Radiance HDR

Overview
Radiance HDR, also known as RGBE or Radiance Picture, is raster image file format associated with the Radiance (or RADIANCE) lighting simulation software by Greg Ward. It supports high dynamic range imaging, and may have been one of the first formats to do so.

Format
Each pixel is encoded as four bytes: three color values (RGB or XYZ), and a shared exponent.

There are at least two versions of the format. The documentation for the ra_rgbe utility says "The file format for RADIANCE pictures was changed between release 1.4 and release 2.0."

Identification
The only confirmed filename extension is .pic. Many sources claim that extensions .rad and .img are also used, though .rad seems unlikely because it is used for Radiance Scene Description format. Other possible filename extensions are .hdr, .rgbe, and .xyze.

Files begin with an ASCII signature of " " or, apparently, " ". Note that some of Radiance's other file formats also use the " " signature.

Specifications

 * http://radsite.lbl.gov/radiance/refer/filefmts.pdf ("Picture" file format)

Software

 * Radiance source code
 * ImageMagick (format named "HDR")

Links

 * Radiance web site
 * Wikipedia: Radiance