RGBE image format


RGBE or Radiance HDR is an image format invented by Gregory Ward Larson for the Radiance rendering system. It stores pixels as one byte each for RGB values with a one byte shared exponent. Thus it stores four bytes per pixel.

Description

RGBE allows pixels to have the dynamic range and precision of floating point values in a relatively compact data structure - often when images are generated from light simulations, the range of per-pixel color intensity values are much greater than will nicely fit into the standard 0..255 range of standard 24-bit image formats. As a result, the bright pixels are either clipped to 255 or end up losing all their precision in dimmer pixels.
By using a shared exponent, the RGBE format gains some of the advantages of floating point values whilst using less than the 32 or 16 bits per color component that would be needed for single precision or half precision data in the IEEE floating-point format; and with a higher dynamic range than half precision. An exponent value of 128 maps integer colors into 0..1) floating point space.
A second variant of [the format uses the XYZ color model with a shared exponent. The mime type and file extension is identical, thus applications reading this file format need to interpret the embedded information on the color model.
Greg Ward provides code to handle RGBE files in his Radiance renderer.

Similar formats

Some GPUs support a '7e3' format, where a 32bit word encodes 3 10bit floating point color channels, each with 7bits of mantissa and 3 bits of exponent.
JPEG XT Part 2 and Part 7 Profile A are based on the RGBE format.
RGBM is a format with the exponent replaced with a shared multiplier, while RGBD stores a divider instead. These formats lack the dynamic range of RGBE and logLUV, but are more amenable to a naive approach of linear interpolation on each component. Like RGBE, they can be packaged in any format that accepts a four-channel color model, including ordinary formats like PNG for 3D textures.