High dynamic range

HDR (high dynamic range) video can represent much brighter highlights and more shadow detail than SDR. It usually comes with a wider color gamut (BT.2020 primaries) and 10-bit or higher precision, but those are separate properties: a 10-bit BT.2020 file is not HDR unless it uses an HDR transfer function. What makes video HDR is the transfer function, the curve that maps code values to light. ITU-R BT.2100 defines the two in use, PQ and HLG.

PQ and HLG

PQ (Perceptual Quantizer, SMPTE ST 2084) is an absolute curve: each code value means a specific luminance, up to 10,000 cd/m². Content is graded on a mastering display (commonly 1,000 or 4,000 cd/m²), and a consumer display that cannot reach those levels has to tone-map the picture down. HDR10, HDR10+ and most Dolby Vision content use PQ.

HLG (Hybrid Log-Gamma, ARIB STD-B67, developed by the BBC and NHK) is relative, like SDR gamma: the display maps the signal to its own peak brightness. Its name describes the curve: a square-root (gamma-like) segment for darker values and a logarithmic segment for highlights. It needs no metadata and looks reasonable on SDR displays that understand BT.2020, which suits live broadcast.

HDR formats

Format	Transfer	Metadata
HDR10	PQ	Static: mastering-display color volume (SMPTE ST 2086) plus MaxCLL and MaxFALL, one set for the whole title.
HDR10+	PQ	HDR10’s static metadata plus dynamic per-scene or per-frame metadata (SMPTE ST 2094-40) for tone mapping.
Dolby Vision	PQ or HLG	Dynamic metadata (the RPU) plus a profile-specific base layer, sometimes with an enhancement layer.
HLG	HLG	None required.

Dynamic metadata tells the display how to tone-map each scene instead of using one compromise for the whole film. HDR10+ falls back to HDR10 on displays that do not support it. Whether a Dolby Vision file still plays as HDR10 or HLG elsewhere depends on its profile; the Dolby Vision section lists them.

Encoding HDR

An HDR encode has to keep the color tags (transfer, BT.2020 primaries and matrix), 10-bit or higher precision, the static metadata, and any dynamic metadata. If the tags are lost, players treat the video as SDR BT.709 and it looks washed out. Encoders do not create HDR10+ or Dolby Vision metadata from the picture; it is extracted from the source and injected into the new stream with hdr10plus_tool or dovi_tool.

Converting HDR to SDR needs real tone mapping, not just new tags. The preserve color and HDR guide has the encoding commands, and the HDR section covers identifying formats, metadata tools and tone mapping.