High dynamic range imaging

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Tone Mapped HDRI photograph of Old Saint Paul's church in New Zealand

The six individual exposures used to create the image above. Exposure times from top left are: 1/40sec, 1/10sec, 1/2sec, 1sec, 6sec, 25sec

In computer graphics and cinematography, high dynamic range imaging (HDRI) is a set of techniques that allow a far greater dynamic range of exposures than normal digital imaging techniques. The intention of HDRI is to accurately represent the wide range of intensity levels found in real scenes ranging from direct sunlight to the deepest shadows.

HDRI provides the opportunity to shoot a scene and have total control of the final imaging from the beginning to the end of the photography project. An example of this control is that it provides the possibility to re-expose. One can capture as wide a range of information as possible on-location and choose what is wanted later.

When preparing for display, a high dynamic range image is often tone mapped and combined with several full screen effects.

History

The use of high dynamic range imaging in computer graphics was pioneered by Paul Debevec. Debevec is thought to be the first person to create computer graphic images using HDRI maps to realistically light and animate computer graphics objects. Gregory Ward is widely considered to be the founder of the file format for high dynamic range imaging.

Comparison with Traditional Digital Images

Information stored in high dynamic range images usually corresponds to the physical values of luminance or radiance that can be observed in the real world. This is different from traditional digital images, which represent colors that should appear on a monitor or a paper print. Therefore, HDR image formats are often called "scene-referred", in contrast to traditional digital images, which are "device-referred" or "output-referred". Furthermore, traditional images are usually encoded for the human visual system (maximizing the visual information stored in the fixed number of bits), which is usually called "gamma encoding" or "gamma correction". The values stored for HDR images are linear, which means that they represent relative or absolute values of radiance or luminance (gamma 1.0).

HDR images require a higher number of bits per color channel than traditional images, both because of the linear encoding and because they need to represent values from [10^] to [10^8] (the range of visible luminance values) or more. 16-bit ("half precision") or 32-bit floating point numbers are often used to represent HDR pixels. However, when the appropriate transfer function is used, HDR pixels for some applications can be represented with as few as 10-12 bits for luminance and 8 bits for chrominance without introducing any visible quantization artifacts [link] [link].

Examples

References