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In photography and videography , multi-exposure HDR capture is a technique allowing to capture high dynamic range HDR images by taking and then combining several different exposures of the same subject matter. Images captured by cameras allow differentiation only within a certain range of luminosity. Outside this range, no features are visible because everything appears pure white in the brighter areas and pure black in the darker areas.
The ratio between the maximum and the minimum of the tonal value in an image is known as the dynamic range. Combining several different, narrower range, exposures results in an image with a greater dynamic range than what is possible by taking one single exposure.
HDR is useful for recording many real-world scenes containing very bright, direct sunlight to extreme shade, or very faint nebulae. Many smartphones have a built-in HDR feature performing the process in a automated way for photo capture.
The technique can also be used to capture video by taking and combining multiple exposures for each frame of the video. Due to the limitations of printing and display contrast , the extended luminosity range of input HDR images has to be compressed to be made visible. The method of rendering a high dynamic range image to a standard monitor or printing device is called tone mapping. This method reduces the overall contrast of an HDR image to facilitate display on devices or printouts with lower dynamic range.
It can be applied to produce images with preserved local contrast or exaggerated local contrast for artistic effect. In photography, dynamic range is measured in exposure value EV differences, known as stops. One aim of HDR is to present a similar range of luminance to that experienced through the human visual system. The human eye, through non-linear response, adaptation of the iris , and other methods, adjusts constantly to a broad range of luminance present in the environment.
The brain continuously interprets this information so that a viewer can see in a wide range of light conditions. Most cameras cannot provide this range of exposure values within a single exposure, due to their low dynamic range. Standard photographic and image techniques allow differentiation only within a certain range of brightness. Outside of this range, no features are visible because there is no differentiation in bright areas as everything appears just pure white, and there is no differentiation in darker areas as everything appears pure black.
Non-HDR cameras take photographs with a limited exposure range, referred to as low dynamic range LDR , resulting in the loss of detail in highlights or shadows. Multi-exposure HDR is used in photography and also in extreme dynamic range applications like welding or automotive work. In security cameras the term used instead of HDR is “wide dynamic range”.
Modern CMOS image sensors can often capture a high dynamic range from a single exposure  reducing the need to perform multi-exposure HDR. Color film negatives and slides consist of multiple film layers that respond to light differently. Original film especially negatives versus transparencies or slides feature a very high dynamic range in the order of 8 for negatives and 4 to 4. High-dynamic-range photographs are generally achieved by capturing multiple standard-exposure images, often using exposure bracketing , and then later merging them into a single HDR image, usually within a photo manipulation program.
Any camera that allows manual exposure control can perform multi-exposure HDR image capture, although one equipped with auto exposure bracketing AEB is far better suited. Images from film cameras are less suitable as they often must first be digitized, so that they can later be processed using software HDR methods. An increase of one EV, or one stop, represents a doubling of the amount of light. Conversely, a decrease of one EV represents a halving of the amount of light.
Therefore, revealing detail in the darkest of shadows requires high exposures , while preserving detail in very bright situations requires very low exposures. Exposure variation in an HDR set is only done by altering the exposure time and not the aperture size; this is because altering the aperture size also affects the depth of field and so the resultant multiple images would be quite different, preventing their final combination into a single HDR image. An important limitation for multi-exposure HDR photography is that any movement between successive images will impede or prevent success in combining them afterward.
Also, as one must create several images often three or five and sometimes more to obtain the desired luminance range, such a full set of images takes extra time. Photographers have developed calculation methods and techniques to partially overcome these problems, but the use of a sturdy tripod is, at least, advised. Some cameras have an auto-exposure bracketing AEB feature with a far greater dynamic range than others, from 0. Information stored in high-dynamic-range images typically corresponds to the physical values of luminance or radiance that can be observed in the real world.
This is different from traditional digital images , which represent colors as they should appear on a monitor or a paper print. Therefore, HDR image formats are often called scene-referred , in contrast to traditional digital images, which are device-referred or output-referred.
Furthermore, traditional images are usually encoded for the human visual system maximizing the visual information stored in the fixed number of bits , which is usually called gamma encoding or gamma correction. The values stored for HDR images are often gamma compressed power law or logarithmically encoded, or floating-point linear values, since fixed-point linear encodings are increasingly inefficient over higher dynamic ranges.
HDR images often don’t use fixed ranges per color channel —other than traditional images—to represent many more colors over a much wider dynamic range multiple channels. For that purpose, they do not use integer values to represent the single color channels e. Common are bit half precision or bit floating-point numbers to represent HDR pixels. However, when the appropriate transfer function is used, HDR pixels for some applications can be represented with a color depth that has as few as 10—12 bits for luminance and 8 bits for chrominance without introducing any visible quantization artifacts.
Tone mapping reduces the dynamic range, or contrast ratio, of an entire image while retaining localized contrast. Although it is a distinct operation, tone mapping is often applied to HDR files by the same software package.
