High Dynamic Range Imaging
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High dynamic range imaging
An example of a rendering of an HDRI image into an 8-bit JPEG. This image is of the Tower Bridge in Sacramento, California.In computer graphics and photography, high dynamic range imaging (HDRI) is a set of techniques that allow a far greater dynamic range of exposures (i.e. a large range of values between light and dark areas) 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 was originally developed for use with purely computer-generated images. Later, methods were developed to produce a HDR image from a set of photos taken with a range of exposures. With the rising popularity of digital cameras and easy to use desktop software, the term “HDR” is now popularly used[1] to refer to the process of tone mapping together with bracketed exposures of normal digital images, giving the end result a high, often exaggerated dynamic range; however, in this case neither the input nor the output qualify as “true” HDRI.
Recently, CMOS image sensor producers have begun to release sensors with HDR up to 110 db for security cameras.[2]
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HDR images were first produced with various renderers, notably Radiance. This allowed for more realistic renditions of modelled scenes because the units used were based on actual physical units e.g watts/steradian/m2. It made it possible for the lighting of a real scene to be simulated and the output to be used to make lighting choices (assuming the geometry, lighting, and materials were an accurate representation of the real scene).
At the 1997 SIGGRAPH, Paul Debevec presented his paper entitled “Recovering High Dynamic Range Radiance Maps from Photographs”[5]. It described photographing the same scene many times with a wide range of exposure settings and combining those separate exposures into one HDR image. This HDR image captured a higher dynamic range of the viewed scene, from the dark shadows all the way up to bright lights or reflected highlights.
A year later at SIGGRAPH 98, Debevec presented “Rendering Synthetic Objects into Real Scenes: