Perceptual Quantizer (PQ) and Hybrid Log-Gamma (HLG): How HDR Standards Encode Brightness and Color Volume

Consumer electronics marketing often pitches High Dynamic Range (HDR) as simply a brighter television—a standard video signal with the backlight turned up to blinding levels. But the underlying mathematics of the ITU-R BT.2100 standard contradict this entirely. "HDR-TV enables more natural images that contain wider variations in brightness," the International Telecommunication Union (ITU) notes in its BT.2390 report, clarifying that "the expectation is that indoor scenes produced in HDR will generally be at a similar brightness as with legacy TV systems." HDR is not a universally brighter signal; it is a complete rewrite of how digital video encodes light and color volume. At the heart of this rewrite are two competing electro-optical transfer functions (EOTFs): the Perceptual Quantizer (PQ) and Hybrid Log-Gamma (HLG).[5][6]

For decades, standard dynamic range (SDR) video relied on the BT.1886 gamma curve, a mathematical legacy of cathode-ray tube (CRT) televisions. This system was relative. A signal value of 100% simply meant "display the maximum brightness this specific screen can produce," whether that screen was a 100-nit reference monitor or a 300-nit consumer TV. But as display technology advanced, this relative mapping broke down. If a director wanted a specular highlight—like the sun reflecting off a car bumper—to be exactly 1,000 nits, the old gamma curve had no vocabulary to specify that absolute value.[2][4]

Enter the Perceptual Quantizer (PQ), standardized in 2014 as SMPTE ST 2084 and developed heavily by Dolby Laboratories. PQ fundamentally changes the relationship between the digital file and the screen. Instead of relative percentages, PQ encodes absolute luminance values. A specific digital code in a PQ signal tells the display to output exactly 1,000 nits, or exactly 4,000 nits. The scale extends all the way to a theoretical peak of 10,000 nits (cd/m²). If a consumer TV can only reach 700 nits, it must use internal tone-mapping to compress the highlights, but the creative intent encoded in the file remains absolute.[4][5]

The genius of PQ lies in its data efficiency, which is based on a model of human visual contrast sensitivity. Human eyes are highly sensitive to small changes in dark shadows but relatively insensitive to variations in blindingly bright highlights. If engineers had simply extended the old SDR gamma curve to cover 10,000 nits, it "would have required 15 bits" of data per color channel to avoid visible banding in the shadows, according to the standard's documentation. By allocating more data to the dark regions where the eye needs it, and less to the highlights, PQ achieves a banding-free 10,000-nit container using only 10 or 12 bits.[2][5]

However, PQ's absolute nature creates a massive logistical headache for live broadcasting. A live sports production cannot meticulously tone-map every camera feed for both HDR and legacy SDR televisions in real-time. This is where Hybrid Log-Gamma (HLG) enters the picture. Co-developed by the BBC and Japan's NHK, HLG was designed specifically to solve the live production puzzle.[1][3]

Unlike PQ, HLG does not use absolute luminance values. It is a scene-referred, relative system, much like traditional SDR. The "hybrid" in its name refers to its mathematical curve: the lower half of the signal (the shadows and midtones) uses a traditional gamma curve, while the upper half (the highlights) uses a logarithmic curve to compress the extended dynamic range. This clever math makes HLG inherently backward-compatible. If you feed an HLG signal into an older SDR television, the TV reads the gamma portion normally and simply ignores the logarithmic highlights, producing a perfectly watchable image without requiring a separate SDR broadcast feed.[1][3][4]

This divergence has split the HDR landscape into two distinct workflows. The film and high-end streaming industry—where content is meticulously color-graded in post-production—has overwhelmingly adopted PQ. It serves as the foundation for HDR10, HDR10+, and Dolby Vision. These formats often add dynamic metadata, giving the TV frame-by-frame instructions on how to tone-map the absolute PQ values to the screen's specific hardware limitations.[4][5]

Conversely, the broadcast industry has rallied around HLG. Because it requires no metadata and allows a single video feed to serve both HDR and SDR viewers, it is the pragmatic choice for live events. The BBC Research & Development team has extensively documented how HLG allows broadcasters to transition to HDR without doubling their transmission bandwidth or alienating viewers with older televisions.[1][3]

Both PQ and HLG are enshrined in the ITU-R BT.2100 standard, which was first published in July 2016 and also introduces the concept of "color volume." In the SDR era, color was often thought of as a two-dimensional triangle. But because HDR vastly expands the luminance range, color becomes a three-dimensional volume. A deeply saturated red can now be displayed at 1,000 nits without washing out to white—a physical impossibility under the old SDR standards.[2][5][6]

The choice between PQ and HLG is not about which is visually superior, but about the delivery mechanism. PQ offers absolute control for pre-recorded cinema, while HLG offers elegant pragmatism for live television. Together, they have dragged video encoding out of the CRT era and into a mathematically rigorous model of human perception.