What is HDR (Radiance)? RGBE High Dynamic Range Format Explained
An early high-dynamic-range image format using shared-exponent RGBE encoding for lighting and rendering.
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What is HDR?
The Radiance HDR format, also known as the RGBE image format, is a high-dynamic-range raster image format invented by Greg Ward around 1985 for the Radiance physically based rendering and lighting-simulation system developed at Lawrence Berkeley National Laboratory. It was one of the earliest HDR image formats.
Instead of storing full floating-point values, RGBE keeps one byte each for red, green, and blue plus a single shared 8-bit exponent, giving 32 bits per pixel. The shared exponent scales the three mantissas, allowing the format to represent a very wide dynamic range compactly, which made it a foundation for later HDR imaging and tone-mapping work.
How RGBE Encoding Works
The Radiance picture format encodes each pixel as four bytes: one mantissa byte each for red, green, and blue, plus a single 8-bit exponent shared across all three channels.[2] To recover a floating-point color, each mantissa is scaled by two raised to the shared exponent (offset by a bias), so the format captures a very wide luminance range in 32 bits per pixel.[1] The trade-off is that all three channels must share one exponent, limiting precision when the channels differ greatly in magnitude.[1]
File Structure
A Radiance HDR file begins with an ASCII header identified by a #?RADIANCE (or #?RGBE) signature, followed by variables describing the format and exposure, a resolution string, and then the scanline data, which is typically compressed with a simple run-length encoding.[2] Files commonly use the .hdr or .pic extension.[4]
History and Legacy
Greg Ward devised the format around 1985 for the Radiance lighting-simulation system at Lawrence Berkeley National Laboratory, making it one of the first practical high-dynamic-range image formats.[1] It remains widely used today for environment maps and image-based lighting and is cataloged in preservation registries such as PRONOM.[3] Its main successor, OpenEXR, replaced the shared exponent with full per-channel floating point.[1]
MKV Technical Specifications
HDR vs Other Image Formats
| Feature | HDR | EXR | TIFF |
|---|---|---|---|
| Type | Raster (HDR) | Raster (HDR) | Raster |
| Encoding | RGBE 32-bit[1] | Float 16/32-bit | Integer/float |
| Compression | RLE[2] | Multiple methods | Lossless or none |
| Origin | Radiance renderer[2] | ILM / OpenEXR | Aldus/Adobe |
| Multi-channel | No[1] | Yes | Limited |
| Best for | Environment lighting | VFX compositing | General imaging |
HDR (Radiance RGBE) is a compact way to store high-dynamic-range light data, while EXR provides richer floating-point and multi-channel options at larger sizes.
Advantages & Disadvantages
Advantages
The shared-exponent RGBE encoding represents a very large range of brightness values in just 32 bits per pixel.
As one of the first HDR formats, Radiance HDR is read by a broad range of rendering and image software.
The format applies simple run-length encoding without discarding the encoded pixel data.
It is well suited to storing illumination measurements and environment maps used in physically based rendering.
Disadvantages
A single shared exponent means the three color channels cannot each have independent floating-point precision, unlike formats such as OpenEXR.
The classic RGBE format does not carry transparency information.
It is used mainly in rendering and lighting workflows rather than for general-purpose images or the web.
Common Use Cases
Radiance HDR is used wherever physically meaningful light values must be stored.
Image-based lighting | FileFormer
HDR environment maps light 3D scenes realistically by capturing the full range of scene illumination.
Lighting simulation | FileFormer
The Radiance system uses HDR images to store and analyze physically based lighting results.
HDR photography source | FileFormer
Merged exposure-bracketed photographs are often stored as Radiance HDR before tone mapping.
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Try Image Converter FreeFrequently Asked Questions
What does RGBE mean?
RGBE stands for Red, Green, Blue, Exponent. Each pixel stores one byte per color channel and a shared 8-bit exponent, encoding high dynamic range in 32 bits per pixel.
Who created the Radiance HDR format?
It was created by Greg Ward as part of the Radiance lighting-simulation system at Lawrence Berkeley National Laboratory, around the mid-1980s.
What file extensions does it use?
Common extensions are .hdr and .pic, and the format is also associated with .rgbe and .xyze.
How is Radiance HDR different from OpenEXR?
Radiance HDR uses a compact shared-exponent RGBE encoding at 32 bits per pixel, while OpenEXR stores true per-channel floating-point data with multiple channels and more precision, at the cost of larger files.
Is Radiance HDR compressed?
Yes, it uses a simple run-length encoding scheme, which is lossless with respect to the encoded RGBE pixel values.