File extension

.hdr

Radiance HDR (RGBE) picture format

.hdr files are Radiance RGBE images: a high dynamic range picture format from the Radiance lighting simulation system, also seen with the extensions .pic and .rgbe.

Known meanings

Formats and reference material for .hdr

Radiance HDR (RGBE) picture format

High dynamic range image format storing RGB with a shared exponent per pixel

  • 2d-graphics 3d-graphics
  • Greg Ward / Lawrence Berkeley National Laboratory
Open link

Details

Format notes

Radiance HDR

The Radiance picture format was created by Greg Ward for the Radiance lighting simulation and rendering system. It was one of the first practical formats for high dynamic range images. Files commonly use the extension .hdr, and the same format also appears as .pic and .rgbe. Other, unrelated formats use .hdr as well, for example ENVI image header files.

Technical Structure

A file begins with a text header, starting with the line #?RADIANCE (or #?RGBE), followed by variable lines such as FORMAT=32-bit_rle_rgbe and optional values like EXPOSURE=. A blank line ends the header, and a resolution line such as -Y 512 +X 1024 gives the image size and pixel order. Pixel data follows in binary.

Each pixel takes four bytes: 8-bit red, green, and blue mantissas and one shared 8-bit exponent (RGBE). This covers a very large range of brightness at a small size per pixel, with limited precision. Scanlines are usually compressed with a simple run-length encoding scheme. The format can also store pixels as XYZE.

Adoption

Radiance HDR is widely used for environment maps and light probes in rendering, game engines, and 3D tools, and many sites distribute HDRI files in this format. Libraries such as stb_image can load it, and ImageMagick and Blender support it. OpenEXR is a more flexible alternative for production work.

Preservation Notes

The shared exponent means values are less precise than 16-bit float formats, and the format has no standard metadata beyond header variables. Record the color primaries and exposure values from the header, since they are needed to interpret absolute levels. Parsers should check resolution lines and run-length data against the declared size.

Further Reading

  • Radiance project: https://www.radiance-online.org/
  • Radiance file formats (PDF): https://radsite.lbl.gov/radiance/refer/filefmts.pdf