HDR10 vs HLG: Comparing HDR Technologies

High Dynamic Range (HDR) has emerged as a transformative force, significantly enhancing the viewing experience by delivering greater contrast, more vivid colors, and increased brightness.

HDR is particularly important in modern displays, enabling content to be presented in a way that is much closer to how the human eye perceives the real world.

Two prominent HDR formats are HDR10 and Hybrid Log-Gamma (HLG). Both formats have unique features and are used in different scenarios, making it essential to understand their differences and applications.

What is HDR10?

HDR10 is one of the most widely adopted HDR formats, introduced in 2015 by the Consumer Electronics Association (CEA).

It is an open standard, meaning that content creators and hardware manufacturers can use it without licensing fees, which has contributed to its widespread adoption across various platforms and devices, from TVs and streaming devices to gaming consoles and Blu-ray players.

HDR10 was developed to improve upon the limitations of Standard Dynamic Range (SDR) content, offering a higher level of detail in both the brightest and darkest parts of an image.

Key Features and Specifications

Open Format: HDR10 is an open standard, which allows for broad adoption without the burden of licensing fees. This has made it the de facto HDR standard for many consumer devices, including televisions, monitors, and streaming platforms.
Static Metadata: HDR10 uses static metadata, meaning the brightness information is set once and remains constant throughout the entire video. This metadata provides information about the maximum and average brightness levels (MaxFALL and MaxCLL), which are then used to adjust the display’s settings.
10-bit Color Depth: HDR10 supports a 10-bit color depth, which translates to over 1 billion colors (compared to 16.7 million in 8-bit SDR). This significantly reduces color banding and allows for smoother gradients between colors.
Brightness Levels: HDR10 content is commonly mastered to brightness levels ranging from 1000 to 4000 nits, though most consumer displays are typically capable of reaching around 1000 nits. This range provides a significant improvement over SDR’s typical 100-nit brightness, offering much more detail in both highlights and shadows.

Advantages of HDR10

Wide Adoption: Due to its open format, HDR10 is widely supported across the industry. Most HDR-enabled displays and content, including Blu-rays and streaming services like Netflix and Amazon Prime Video, support HDR10.
Compatibility with Most HDR Displays: Nearly all HDR-capable displays are compatible with HDR10, making it a safe choice for content creators and consumers alike.

Limitations of HDR10

Static Metadata Limitations: The use of static metadata in HDR10 can lead to suboptimal image quality in scenes with varying brightness levels. For example, a scene with both very bright and very dark areas may not be displayed as effectively because the same settings are applied across the entire scene.
Potential Issues with Incorrect Mastering Data: If the content is not mastered correctly, HDR10 can suffer from issues like clipped highlights or crushed shadows, where details in the brightest or darkest parts of the image are lost.

What is HLG (Hybrid Log-Gamma)?

Hybrid Log-Gamma (HLG) is a royalty-free HDR format developed jointly by the BBC and NHK (Japan’s national broadcaster) in 2015.

Unlike HDR10, which is primarily intended for pre-recorded content, HLG was designed with live broadcasts in mind. It is unique in its ability to deliver HDR content that is also compatible with Standard Dynamic Range (SDR) displays, which has made it particularly popular in the broadcasting industry.

It achieves this through a Hybrid Log-Gamma curve that adjusts to the display’s capabilities, allowing the same signal to be displayed on both HDR (logarithmic curve) and SDR (gamma curve) TVs without requiring separate distributions.

In practice, SDR displays interpret the signal as usual, while HLG-compatible displays can correctly interpret the logarithmic portion of the signal curve to provide a wider dynamic range.

Key Features and Specifications

Developed by BBC and NHK: HLG was created as a solution for broadcasters to deliver HDR content without disrupting existing SDR infrastructure. It was designed to be backward-compatible with SDR displays while still offering an enhanced viewing experience on HDR-capable devices.
Backward Compatibility with SDR Displays: One of HLG’s key features is its backward compatibility. HLG-encoded content can be displayed on SDR displays without the need for additional metadata or complex tone mapping. This makes it an ideal choice for live broadcasts, where a wide range of display types may be used by viewers.
Hybrid Log-Gamma Transfer Function: HLG employs a transfer function that combines a logarithmic curve for the upper portion of the signal range with a traditional gamma curve for the lower portion. This hybrid approach allows HLG to effectively represent a wide range of brightness levels, ensuring compatibility with both HDR and SDR displays without the need for additional metadata. The logarithmic part efficiently handles high luminance levels, while the gamma portion manages lower luminance, resulting in a balanced and versatile HDR solution suitable for broadcasting.
10-bit Color Depth: Similar to HDR10, HLG supports a 10-bit color depth, offering over 1 billion colors. This allows for smoother color transitions and significantly reduces the occurrence of color banding, a common issue in lower bit-depth formats.

