-based HDR — the family that includes HDR10 and Dolby Vision — is display-referred: the signal is encoded against the assumption of a known reference mastering display, and metadata describing that display travels with the content. HLG takes the opposite approach. It is scene-referred, meaning the rendering intent is applied at the display rather than at the camera.

That choice is what makes HLG "backward compatible" with SDR. The lower half of an HLG signal uses a conventional gamma curve very close to SDR, so a legacy display that knows nothing about HDR can be fed an HLG stream and still produce a watchable picture without any conversion step in the pipeline.

The transfer function: hybrid gamma + log

The name is literal. HLG defines a non-linear opto-electronic transfer function in which the lower half of signal values use a gamma curve and the upper half use a logarithmic curve. The gamma segment is what gives HLG its SDR-friendly tonality on legacy displays; the log segment is what gives it HDR headroom in the highlights.

Mechanically, the OETF runs at the camera, mapping the sensor's linear scene light into a video signal. The OOTF — the actual rendering of that signal into display light — is implemented solely in the display, not baked into the signal. By contrast, display-referred systems require the inverse EOTF to be applied at the camera, which means a reference final display has to be known up front. That works for movie production; it works less well for live broadcast.

Because the rendering happens at the display, HLG can adapt to the device it runs on. The system gamma is adjusted depending on the actual peak luminance and viewing environment, with a nominal value at a 1,000 cd/m² peak. One signal can be deployed across a wide range of display luminances and viewing environments by adjusting the OOTF gamma inside the display itself, with no metadata required to make that work.

Why broadcast: BBC, NHK, ATSC 3.0, YouTube

HLG was designed for live broadcast workflows, and you can read its design constraints back from where it gets used. Live television cannot insert frame-accurate metadata, and broadcasters cannot replace their existing tooling — mixers, switchers, compressors, captioning chains — for a new color pipeline. HLG signals can be mixed, resized, and compressed using conventional tools. Scene-referred is best suited for real-time television; display-referred works fine for non-real-time, file-based workflows.

Adoption follows that split. Among TV broadcasters, the vast majority have adopted HLG. BBC iPlayer launched 4K HDR with Planet Earth II using HLG. NHK and SKY Perfect JSAT use HLG for Japanese satellite HDR services. DirecTV uses HLG for live 4K HDR sports. ATSC 3.0, the next-generation US broadcast standard, specifies HLG.

The streaming side is more mixed. HLG appears on platforms that handle linear/broadcast HDR feeds — YouTube delivers HDR in either HLG or PQ, for instance — while a great deal of on-demand streaming sits on PQ-based HDR formats instead. The pattern is consistent with HLG's design brief: it is the format that shows up wherever a live broadcast pipeline is involved.

Metadata-free operation and display behavior

HLG carries no metadata at all — no static MaxCLL/MaxFALL of the HDR10 kind, no dynamic per-scene metadata of the kind used by other HDR formats. Every HLG-aware display interprets the signal using only the standardized transfer function plus its own knowledge of its peak luminance.

That omission is not a shortcut; it is the consequence of being scene-referred. A scene-referred signal does not need to be told about the mastering display because there isn't one — the display already knows its own peak luminance and viewing environment, and it picks the correct system gamma locally. Metadata-based HDR formats need that information precisely because they assume a known reference mastering display.

The rest of the envelope sits inside the broader BT.2100 specification. HLG is specified as a 10-bit signal in the BT.2020 wide color gamut. On a 2,000 cd/m² 10-bit display this gives roughly 17.6 stops of dynamic range without visible banding, against about 6 stops for 8-bit SDR.