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What is High Dynamic Range Imaging?

High dynamic range imaging, or more commonly know as HDR, is a technique used in computer graphics, film making, photography and other aspects of digital imaging. This system allows technicians to manipulate images allowing a greater range of luminosity in each scene. Most imaging equipment has a handicap called a graduated ND filter that acts as an optical filter monitoring light transmissions. This handicap restricts the light sensitivity range, the opacity range, and the reflectance range. HDR improves the quality of the image by bringing together the varying light levels of separate images of the same subject, taken at distinct light intensities to create an open-ended range of absolute values of light in one image. The process depicts a more complete rendering, ranging from direct sunlight to the deepest shadows, rather than an image of brightness.

The History Behind HDR:

Before HDR was put into practice, photographers and film makers needed to find ways around the limitations of modern digital imaging which fail to capture the total perceptual display that the human eye is capable of seeing. The problem is that real-world scenes contain light ranges that exceed a 50,000:1 dynamic range while digital equipment is limited to around 300:1 dynamic range. Images tend to result in the subject seen and the surrounding blanked out.

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In this image the plant and the bench are the subject, and the window is the surrounding. Not much is seen outside the window, but the details on the back of the bench and the colors of the plant are seen pretty clearly.

In the past, photographers dealt with the limitation of dynamic range by experimenting with media. Ansel Adams was perhaps the first to systematically measure the sensitivity range of all of the equipment he used to precisely indicate what the photograph would display depending on the length of time the shutter was open. Also, through Adams' work it was discovered that one way to get really great dynamic range with color photography is to use black-and-white film and color filters. Although this was a practical solution, as technology advanced so did the digital imaging market, thus creating HDR.

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HDR Images VS Traditional Digital Images

Digital HDR allows professionals to store a great quantity of images, the time to return to them later, and still get the quality of the picture they desired. Images stored in HDR vary from the traditional term of "digital pictures&" by the quality of the image that is stored. HDR images usually possess clear values of luminance and radiance which are observed in the real world situations. Whereas, digital images must accommodate to the limitations of the monitor or the photo paper. One other important distinction is the color channel discrepancies. Traditional digital images use eight bits values per pixel channel. However, HDR uses 16 to 32 bits values. This jump in resolution allows for a much greater range of colors, greater than the monitor can display at once. As a result, to view these images, the computer breaks the images into levels of brightness and darkness. The brightest part of the image are viewed with the brightest colors the monitor can display the darkest are viewed with the darkest the screen can display. This allows for each image to display details, bright highlights and dark shadows.

Tone Mapping

After creating the HDR image, technician and users must be able to print or display the creations. To do this, people use a technique called tone mapping. With this, the wide intensity ranges in the image are converted to the lower range which are able to be displayed on the photo paper or LCD screen. This conversion of wide range to low range is similar to Ansel Adams' technique of "dodge and burn" using silver halide photography.

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Without Tone Mapping

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With Tone Mapping

The right image above demonstrates the tone mapping of the HDR photo of Stanford Memorial Church. Whereas the left image shows what you get if you display the image without tone mapping.

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This scene of the Grand Canyon from the North Rim is an example of exposures of 4 f stops

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Old Saint Pauls Cathedral, New Zealand, Dean S. Pemberton

This HDR image was shot in eight exposures ranging from 1/20th of a second to 30 seconds. In the image on can see the levels of the photograph. The shadows at the roof show the length of the cathedral, the patches of light at the altar give clue to the time of day. The image, although looking unnatural, does display an accurate representation of the cathedral.