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Radiance HDR

Radiance HDR is a classic high-dynamic-range raster image format commonly used for lighting environments, skyboxes, reflections, and tone-mapping test images in graphics pipelines.

Origin And History​

The format was defined by Greg Ward as part of the Radiance lighting simulation and rendering system. Radiance development began in the mid-1980s at Lawrence Berkeley National Laboratory, and the system required an image format that could preserve photometric values rather than quantizing everything down to conventional 24-bit display RGB.

That led to the RGBE representation used by Radiance picture files:

  • three 8-bit mantissas for red, green, and blue
  • one shared 8-bit exponent
  • compact storage compared to full 96-bit float RGB

The RGBE encoding was later documented in Graphics Gems II ("Real Pixels", 1991), and the format shipped as part of the freely available Radiance system. This is the main historical reason .hdr and .pic files became common in rendering, lighting, and image-based-lighting workflows.

Format Revisions​

Radiance's own documentation notes an important format change between Radiance release 1.4 and release 2.0:

Revision RangeEncoding StyleCompatibility Notes
Radiance 1.4 and earlierOlder flat or simpler RLE layoutOlder files remained readable after later format changes.
Radiance 2.0 and laterPer-component scanline RGBE RLEBecame the standard form produced by Radiance tools.

This is why modern loaders usually target the FORMAT=32-bit_rle_rgbe form and support the older flat/non-RLE fallback only for compatibility.

The most common .hdr files used in realtime rendering are Radiance RGBE images:

  • 3 color channels plus a shared exponent
  • High dynamic range values, including values above 1.0
  • Efficient storage for bright light sources such as suns, studio lights, and emissive skies

Why .hdr Is Used In Graphics​

Unlike PNG and JPEG, Radiance HDR preserves scene-referred light intensity instead of clamping everything into low-dynamic-range display values.

That matters for:

  • image-based lighting
  • physically based rendering
  • reflections
  • exposure and tone-mapping validation

File Structure​

A Radiance HDR file typically contains:

  • a magic header such as #?RADIANCE or #?RGBE
  • metadata lines including FORMAT=32-bit_rle_rgbe
  • an image size line such as -Y 256 +X 512
  • scanline pixel data, commonly encoded with Radiance RLE

The pixel payload is usually stored as RGBE:

  • R, G, B are mantissas
  • E is a shared exponent for the pixel

RadianceHDRLoader decodes that representation into linear floating-point RGBA data with alpha synthesized to 1.0.

The file is typically recognizable by:

  • a magic header such as #?RADIANCE
  • a format line such as FORMAT=32-bit_rle_rgbe
  • a resolution line such as -Y 256 +X 512

loaders.gl Support​

Use RadianceHDRLoader to load standard 2D Radiance RGBE .hdr textures.

Format Featureloaders.gl SupportNotes
Standard 2D Radiance .hdr images✅Returned as a Texture with shape: 'texture' and type: '2d'.
Radiance RGBE pixel decoding✅Decoded to linear floating-point RGBA data.
Standard Radiance scanline RLE✅Supports FORMAT=32-bit_rle_rgbe.
Older flat / non-RLE pixel data✅Supported as a compatibility fallback.
Float output✅Exposed as Float32Array.
Cube map helpers❌Applications must assemble cube textures separately.
Environment-map preprocessing❌No PMREM, convolution, or other lighting preprocessing utilities are included.
Half-float output❌Output is currently Float32Array only.
Non-Radiance HDR container formats❌This loader targets Radiance RGBE .hdr files, not EXR or other HDR image formats.

References​