Advantages of HLG

Compatibility with Both HDR and SDR Displays: HLG’s most significant advantage is its ability to be displayed on both HDR and SDR screens without the need for separate streams or complex conversion processes. This makes it a versatile choice for broadcasters who need to reach a broad audience with diverse display capabilities.
Preferred by Broadcasters: Due to its backward compatibility and lack of reliance on metadata, HLG is favored by broadcasters, particularly for live events like sports and news. It simplifies the production process and ensures that all viewers, regardless of their display type, receive an optimal viewing experience.

Limitations of HLG

Limited to 10-bit Color Depth: While 10-bit color depth is a significant improvement over 8-bit SDR, it is still a limitation compared to some newer HDR formats that support higher bit depths, such as Dolby Vision, which offers 12-bit color depth. This could limit HLG’s ability to deliver the most nuanced color gradations.
Lagging Behind HDR10 in Display Integration: Despite its advantages in broadcasting, HLG has been slower to be integrated into consumer electronics compared to HDR10. While newer television models and streaming platforms are beginning to support HLG, it does not yet have the same level of industry-wide adoption as HDR10.

Comparison: HDR10 vs HLG

Feature	HDR10	HLG
Metadata	Static	Dynamic
Compatibility	HDR Displays Only	HDR and SDR Displays
Use Cases	Consumer Electronics (Blu-ray, Streaming)	Broadcasting, Live Events
Color Depth	10-bit	10-bit
Brightness	Typically mastered to 1000-4000 nits	Adapts to display capabilities, typically lower

Metadata: Static vs Dynamic

A key difference between HDR10 and HLG lies in how they manage brightness and contrast. HDR10 uses static metadata, meaning that the same brightness and color settings are applied throughout an entire piece of content.

This can be limiting in scenes with varying brightness levels, as it might not optimize the image quality for each scene.

In contrast, HLG does not use metadata at all, relying instead on its electro-optical transfer function (EOTF), which automatically adjusts the image based on the display’s capabilities. This gives HLG an edge in situations where content has a wide range of brightness and contrast levels, particularly in live broadcasts where scene-by-scene adjustment is impractical.

Compatibility: HDR-Only vs HDR and SDR

A major advantage of HLG is its backward compatibility with SDR displays (SDR vs HDR). HLG content can be viewed on SDR displays without requiring any additional processing or metadata, making it highly versatile for broadcasters who need to deliver content to a diverse audience with varying types of displays.

On the other hand, HDR10 is designed exclusively for HDR displays. While it provides a superior viewing experience on HDR-capable devices, it lacks the versatility of HLG when it comes to content delivery across different types of displays.

Use Cases: Consumer Electronics vs Broadcasting

HDR10 is the go-to format for consumer electronics, particularly for pre-recorded content like movies, TV shows, and video games. It’s supported by a wide range of devices including 4K TVs, Blu-ray players, and streaming services.

HLG, however, is primarily used in broadcasting, particularly for live events like sports and news. Its ability to deliver an HDR experience while maintaining compatibility with SDR makes it ideal for situations where content is delivered to a mixed audience.

Color Depth and Brightness Levels

Both HDR10 and HLG support a 10-bit color depth, which offers over 1 billion colors and significantly reduces color banding compared to the 8-bit color depth used in SDR.

However, HDR10 content is often mastered to higher brightness levels, typically ranging from 1000 to 4000 nits. This allows HDR10 to deliver more vibrant highlights and deeper shadows, provided the display supports these brightness levels.

HLG, while also capable of impressive brightness, generally operates within the limitations of broadcast standards, making it more suitable for live content but less optimal for high-brightness HDR content.

Adoption and Industry Support

HDR10 enjoys widespread industry support and is the most commonly used HDR format in consumer electronics. It is supported by virtually all HDR-capable devices and content platforms.

HLG, while widely adopted in broadcasting, has been slower to gain traction in consumer electronics. However, it has seen increasing support in recent years, particularly in Europe and Asia, where broadcasters are adopting HDR for live events.

Streaming platforms and newer television models are also beginning to support HLG, though it still lags behind HDR10 in overall adoption.

Future Trends and Developments

Display technology is constantly evolving, advancements such as HDR10+ and Dolby Vision, which utilize dynamic metadata, are gaining traction, offering even more precise control over brightness and color.

While HDR10 and HLG remain relevant forces in the current HDR ecosystem, these newer formats represent the ongoing evolution of display technology, promising even more immersive and realistic viewing experiences in the future.

The development of these formats highlights the continual push for improved image quality and a more nuanced representation of visual content. Furthermore, the increasing adoption of higher bit depths and wider color gamuts will further enhance the capabilities of HDR displays, ultimately bringing us closer to a truly lifelike viewing experience.