HDR and HDRI: High Dynamic Range Images and Environment Maps Explained

What are HDR and HDRI

HDR (High Dynamic Range) refers to technology that can represent a wider luminance range than standard images. Traditional LDR images typically have 8 bits per channel with maximum brightness of 1 (or 255). HDR images use floating-point or higher bit depths, capturing details from darkest shadows to brightest highlights. HDRI (High Dynamic Range Image) is a high dynamic range image file, commonly used for environment maps and light probes.

Advantages of HDR

• **Detail preservation**: Retains details in both bright and dark areas, avoiding overexposure and underexposure.
• **Physically correct lighting**: In rendering, HDR lighting values can exceed 1, simulating real light source intensities.
• **Post-processing flexibility**: Tone mapping and exposure adjustments can operate on HDR buffers without color loss.

HDRI Environment Maps

The most common application of HDRI is as an environment map for image-based lighting (IBL). An environment map is a 360-degree panoramic image recording surrounding lighting information. In PBR rendering, HDRI provides ambient light and reflections, seamlessly integrating objects into the environment.

Generating HDRI

• **Photography**: Capture the same scene with a camera at different exposures, then composite into an HDR panorama using software.
• **Procedural generation**: Use software like Blender or Terragen to render sky and lighting.
• **Download**: Obtain free HDRIs from websites like Poly Haven, HDRI Haven.

Using HDRI

In Unity, import HDRI as a Cubemap or 2D texture and set it in the skybox material. In Unreal, load HDRI via a Sky Light. Engines automatically handle IBL, providing diffuse and specular lighting to the scene.

HDR in the Rendering Pipeline

Real-time rendering typically uses HDR framebuffers. Pipeline:

1. Render scene to HDR buffer (floating-point texture).
2. Apply post-processing (tone mapping, bloom) in HDR space.
3. Tone map to convert HDR to LDR.
4. Gamma correct and output to screen.

HDR-capable displays (HDR TVs, phone screens) can directly show HDR content with proper color management and metadata.

FAQ

What's the difference between HDR and HDRI?

HDR is a technical concept indicating high dynamic range; HDRI is a specific high dynamic range image file, often used for environment maps.

Why does rendering need HDR?

HDR allows lighting values above 1 for physically correct calculations, and post-processing doesn't lose details.

How to view HDRI files?

Use HDR-capable viewers like Photoshop, HDRView, or online preview tools. Normal monitors cannot display full dynamic range, so tone mapping is needed.

Does mobile support HDR rendering?

Modern mobile GPUs support floating-point textures for HDR rendering, but bandwidth and performance should be considered; compressed formats like R11G11B10 are often used.

Can HDRI be used in non-PBR rendering?

Yes, HDRI as an environment map provides lighting; any shader can sample it for reflections and diffuse.