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Tech Primer
High Dynamic Range Imaging
 High
dynamic range imaging (HDRI) is a set of techniques that allow greater dynamic
range of exposure than normal digital imaging. The aim of HDRI is to extend
the dynamic range of digital imaging technologies far beyond the traditional
method. HDRI is used to accurately represent wide range of intensity levels
found in real scenes from direct sunlight to the deepest shadows. ‘Dynamic
Range’ refers to the ratio between high and low extremes in a set of intensity
values. HDRI was initially developed for only computer-generated images. Later,
methods were developed to generate high dynamic range images from photos clicked
with a range of exposures.
History
Paul Debevec pioneered the use of high dynamic range imaging in computer graphics.
He was the first person to create computer graphic images using HDRI maps. The
most commonly used format for dynamic range imaging, Radiance RGBE, was introduced
in 1985 by Gregory Ward. Paul Debevec also demonstrated the way to take multiple
photographs in different exposures and the procedure to create a single high
dynamic range image. This technique is incorporated in software such as Photoshop
and Photogenics.
HDRI vs. traditional images
High Dynamic Range images correspond to the physical value of luminance or radiance
that can be observed in the real world. A traditional digital image represents
colors that should appear on paper or on a monitor. Traditional images are known
as device- or ‘output-referred, and HDR images format are called scene-referred.
Traditional images are linear in nature i.e. they represent relative or absolute
value of radiance or luminance. These images are encoded for the human visual
system called gamma encoding or gamma correction. This maximizes the visual
information stored in a fixed number of bits. In comparison to traditional images,
HRD images require a large number of bits per color because the need to represent
both linear encoding and values from 10-4 to 10-8 or more that are referred
as the range of visible luminance values.
Applications of High Dynamic Range
Some applications that require High Dynamic Range images as input are digital
cinema, Human vision simulation and psychophysics, Global illumination techniques,
Digital compositing for film, Reconnaissance and satellite imaging i.e. remote
sensing, mixed reality rendering
For more information visit: www.cybergrain.com/tech/hdr/
—Garima Grover
grover_garima@hotmail.com
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