High Dynamic Range (HDR Images)

High-dynamic-range imaging (HDRI) is a high-dynamic-range (HDR) technique used in photographic imaging and films, and in ray-traced computer-generated imaging, to reproduce a greater dynamic range of luminosity than what is possible with standard digital imaging or photographic techniques. Standard techniques allow differentiation only within a certain range of brightness. Outside this range, no features are visible because there is no differentiation in bright areas as everything appears just pure white, and there is no differentiation in darker areas as everything appears pure black.The ratio between the maximum and the minimum of the tonal value in an image is known as the dynamic range.

HDR images can record and represent a greater range of luminance levels than can be achieved using more traditional methods, such as many real-world scenes containing very bright, direct sunlight to extreme shade, or very faint nebulae. This is often achieved by capturing and then combining several different, narrower range, exposures of the same subject matter. Non-HDR cameras take photographs with a limited exposure range, referred to as low dynamic range (LDR), resulting in the loss of detail in highlights or shadows.

The two primary types of HDR images are computer renderings and images resulting from merging multiple low-dynamic-range (LDR) or standard-dynamic-range (SDR) photographs. HDR images can also be acquired using special image sensors, such as an oversampled binary image sensor.

Due to the limitations of printing and display contrast, the extended luminosity range of input HDR images has to be compressed to be made visible. The method of rendering an HDR image to a standard monitor or printing device is called tone mapping. This method reduces the overall contrast of an HDR image to facilitate display on devices or printouts with lower dynamic range, and can be applied to produce images with preserved local contrast (or exaggerated for artistic effect).