Tone mapping is often need because the dynamic range of the electronic representation that display can receive is often lower than the dynamic range of the captured image. Notable titles include:. As the popularity of this imaging method grows, several camera manufacturers are now offering built-in multi-exposure HDR features. Some smartphones provide HDR modes, and most mobile platforms have apps that provide multi-exposure HDR picture taking.
Some of the sensors on modern phones and cameras may even combine the two images on-chip so that a wider dynamic range without in-pixel compression is directly available to the user for display or processing.
This is an example of four standard dynamic range images that are combined to produce three resulting tone mapped images:. A fast-moving subject or unsteady camera will result in a “ghost” effect or a staggered-blur strobe effect, as a result of the merged images not being identical, but each capturing the moving subject at a different moment in time, with its position changed.
Sudden changes in the lighting conditions strobed LED light can also interfere with the desired results, by producing one or more HDR layers that do have the luminosity expected by an automated HDR system, though one might still be able to produce a reasonable HDR image manually in software by rearranging the image layers to merge in order of their actual luminosity. Camera characteristics such as gamma curves , sensor resolution, noise, photometric calibration and color calibration affect resulting high-dynamic-range images.
Although not as established as for still photography capture, it is also possible to capture and combine multiple images for each frame of a video in order to increase the dynamic range captured by the camera. Some cameras designed for use in security applications can automatically provide two or more images for each frame, with changing exposure.
The idea of using several exposures to adequately reproduce a too-extreme range of luminance was pioneered as early as the s by Gustave Le Gray to render seascapes showing both the sky and the sea.
Such rendering was impossible at the time using standard methods, as the luminosity range was too extreme. Le Gray used one negative for the sky, and another one with a longer exposure for the sea, and combined the two into one picture in positive. Manual tone mapping was accomplished by dodging and burning — selectively increasing or decreasing the exposure of regions of the photograph to yield better tonality reproduction.
This was effective because the dynamic range of the negative is significantly higher than would be available on the finished positive paper print when that is exposed via the negative in a uniform manner. An excellent example is the photograph Schweitzer at the Lamp by W. The image took five days to reproduce the tonal range of the scene, which ranges from a bright lamp relative to the scene to a dark shadow.
Ansel Adams elevated dodging and burning to an art form. Many of his famous prints were manipulated in the darkroom with these two methods. Adams wrote a comprehensive book on producing prints called The Print , which prominently features dodging and burning, in the context of his Zone System. With the advent of color photography, tone mapping in the darkroom was no longer possible due to the specific timing needed during the developing process of color film.
Photographers looked to film manufacturers to design new film stocks with improved response, or continued to shoot in black and white to use tone mapping methods. The film was processed in a manner similar to color films , and each layer produced a different color.
The concept of neighborhood tone mapping was applied to video cameras in by a group from the Technion in Israel, led by Oliver Hilsenrath and Yehoshua Y. Technion researchers filed for a patent on this concept in ,  and several related patents in and This process is known as bracketing used for a video stream.
In , another commercial medical camera producing an HDR video image, by the Technion. Modern HDR imaging uses a completely different approach, based on making a high-dynamic-range luminance or light map using only global image operations across the entire image , and then tone mapping the result.
Global HDR was first introduced in  resulting in a mathematical theory of differently exposed pictures of the same subject matter that was published in by Steve Mann and Rosalind Picard. It consisted of four film images of the space shuttle at night that were digitally composited with additional digital graphic elements.
The advent of consumer digital cameras produced a new demand for HDR imaging to improve the light response of digital camera sensors, which had a much smaller dynamic range than film. Second, convert this image array, using local neighborhood processing tone-remapping, etc. The image array generated by the first step of Mann’s process is called a lightspace image , lightspace picture , or radiance map. Another benefit of global-HDR imaging is that it provides access to the intermediate light or radiance map, which has been used for computer vision , and other image processing operations.
In February , the Dynamic Ranger technique was demonstrated, using multiple photos with different exposure levels to accomplish high dynamic range similar to the naked eye. In the early s, several scholarly research efforts used consumer-grade sensors and cameras. The “x” channel can be merged with the normal channel in post production software. The Arri Alexa camera uses a dual-gain architecture to generate an HDR image from two exposures captured at the same time.
With the advent of low-cost consumer digital cameras, many amateurs began posting tone-mapped HDR time-lapse videos on the Internet, essentially a sequence of still photographs in quick succession. In , the independent studio Soviet Montage produced an example of HDR video from disparately exposed video streams using a beam splitter and consumer grade HD video cameras.
From Wikipedia, the free encyclopedia. Technique to capture HDR images and videos. For the technology related to HDR displays, see High-dynamic-range video. For other uses, see High dynamic range. This article has multiple issues.
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Luminosity Masking. Wireless tethering.