Reflection Probe

What is a Reflection Probe

A reflection probe is a component placed in a 3D scene that captures the surrounding environment information (typically stored as a cubemap) at that location, providing reflection and indirect lighting data for nearby dynamic or static objects. It is an essential tool for achieving global illumination and high-quality reflections, especially for metallic or glossy surfaces.

Reflection probes are widely used in game engines, such as Unity's Reflection Probe and Unreal Engine's Reflection Capture.

How Reflection Probes Work

A reflection probe renders or captures the scene view in six directions (up, down, left, right, forward, back) at its position, generating a cubemap. This cubemap is then used in shaders: when objects need reflections, they sample this cubemap to simulate environment reflections. For indirect diffuse lighting, a pre-convolved irradiance map can be used.

Reflection probes can be static (baked at edit time) or dynamic (updated at runtime). Dynamic probes render the scene in real time, which is more expensive and typically used for important areas or moving objects.

Types of Reflection Probes

• **Static Reflection Probes**: Baked at build time, do not change at runtime, low performance overhead. Suitable for fixed scenes.
• **Dynamic Reflection Probes**: Updated at runtime, can reflect moving objects and lighting changes, but high performance cost. Often update frequency and resolution are limited.
• **Real-time Reflection Probes**: Updated every frame for high-fidelity reflections, such as mirrors, but highest cost.

Placement and Influence Range of Reflection Probes

Reflection probes can be placed at key positions in the scene with defined influence ranges (e.g., spherical or box-shaped areas). Objects select the most relevant probe or blend data from multiple probes based on their position for smooth transitions.

Some engines support probe priority and blending weights.

Reflection Probes vs. Screen Space Reflections

Reflection probes provide static or low-frequency environment reflections but cannot capture off-screen details or precise reflections of dynamic objects. Screen Space Reflections (SSR) can supplement this, but limited to what is on screen. Both are often used together.

Performance Optimization

• **Reduce Resolution**: The cubemap resolution of reflection probes need not be very high; usually 128-256 is sufficient for most reflections.
• **Limit Update Frequency**: Dynamic probes can update a few times per second instead of every frame.
• **Use Time Slicing**: Render the six faces across frames to reduce per-frame cost.
• **Reasonably Layout**: Avoid too many probes; use blending to reduce count.
• **Use Pre-filtering**: Pre-generate mipmaps for different roughness levels to reduce runtime computation.

Conclusion

Reflection probes are practical tools for achieving environmental reflections and indirect lighting in real-time rendering. Proper use can significantly enhance scene realism.