The Science Behind HDRi Lighting - Understanding Dynamic Range

HDR vs LDR: Definition and Key Differences

Photorealistic rendering pipelines rely on handling a wide dynamic range: modern game engines and architectural rendering software support HDR to simulate very bright light sources (such as the sun) alongside deep shadow regions. High Dynamic Range (HDR) expands the tonal range of an image – the ratio between the brightest and darkest areas in the scene. Unlike traditional Low Dynamic Range (LDR) images, typically encoded with 8 bits per channel, HDR images store linear light values across a much broader spectrum, often using floating-point pixels at 16 or 32 bits per channel. This allows details to be preserved in both highlights and deep shadows, overcoming the limitations of conventional LDR formats.

Capturing and Representing HDR Data

To create an HDR image, multiple shots of the same subject are taken at different exposure settings, covering the full range of brightness present in the scene. These exposures are then merged into a single HDR image that retains detail in both the brightest and darkest areas. Each pixel stores floating-point values (for example, 16- or 32-bit per component), allowing extremely high precision. This enables encoding of light values from extremely low (shadows) to very high (direct sunlight) levels, resulting in a final image with a much broader range than standard LDR imagery.

Dynamic Range and Realistic Lighting

HDR is crucial in physically based rendering (PBR) engines to correctly simulate intense light sources. Using color buffers at 16 or 32 bits per channel allows representation of light values above [1.0], avoiding clipping in highlights. This ensures that PBR materials react naturally to bright sources like sunlight or strong artificial lights. Without HDR, such pixels would quickly max out, losing highlight detail. HDR preserves information in both light peaks and deep shadows, resulting in more believable global illumination.

HDRi and Image-Based Lighting (IBL)

Image-Based Lighting (IBL) uses panoramic HDR images as a source of ambient illumination. The HDRi is mapped onto a surrounding sphere or cube (sky dome or skybox), so each pixel contributes light and color from a specific direction. This allows 3D objects to receive realistic lighting and reflections based on real-world measurements, accurately reproducing environmental light and greatly improving realism in renders.

Impact on Global Illumination and Reflections

HDRi maps play a decisive role in both reflections and global illumination (GI). They provide the renderer with a full ambient light field around 3D objects, simulating indirect lighting from all directions. As a result, surfaces are lit not only by direct lights but also by subtle ambient contributions captured in the HDRi, enhancing shadow depth and tonal gradations. Specular reflections also benefit: reflective surfaces reproduce the HDRi environment with accurate colors and light spots, making PBR materials such as metals and glass look far more convincing.

File Formats, Bit Depth, and Workflow

HDRi creation and usage rely on specialized formats and a linear color workflow. Common formats include:

  • .hdr (Radiance HDR): A classic HDR format encoding brightness with 32-bit (RGBE) pixels. Widely supported and simple to use for environment maps.
  • .exr (OpenEXR): An open-source format common in VFX, supporting 16- or 32-bit floating-point channels, alpha, depth, and multiple layers. Offers efficient lossless compression.
  • Bit depth: A 32-bit HDR image can represent about 4.29 billion values per channel (versus 256 in an 8-bit LDR), but file size is significantly larger. The 16-bit half-float option reduces storage requirements with minimal precision loss in extreme highlights.

Rendering with HDR requires a linear color workflow. For example, in Unity, HDR rendering stays in linear space until tone mapping is applied in the final stage.

Optimization and Performance in Game Engines

Real-time engines like Unity and Unreal integrate HDR lighting to deliver high-quality visuals efficiently. In Unity, reflection probes (static or dynamic) simulate environment reflections from an HDRi, saving real-time computation by updating only when necessary. Unreal Engine uses the Sky Light component to feed an HDRi into global illumination, while Reflection Capture actors generate HDR cubemaps for accurate reflections. Performance can be improved by compressing HDR textures (e.g., BC6H) and limiting resolution to what’s visually necessary. Both engines require a linear-space workflow with final tone mapping to produce correct results.

Conclusion

Mastering high dynamic range is essential for realistic lighting in modern 3D graphics and games. HDRi maps transfer the full range of real-world brightness into 3D scenes, improving reflections, shadows, and overall detail. With proper formats, a linear workflow, and tone mapping, developers and CGI professionals can achieve realism far beyond what LDR techniques offer. While HDR requires extra care for performance, the visual payoff makes it an invaluable tool for photorealistic lighting in Unity, Unreal, and professional rendering software.