The RGBE trick
Ordinary images give each colour channel eight bits, which is fine for a screen but hopeless for representing real-world light, where a sunlit cloud can be thousands of times brighter than a shadow. HDR’s answer is RGBE: it stores red, green and blue mantissas plus a single shared exponent byte, so four bytes per pixel cover an enormous brightness range at modest cost. The shared exponent is the clever part — it stretches the dynamic range without the storage of full floating-point per channel. The result is a file that holds physically meaningful radiance values rather than gamma-encoded display colours.
This encoding is lossy in a gentle way: the shared exponent sacrifices some per-channel precision, particularly when the three channels differ wildly in magnitude. For lighting work that imprecision is rarely visible, which is why the format served the field for decades.
Lighting, IBL and bracketed photography
HDR images are the raw material of image-based lighting: a captured environment, often a mirrored-sphere or panoramic shot, illuminates a 3D scene with realistic reflections and highlights. Photographers also produce HDR by merging several bracketed exposures into one file that holds the full tonal range of a scene. To show such an image on a normal screen it must be tonemapped, compressing that vast range down into the eight bits a display can render.
Strengths
- Captures a huge brightness range in physically meaningful values.
- Compact RGBE encoding keeps files reasonable for the range they cover.
- A long-standing standard for image-based lighting and HDR photography.
Limitations
- RGBE’s shared exponent limits per-channel precision.
- No alpha channel.
- Must be tonemapped before it can be shown on an ordinary display.
When to use Radiance HDR
Use Radiance HDR for lighting work, image-based lighting environments and merged bracketed photography where you need to preserve real light values rather than screen colours.
Radiance HDR FAQ
- What does HDR store that a JPEG cannot?
- A Radiance HDR file holds real radiance values across a brightness range far wider than a screen can show, whereas a JPEG only stores eight-bit, display-ready colour. That extra range is what lets HDR drive realistic lighting and survive heavy exposure adjustments.
- What is the difference between HDR and EXR?
- Both store high-dynamic-range imagery, but HDR uses the compact RGBE scheme with limited precision and no alpha, while OpenEXR stores true floating-point channels, supports alpha and multiple channels, and is the heavier VFX standard. HDR is simpler; EXR is more capable.
- Can I convert an HDR file to a normal JPEG or PNG?
- Yes, but the wide range has to be tonemapped down to eight bits first. Morphr does this in the browser, compressing the light values into a standard JPEG, PNG, WebP or TIFF you can view anywhere.