High dynamic range imaging (HDRI or HDR) is a set of methods used in imaging and photography, to allow a greater dynamic range between the lightest and darkest areas of an image than current standard digital imaging methods or photographic methods. HDR images can represent more accurately the range of intensity levels found in real scenes, from direct sunlight to faint starlight, and is often captured by way of a plurality of differently exposed pictures of the same subject matter.
In simpler terms, HDR is a range of methods to provide higher dynamic range from the imaging process. Non-HDR cameras take pictures at one exposure level with a limited contrast range. This results in the loss of detail in bright or dark areas of a picture, depending on whether the camera had a low or high exposure setting. HDR compensates for this loss of detail by taking multiple pictures at different exposure levels and intelligently stitching them together to produce a picture that is representative in both dark and bright areas.
HDR is also commonly used to refer to display of images derived from HDR imaging in a way that exaggerates contrast for artistic effect. The two main sources of HDR images are computer renderings and merging of multiple low-dynamic-range (LDR)or standard-dynamic-range (SDR) photographs. Tone mapping methods, which reduce overall contrast to facilitate display of HDR images on devices with lower dynamic range, can be applied to produce images with preserved or exaggerated local contrast for artistic effect.
Photography
In photography, dynamic range is measured in EV differences (known as stops) between the brightest and darkest parts of the image that show detail. An increase of one EV or one stop is a doubling of the amount of light.
| Device | Stops | Contrast |
|---|---|---|
| LCD display | 9.5 | 700:1 (250:1 – 1750:1) |
| DSLR camera (Canon EOS-1D Mark II) | 12 | 2048:1 |
| Negative film (Kodak Vision3) | 13 | 128:1 |
| Human eye | 10–14 | 1024:1 – 16384:1 |
High-dynamic-range photographs are generally achieved by capturing multiple standard photographs, often using exposure bracketing, and then merging them into an HDR image. Digital photographs are often encoded in a camera's raw image format, because 8 bit JPEGencoding doesn't offer enough values to allow fine transitions (and introduces undesirable effects due to the lossy compression).
Any camera that allows manual over- or under-exposure of a photo can be used to create HDR images. This includes film cameras, though the images may be digitized for processing with software HDR methods.
Some cameras have an auto exposure bracketing (AEB) feature with a far greater dynamic range than others, from the 3 EV of the Canon EOS 40D, to the 18 EV of the Canon EOS-1D Mark II.[9] As the popularity of this imaging method grows, several camera manufactures are now offering built-in HDR features. For example, the Pentax K-7 DSLR has an HDR mode which captures an HDR image and then outputs (only) a tone mapped JPEG file.[10] The Canon PowerShot G12, Canon PowerShot S95 and Canon PowerShot S100 offer similar features in a smaller format.[11] Even some smartphones now include HDR modes[12].
Editing
Of all imaging tasks, editing is the one that demands the highest dynamic range. Editing operations need high precision to avoid aliasing artifacts such as banding and jaggies. Photoshop users are familiar with the issues of low dynamic range today. With 8 bit channels, if you brighten an image, information is lost irretrievably: darkening the image after brightening does not restore the original appearance. Instead, all of the highlights appear flat and washed out. One must work in a carefully planned work-flow to avoid this problem.
Scanning film
In contrast to digital photographs, color negatives and slides consist of multiple film layers that respond to light differently. As a consequence, transparent originals (especially positive slides) feature a very high dynamic range.
| Material | Dynamic range (F stops) | Object contrast |
|---|---|---|
| photograph | 5 | 1:32 |
| color negative | 8 | 1:256 |
| positive slide | 12 | 1:4096 |
When digitizing photographic material with an image scanner, the scanner must be able to capture the whole dynamic range of the original, or details are lost. The manufacturer's declarations concerning the dynamic range of flatbed and film scanners are often slightly inaccurate and exaggerated.
Despite color negative having less dynamic range than slide, it actually captures considerably more dynamic range of the scene than does slide film. This dynamic range is simply compressed considerably.
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