5.4 Field of view, resolution, and refresh rates

AR/VR displays are all about immersion and performance. Field of view, resolution, and refresh rates are key factors that make or break the experience. These elements determine how realistic and smooth the virtual world appears to users.

Getting these display characteristics right is crucial for creating believable virtual environments. High refresh rates and frame rates reduce motion sickness, while good resolution and pixel density make images sharp and clear. It's a balancing act to optimize all these factors.

Display Characteristics

Field of View and Angular Resolution

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• Field of view (FOV) refers to the extent of the observable world that is seen at any given moment
  • Measured in degrees, with a larger FOV providing a more immersive experience
  • Human binocular FOV is approximately 114 degrees vertically and 190 degrees horizontally
  • AR/VR devices aim to match or exceed the human FOV for increased immersion (Oculus Rift has a 110-degree FOV)
• Angular resolution is the amount of detail that can be seen within the FOV
  • Determined by the number of pixels per degree (PPD) of the display
  • Higher angular resolution results in sharper and more detailed images
  • Insufficient angular resolution can lead to a pixelated appearance and reduced visual fidelity (PlayStation VR has a PPD of around 10)

Pixel Density and Screen-Door Effect

• Pixel density refers to the number of pixels per unit area on a display
  • Measured in pixels per inch (PPI) or pixels per centimeter (PPC)
  • Higher pixel density displays produce smoother and more detailed images
  • Low pixel density can result in visible individual pixels and reduced image quality (HTC Vive has a PPI of 447)
• Screen-door effect (SDE) is a visual artifact caused by the visible gaps between pixels on a display
  • More noticeable on displays with lower pixel densities
  • Can create a mesh-like appearance, similar to looking through a screen door
  • SDE can be mitigated by increasing pixel density or using advanced display technologies (OLED displays have reduced SDE compared to LCD)

Rendering Performance

Refresh Rate and Frame Rate

• Refresh rate is the number of times per second that a display updates its image
  • Measured in Hertz (Hz), with higher refresh rates resulting in smoother motion and reduced motion blur
  • AR/VR devices typically require high refresh rates (90Hz or higher) to minimize motion sickness and provide a comfortable user experience
  • Low refresh rates can cause flickering, judder, and increased motion-to-photon latency (Valve Index supports refresh rates up to 144Hz)
• Frame rate refers to the number of images (frames) that are rendered and displayed per second
  • Measured in frames per second (FPS), with higher frame rates resulting in smoother animation and motion
  • AR/VR applications should aim to maintain a consistent frame rate that matches the display's refresh rate to avoid judder and visual artifacts
  • Inconsistent or low frame rates can break immersion and cause discomfort (Oculus Rift S runs at 80FPS)

Foveated Rendering

• Foveated rendering is a technique that reduces the rendering workload by decreasing image quality in the peripheral vision
  • Exploits the fact that human visual acuity is highest in the central region of the retina (fovea) and decreases towards the periphery
  • Renders the center of the image at full resolution while progressively reducing the resolution towards the edges
  • Can significantly improve rendering performance and allow for higher resolutions and frame rates in the user's area of focus
  • Eye-tracking technology is often used to determine the user's gaze direction and adjust the foveated rendering accordingly (FOVE 0 headset incorporates eye-tracking for foveated rendering)