RGBE or Radiance HDR is an image format invented by Gregory Ward Larson for the
Radiance
In radiometry, radiance is the radiant flux emitted, reflected, transmitted or received by a given surface, per unit solid angle per unit projected area. Radiance is used to characterize diffuse emission and reflection of electromagnetic radiati ...
rendering system. It stores pixels as one byte each for
RGB
The RGB color model is an additive color model in which the red, green and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three addi ...
(red, green, and blue) values with a one byte shared exponent. Thus it stores four bytes per pixel.
Description
RGBE allows pixels to have the
dynamic range
Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base ...
and precision of
floating-point
In computing, floating-point arithmetic (FP) is arithmetic that represents real numbers approximately, using an integer with a fixed precision, called the significand, scaled by an integer exponent of a fixed base. For example, 12.345 can ...
values in a relatively compact data structure (32bits per pixel) - 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 (8-bit) 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
Single-precision floating-point format (sometimes called FP32 or float32) is a computer number format, usually occupying 32 bits in computer memory; it represents a wide dynamic range of numeric values by using a floating radix point.
A floatin ...
or
half-precision
In computing, half precision (sometimes called FP16) is a binary floating-point computer number format that occupies 16 bits (two bytes in modern computers) in computer memory. It is intended for storage of floating-point values in applications w ...
data in the
IEEE floating-point
The IEEE Standard for Floating-Point Arithmetic (IEEE 754) is a technical standard for floating-point arithmetic established in 1985 by the Institute of Electrical and Electronics Engineers (IEEE). The standard addressed many problems found in ...
format and with a higher dynamic range than half-precision. An exponent value of 128 maps integer colors
..255into
XYZ color model with a shared exponent. The mime type and file extension are 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
In radiometry, radiance is the radiant flux emitted, reflected, transmitted or received by a given surface, per unit solid angle per unit projected area. Radiance is used to characterize diffuse emission and reflection of electromagnetic radiati ...
renderer.
Similar formats
OpenGL mandates support for an analogous RGB9_E5 color (not render) format, where three channels have 9 bits of mantissa each and share 5 bits of exponent.
JPEG XT Part 2 (Dolby JPEG-HDR) 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 (appropriating the RGBA structure) for 3D textures.
A wider variety of color formats take the more conventional route of storing separate floating-point numbers. These include the Xbox '
7e3' format (3 10bit
floating point
In computing, floating-point arithmetic (FP) is arithmetic that represents real numbers approximately, using an integer with a fixed precision, called the significand, scaled by an integer exponent of a fixed base. For example, 12.345 can ...
color channels, each with 7bits of mantissa and 3 bits of exponent) and the OpenGL R11F_G11F_B10F format.
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See also
* Radiance software
* High dynamic range imaging
In photography and videography, multi-exposure HDR capture is a technique that creates extended or high dynamic range (HDR) images by taking and combining multiple exposures of the same subject matter at different exposure levels. Combining m ...
* FP16
References
External links
Radiance file formats
{{Graphics file formats
High dynamic range file formats
Graphics file formats