HDR Rendering Pipeline

What is HDR Rendering Pipeline

HDR (High Dynamic Range) rendering pipeline is a rendering workflow that uses floating-point textures (such as RGBA16F) to store scene lighting information. Unlike traditional LDR (Low Dynamic Range) rendering which uses 8-bit integers, HDR allows storing brightness values exceeding 1.0, thereby preserving details in bright areas and avoiding overexposure. Finally, tone mapping converts the HDR image to an LDR image suitable for display devices.

Why Use HDR Rendering

• **Preserve details**: In high-contrast scenes, HDR retains details in both bright and dark areas, avoiding pure white or pure black.
• **Physical correctness**: Paired with PBR materials and lighting, light intensity can exceed 1.0, making rendering more realistic.
• **Post-processing advantages**: Effects like Bloom require HDR information to correctly identify bright areas.
• **Avoid banding**: Floating-point formats provide higher precision, reducing banding artifacts in color gradients.

Key Components of HDR Rendering Pipeline

1. HDR Render Target

Use floating-point texture formats for color storage, such as RGBA16F (16-bit half-float per channel) or R11G11B10F (compact format suitable for color). These formats allow storing values greater than 1.0.

2. Linear Space Workflow

HDR rendering is often combined with linear space; lighting calculations are performed in linear space to ensure physical correctness. sRGB textures are converted to linear on sampling.

3. Tone Mapping

Tone mapping is the operator that maps HDR image to LDR display range. Common algorithms include:

• **Reinhard**: Simple and soft, but may reduce contrast.
• **ACES**: Cinematic tone mapping, providing more natural color and contrast.
• **Filmic**: Simulates film response.

4. Post-processing

HDR provides better input for effects like bloom and lens flares, as these effects need to identify bright areas.

Implementing HDR Rendering Pipeline

In game engines, you usually just enable the HDR option:

• **Unity**: Enable HDR in the render pipeline asset, choose HDR format and tone mapping algorithm. Linear space and HDR are default.
• **Unreal Engine**: HDR rendering is default, with built-in tone mappers.

For custom engines:

1. Create floating-point render targets.
2. Ensure lighting calculations in linear space with floating-point precision.
3. Apply tone mapping in post-processing stage.
4. Output to LDR back buffer or directly display if monitor supports HDR.

HDR and Display Devices

Traditional monitors are LDR, only able to display 0-1 range. Modern HDR monitors support higher brightness ranges (e.g., HDR10) and can directly display HDR content without tone mapping (or with lighter tone mapping). Engines need to detect display capabilities and adapt.

Performance Considerations

HDR rendering uses floating-point textures, increasing bandwidth and memory consumption. R11G11B10F is a balanced format for color render targets. On mobile platforms, HDR may introduce extra overhead; trade-offs are needed.

Summary

The HDR rendering pipeline is fundamental to modern real-time rendering. By using floating-point formats and tone mapping, it provides more realistic lighting and post-processing effects.