HDR, Wide Gamut, PQ, HLG, and Modern Displays

HDR delivery is a chain of constraints: transfer function, mastering peak, display peak, average light level, gamut container, panel technology, metadata, ambient surround, bit depth, and tone mapping. This console lets you stress-test that chain before the article breaks each piece apart.

HDR - High Dynamic Range - is a deliberately broad term. In consumer marketing, it can mean any of "wider gamut," "brighter highlights," "deeper blacks," "more bits per channel," or "Dolby Vision logo on the box." In the engineering sense that this article uses, HDR specifically refers to delivery pipelines where:

1. Peak luminance reaches at least 1000 cd/m² (vs SDR's ~100 cd/m² target)
2. Black point reaches below 0.05 cd/m² (vs SDR's typical 0.1-0.3 cd/m²)
3. Bit depth is at least 10 bits per channel (8-bit cannot encode the wider range without visible banding)
4. A non-traditional transfer function maps file values to luminance - PQ (absolute) or HLG (relative)
5. The signal usually targets a wider color gamut than Rec.709 - typically Rec.2020 container with P3-D65 actual coverage

The "high" in High Dynamic Range refers to the ratio of peak to floor. SDR operates at roughly 100:1 to 1000:1. HDR operates at 10 000:1 to 100 000:1 - two to three orders of magnitude more.

"HDR" is really four independent upgrades to SDR that usually ship together. Understanding each separately makes the standards easier to navigate.

PQ - Perceptual Quantizer - is the dominant HDR transfer function. Standardized as SMPTE ST 2084 and integrated into ITU-R BT.2100, PQ maps file values 0-1 directly to absolute luminance values 0 to 10 000 cd/m². File value 0.5 represents a specific physical brightness (about 100 cd/m²); file value 0.75 represents about 1000 cd/m². The mapping never depends on what the display can do - the signal pins to a real-world brightness.

The curve is non-linear by design. It allocates more code values where the eye is sensitive (the dark to mid-tones range) and fewer in the brightest highlights, following the Barten contrast-sensitivity model. This is what lets 10 bits of PQ describe 0-10 000 cd/m² without visible banding - far more dynamic range than a 12-bit linear encoding could fit.

HLG - Hybrid Log-Gamma - is the broadcast HDR transfer function, jointly developed by the BBC and NHK and standardized in ITU-R BT.2100. Unlike PQ's absolute encoding, HLG uses a relative system: the lower half of the file range is a conventional gamma curve compatible with BT.709 SDR receivers; the upper half extends into logarithmic encoding to fit the additional HDR range.

The key design goal was backward compatibility. An HLG broadcast feeds an SDR receiver a usable picture without any decoder upgrade - the SDR set just sees the lower-gamma half of the signal. HDR receivers decode the full range. One signal serves both audiences.

Four named delivery formats dominate the HDR ecosystem. They differ in transfer function, metadata model, codec requirements, and licensing.

Modern HDR almost always ships in a Rec.2020 container - the widest broadcast color space, with primaries on the spectral locus. But few consumer displays cover the full Rec.2020 triangle. Most modern wide-gamut panels reach P3-D65 (the same primaries as Display P3) which is significantly wider than Rec.709 but well short of full Rec.2020.

The result is a common authoring pattern: master in P3-D65 inside a Rec.2020 container. The signal claims Rec.2020; the actual content rarely touches the extreme Rec.2020 corners; consumer displays render the P3-D65 portion accurately and tone-map anything that strays beyond their gamut.

The "modern" panels that earn the HDR label rely on different physics to reach their peaks. Each technology has its own trade-offs in contrast, peak brightness, gamut, and burn-in risk.

HDR signals carry metadata the display uses to tone-map sensibly to its own peak. Two layers exist: static metadata (one set of values for the whole asset) and dynamic metadata (per-scene or per-frame).

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A 4000-nit master arrives on a 800-nit display. Something has to give. Tone mapping is the function that compresses the input range into the output range while preserving as much perceptual fidelity as possible. It happens at the display, in the player, or both - and different choices produce visibly different pictures.

HDR authoring touches every stage. The mastering monitor must be calibrated and certified at the target peak. The grade has to allocate brightness budget across the scene rather than crank everything up. Highlights need physical-sense values (sun ≈ 4000 nits, light bulb ≈ 1500, paper white ≈ 200) so the result feels natural rather than radioactive.

Six questions on HDR fundamentals, PQ/HLG, and formats. Wrong answers come with brief explanations.