High Dynamic Range Imaging: what is it?  
# Friday, January 02, 2009

The intention of High Dynamic Range Imaging (HDRI) is to accurately represent the wide range of light to dark intensity levels found in real life scenes that include bright highlights, dark shadows and every shade between light and dark.

Jean-Baptiste Gustave Le Gray (1820-1884) as been called "the most important French photographer of the nineteenth century because of his technical innovations in the still new medium of photography, his role as the teacher of other noted photographers, and the extraordinary imagination he brought to picture making"

His most important technical innovation:

Combination Printing, creating seascapes by using one negative for the water and one negative for the sky at a time when it was impossible to photograph both the sky and the sea in one image because of the wide luminosity range. He invented the photograph technique currently known as High Dynamic Range imaging or HDRI.


Today; one problem with HDRI is viewing the images. Typical computer monitors (CRTs, LCDs), prints, and other methods of displaying images only have a limited dynamic range. Thus various methods of converting HDR images into a viewable format have been developed, generally called "tone mapping".

Early methods of tone mapping were simple. They simply showed a "window" of the entire dynamic range, clipping to set minimum and maximum values. However, more recent methods have attempted to compress the dynamic range into one reproducible by the intended display device. The more complex methods tap into research on how the human eye and visual cortex perceive a scene, trying to show the whole dynamic range while retaining realistic colour and contrast.

Images with too much "HDR" processing have their range over-compressed, creating a surreal low-dynamic-range rendering of a high-dynamic-range scene.

Synthetic HDR images

Computer-created 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". 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: Bridging Traditional and Image-Based Graphics with Global Illumination and High Dynamic Range Photography". In this paper he used his previous technique to photograph a shiny chrome ball to produce what he called a "light probe", essentially an HDR environment map. This light probe could then be used in the rendering of a synthetic scene. Unlike a normal environment map that simply provides something to show in reflections or refractions, the light probe also provided the light for the scene. In fact, it was the only light source. This added an unprecedented level of realism, supplying real-world lighting data to the whole lighting model.

HDRI lighting plays a great part in movie making when computer 3D objects are to be integrated into real-life scenes.

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