HDR a basic overview

HDR

High dynamic range (HDR) imaging is a set of techniques that allows a photographer to capture and display a greater range of exposure than can be captured in traditional film or digital imaging techniques. The consept of exposure range to a photographer represents the values between light and dark areas in the scene being photographed. Factors effecting the range which a photographer can capture is a function of the medium being used; the sesitivity of the film, paper, and digital sensors. HDR has been recognised for decades but its use was limited due to a lack of computer processing power. However with todays powerful computers HDR is now emerging into the general population.

HDR images contain digital information which corasponds to the physical luminance that can be obsevered in the real world. This is differant than traditional digital images which represent information as it should appear on a monitor or papaer. Thus the data stored in an HDR image is often called scene-referred and the data stored in traditional digital images is called device-referred.

The human eye can see a much greater range of lumanace than a camera can. For an image to come close to this range the photographer must make a minimum of 3 exposures of the scene to collect all the data neaded. A photo to record the bright areas one to record the dark areas and one the records inbetween the two. The more exposures made the higher the dynamic range.

in this example we see that 4 images were used with exposure values of -5, -2, +2, and +5.

These exposures are then blended together using computer softwere such as photoshop or photomatrix. However there is one problem, that is in viewing the image. CRT, LCD, prints, and other ways of viewing images have a limited dynamic range. There are however various methods that have been developed to convert the HDR image into a viewable format, the most common of which is called Tone Mapping.

Tone mapping is a technique which employs a computer to map one set of colors to another with the result of approximating the appearance of HDR images. When an HDR image is displayed on a non-HDR display it has a strong loss of contrast, tone mapping adds contrast to the image while retaining the image details and color appearance.


here we see the above images blened together and tone mapped.

Here are the other HRD images I have made.