RGBE

RGBE is a pixel encoding used for high-dynamic-range (HDR) imaging, short for Red, Green, Blue, Exponent. In this scheme, each pixel is stored as four bytes: one byte for each color channel (R, G, B) and one byte for a shared exponent (E). The four-byte value represents a color in the form (R/255, G/255, B/255) × 2^(E−128). If the pixel is black, all four bytes are zero. The exponent provides a common scale for the three color channels, allowing a wide dynamic range to be represented with only 8 bits per channel.

Encoding and decoding

To decode, read the four bytes. If E is zero, the color is (0, 0, 0). Otherwise, compute scale = 2^(E − 128) / 255, and set the color to (R × scale, G × scale, B × scale). Encoding converts floating-point RGB values into this compact form by choosing a suitable shared exponent so that the largest color component fits within 0–255, then storing the scaled mantissas in the R, G, B bytes and the exponent in E.

Origin and use

RGBE was popularized by the Radiance HDR format, developed for lighting and rendering workflows. It enables efficient storage of HDR images for environments, global illumination maps, and other scenes where luminance spans many orders of magnitude. The format is widely supported by older HDR tools and some modern processors, though many workflows now prefer alternative HDR formats with higher per-channel precision, such as OpenEXR.

Advantages and limitations

RGBE provides a compact four-byte-per-pixel representation with a broad dynamic range. However, the 8-bit mantissas introduce quantization and potential color banding, and the format is not ideal for precise color editing or workflows requiring full floating-point accuracy.