RGBE image format
Encyclopedia
RGBE is an image format invented by Gregory Ward Larson. It stores pixels as one byte each for RGB
(red, green, and blue) values with a one byte shared exponent. Thus it stores four bytes per pixel.
RGBE's biggest advantage is that it allows pixels to have the extended range and precision of floating point
values. Often when images are generated from light simulations, the range of pixels values is 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 without the 12 bytes per pixel needed for single precision IEEE floating-point values, or 6 bytes in half precision (and which would cover smaller range). It can handle very bright pixels without loss of precision for darker ones.
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
rendering system.
RGB color model
The RGB color model is an additive color model in which red, green, and blue light is added together in various ways to reproduce a broad array of colors...
(red, green, and blue) values with a one byte shared exponent. Thus it stores four bytes per pixel.
RGBE's biggest advantage is that it allows pixels to have the extended range and precision of floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...
values. Often when images are generated from light simulations, the range of pixels values is 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 without the 12 bytes per pixel needed for single precision IEEE floating-point values, or 6 bytes in half precision (and which would cover smaller range). It can handle very bright pixels without loss of precision for darker ones.
A second variant of the format uses the XYZ color model
CIE 1931 color space
In the study of color perception, one of the first mathematically defined color spaces is the CIE 1931 XYZ color space, created by the International Commission on Illumination in 1931....
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
Radiance (software)
Radiance is a suite of tools for performing lighting simulation originally written by Greg Ward. It includes a renderer as well as many other tools for measuring the simulated light levels. It uses ray tracing to perform all lighting calculations, accelerated by the use of an octree data structure...
rendering system.
See also
- Radiance HDR image format
- High dynamic range imagingHigh dynamic range imagingIn image processing, computer graphics, and photography, high dynamic range imaging is a set of techniques that allows a greater dynamic range between the lightest and darkest areas of an image than current standard digital imaging techniques or photographic methods...