Raw files
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Raw Files image format Type of format. A raw image files contains minimally processed data from the image sensor of a digital camera or image scanner. Raw files are so named because they are not yet processed and therefore are not ready to be used with a bitmap graphics editor or printed. Normally, the image is processed by a raw converter in a wide-gamut internal colorspace where precise adjustments can be made before conversion to an RGB files format such as TIFF or JPEG for storage, printing, or further manipulation. These images are often described as "RAW image files" (note capitalization) based on the erroneous belief that they represent a single files format, and thus deserve a common filename extension, .RAW. In fact there are dozens if not hundreds of raw image formats in use by different models of digital cameras. Raw image files are sometimes called digital negatives, as they fulfill the same role as film negatives in traditional chemical photography: that is, the negative is not directly usable as an image, but has all of the information needed to create an image. Likewise, the process of converting a raw image files into a viewable format is sometimes called developing a raw image, by analogy with the film development process used to convert photographic film into viewable prints. Like a photographic negative, a raw digital image may have a wider dynamic range or color gamut than the eventual final image format. The selection of the final choice of image rendering is part of the process of white balancing and colour grading. Raw formats' purpose is to faithfully record both 100% of exactly what the sensor "saw" (the data), and the conditions surrounding the recording of the image (the metadata). Rationale. Providing a detailed and concise description of the content of raw files is highly problematic. There is no single raw format; formats can be similar or radically different. Different manufacturers use their own proprietary and typically undocumented formats, which are collectively known as raw format. Often they also change the format from one camera model to the next. Multiple major manufacturers, including Nikon, Canon, and Sony encrypt portions of the files in an attempt to prevent third-party tools from accessing them. This industry-wide situation of inconsistent formatting has concerned many photographers who worry that their valuable raw photos may someday become inaccessible, as operating systems and applications become obsolete and abandoned raw formats are dropped from new software. The availability of high-quality open source software which decodes raw image format files, particularly draw, has helped to alleviate these concerns. There are several would-be standards competing for digital camera industry buy-in. Files contents Raw files contain, by necessity, the information required to produce a RGB format files from the camera's sensor data. Although there is no standard raw files format, the structure of raw files often follows a standard pattern, that is: * a short file header which typically contains the byte-ordering of the files, a files identifier and an offset into the main files data * camera sensor metadata which is required to interpret the sensor image data. This includes the size of the sensor, the attributes of the CFA and its colour profile * image metadata which is required for inclusion in any generated JPEG file. These include the exposure settings and other EXIF data * an image thumbnail * optionally a reduced size image in JPEG format * the sensor image data Many raw files formats (including 3FR(Hasselblad), DCR,K25,KDC(Kodak), CR2(Canon), ERF(Epson), MEF,(Mamiya), MOS(Leaf), NEF(Nikon), ORF(Olympus), PEF(Pentax), RAW,RW2(Panasonic) and ARW,SRF,SR2(Sony)) are based on the TIFF files format. These files may deviate from the TIFF standard in a number of ways, including: the use of a non-standard file header, the inclusion of additional image tags and the encryption of some of the tagged data. The Adobe digital negative format is an extension of the TIFF 6.0 format. Sensor image data. Digital camera raw files contain the pixel data from a rectangular image sensor, the modern equivalent of photographic film, usually at 12 or 14 bits per pixel. The sensor is almost invariably overlaid with a colour filter array, usually a Bayer filter, consisting of a mosaic of red, blue and green filters. Given that three colours fit uncomfortably in a rectangular grid, green was chosen to be doubly present, since the human eye is more sensitive to it. Green also often serves as the luminance channel, and as the dominant channel for in-camera black-and-white conversions. To retrieve an image from a raw files, this mosaic of data must be converted into a full RGB image. This is often referred to as digital development. One variation on the Bayer filter is the RGBE filter of the Sony Cyber-shot DSC-F828, which exchanged the green in the RG rows with "emerald" (a blue-green or cyan, colour). Other sensors, such as the Foveon X3 sensor capture information directly in RGB form, having three pixel sensors in each location, one for each colour component; these camera RGB raw data still need to be processed to make an image files. Flatbed and film scanner sensors are typically straight narrow RGB or RGBI (where "I" is infrared) strips that are swept across an image; other than that, the remainder of the discussion about raw files applies to them as well. (Some scanners do not allow the user access to the raw data at all, as a speed compromise. The raw data are processed very rapidly inside the scanner to select out the best part of the available dynamic range so only the result is passed to the computer for permanent storage.) Benefits. Nearly all digital cameras can process the image from the sensor into a JPEG file using settings for white balance, colour saturation, contrast, and sharpness that are either selected automatically or entered by the photographer before taking the picture. Cameras that support raw files save these settings in the files, but defer the processing. This results in an extra step for the photographer, so raw is normally only used when additional computer processing is intended. However, raw has numerous advantages over JPEG such as: * Higher image quality. Because all the calculations (such as applying the gamma curve, demosaicing, white balance, brightness, contrast, etc...) used to generate pixel values (in RGB format for most images) are performed in one step on the base data, the resultant pixel values will be more accurate and exhibit less posterization. * JPEG are typically saved using a lossy compression format (a lossless JPEG compression is available). Raw formats are either uncompressed or use lossless compression, so the maximum amount of image detail is always kept within the RAW file. * Finer control. Using RAW conversion software allows users to manipulate more parameters (such as lightness, white balance, hue, saturation, etc...) and do so with greater variability. For example, the white point can be set to any value, not just discrete values like "daylight" or "incandescent". * Camera raw files have 12 or 14 bits of intensity information, not the gamma-compressed 8 bits stored in JPEG files (and typically stored in processed TIFF files); since the data is not yet rendered and clipped to a colour space gamut, more precision may be available in highlights, shadows, and saturated colours. * The colour space can be set to whatever is desired. * Different demosaicing algorithms can be used, not just the one coded into the camera. * The contents of raw files include more information, and potentially higher quality, than the converted results, in which the rendering parameters are fixed, the colour gamut is clipped, and there may be quantization and compression artifacts. * Large transformations of the data, such as increasing the exposure of a dramatically under-exposed photo, result in less visible artifacts when done from raw data than when done from already rendered image files. Raw data leave more scope for both corrections and artistic manipulations, without resulting in images with visible flaws such as posterization. Drawbacks. Camera raw files are typically 2–6 times larger than JPEG files. Some raw formats do not use compression, some implement lossless data compression to reduce the size of the files without affecting image quality and others use lossy data compression where quantization and filtering is performed on the image data. While use of raw formats avoids the compression artifacts inherent in JPEG, fewer images can fit on a given memory card. It also takes longer for the camera to write raw images to the card, so fewer pictures can be taken in quick succession (affecting the ability to take, for example, a sports sequence). There is still no widely accepted standard raw format. Three potential candidates for a standard format have been put forward, but none has been adopted by many major camera companies. Numerous different raw formats are currently in use and new raw formats keep appearing, while others are abandoned. Because of the lack of a standard raw format, more specialized software may be required to open raw files than for standardized formats like JPEG or TIFF. Software developers have to frequently update their products to support the raw formats of the latest cameras.The time taken in the image workflow is an important factor when choosing between raw and ready-to-use image formats.
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