EOTF Curve

EOTF Curve

The EOTF, or Electro-Optical Transfer Function, is one of the core concepts in display technology. It describes the mapping between digital signal input and screen brightness output. Specifically, the EOTF defines the rules that translate input signal levels into light intensity emitted by the display device, ensuring that the screen can accurately render brightness and contrast that match the creator’s intention. The EOTF curve is especially important in modern HDR displays, as it helps preserve fine details across varying brightness ranges.

Common EOTF standards include PQ (Perceptual Quantizer) and HLG (Hybrid Log-Gamma). These standards are designed to optimize visual effects based on human perception, but they differ in their approach to ambient light adaptation.

Ambient Light and Its Impact

Ambient light is the external light source surrounding the display device, such as indoor lighting or sunlight. Ambient light plays a significant role in visual perception because it directly influences the eye's contrast sensitivity, as well as the perceived brightness and color of the content on the screen. For instance, higher ambient light may cause the screen to appear darker, while low ambient light may make the screen appear brighter, even blinding.

EOTF curves are typically designed with a specific viewing environment in mind. For example, the PQ curve generally assumes a dark-room viewing setup. However, in real-world settings, viewing conditions are rarely standardized, which means the EOTF curve may not account for changing ambient light. In bright environments, displays need to increase brightness or adjust contrast to compensate for the light’s impact on visual perception. Conversely, in dark environments, the brightness needs to be lowered to avoid excessive brightness causing visual fatigue.

Ambient Light Adaptation in PQ, HLG, and Dolby Vision IQ

Currently, HDR display standards employ three primary EOTF curves: PQ, HLG, and Dolby Vision IQ. PQ's advantage lies in its ability to display highly accurate details in dark environments, though it struggles with adapting to ambient light. Thus, PQ performs best in controlled viewing conditions.

An additional feature of the PQ curve is peak clipping. Since display devices have limited brightness ranges, PQ compresses the dynamic range of the content to fit within the display’s maximum brightness. As a result, high-brightness details may be clipped in some instances, leading to detail loss in extremely bright scenes. This clipping design ensures the preservation of as much perceptible detail as possible in controlled environments but may limit performance in non-standard lighting conditions.

HDR Standards and Clipping: HDR400, HDR600, and HDR1000

In HDR content, clipping is especially significant, as HDR is designed to display very high brightness and deep shadow details. Display devices with different peak brightness levels—HDR400, HDR600, and HDR1000—can result in varying degrees of detail loss in bright scenes.

• HDR400: Suitable for mid-range displays with a peak brightness of 400 nits. While it offers better brightness and contrast than SDR (Standard Dynamic Range), its performance is limited in high-brightness scenes, where detail loss becomes apparent.
• HDR600: Provides higher peak brightness (600 nits) and better highlights in HDR content, though extreme brightness in some scenes may still lead to detail loss.
• HDR1000: A higher-end standard with a peak brightness of 1000 nits, suitable for detailed HDR content. However, even HDR1000 displays may experience clipping in extreme brightness situations, where the display hardware cannot meet the content’s requirements.

To achieve full HDR content display, the peak brightness of the display should ideally match or exceed the brightness limit set by content creators. Many creators target brightness levels of 4000 nits or higher, which current consumer devices cannot fully replicate. Thus, proper clipping lines must be used to ensure as much detail as possible is retained without compromising the visual effect.

Advanced EOTF Mapping Techniques

To overcome clipping issues, several methods can be employed to optimize HDR content display on devices with a peak brightness of 1000 nits:

1. Dynamic Range Compression and Local Mapping: This technique compresses the brightness beyond 1000 nits and maps it to the displayable range, reducing clipping loss and preserving details in both highlights and shadows.
2. Scene-Based Adaptive Brightness Adjustment: By adjusting brightness based on scene characteristics, displays can prevent clipping by lowering brightness peaks when necessary.
3. Dynamic Brightness Management: Displays can dynamically adjust the EOTF curve based on the content’s brightness distribution, ensuring high brightness details are preserved without sacrificing overall contrast.

HLG and Dolby Vision IQ for Ambient Light Adaptation

HLG (Hybrid Log-Gamma) is better suited for varying ambient light conditions, as it handles signals using a logarithmic and linear combination, providing better adaptability to light changes. HLG excels at preserving image details and dynamic range in high-brightness environments, providing a more consistent viewing experience across different settings.

Dolby Vision IQ integrates advanced HDR technology with ambient light sensors. This solution uses sensors to detect surrounding light intensity and dynamically adjust the EOTF curve and image parameters, ensuring optimal display performance regardless of the lighting environment.

EOTF Dynamic Adjustment in DarkVision Series

The DarkVision Series, equipped with GeekColor HDR color management, supports peak clipping management from HDR1000 to HDR4000, along with six optimized EOTF curves. This system enhances HDR display by ensuring better detail retention, especially in HDR4000 peak brightness scenarios, and guarantees accurate color reproduction and brightness even in SDR content.