Jpeg files
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The JPEG standard The name "JPEG" stands for Joint Photographic Experts Group, the name of the committee that created the standard. The group was organized in 1986, issuing a standard in 1992, which was approved in 1994 as ISO 10918-1. JPEG is distinct from MPEG (Moving Picture Experts Group), which produces compression schemes for video. The JPEG standard specifies both the codec, which defines how an image is compressed into a stream of bytes and decompressed back into an image, and the file format used to contain that stream. Typical usage. The JPEG compression algorithm is at its best on photographs and paintings of realistic scenes with smooth variations of tone and colour. For web usage, where the bandwidth used by an image is important, JPEG is very popular. And JPEG/Exif is the most common format saved by digital cameras. On the other hand, JPEG is not as well suited for line drawings and other textual or iconic graphics, where the sharp contrasts between adjacent pixels cause noticeable artifacts. Such images are better saved in a lossless graphics format such as TIFF, GIF, PNG, or a raw image format. JPEG is also not well suited to files that will undergo multiple edits, as some image quality will usually be lost each time the image is decompressed and recompressed (generation loss). To avoid this, an image that is being modified or may be modified in the future can be saved in a lossless format such as PNG, and a copy exported as JPEG for distribution. As JPEG is a lossy compression method — it removes information from the image — it must not be used in astronomical or medical imaging or other purposes where the exact reproduction of the data is required. Lossless formats such as PNG must be used instead. JPEG compression. A chart showing the relative quality of various JPEG encoding settings and also compares saving a file as a JPEG normally and using Photoshop's "save for web" option The compression method is usually lossy, meaning that some visual quality is lost in the process and cannot be restored. There are variations on the standard baseline JPEG that are lossless; however, these are not widely supported.There is also an interlaced "Progressive JPEG" format, in which data is compressed in multiple passes of progressively higher detail. This is ideal for large images that will be displayed while downloading over a slow connection, allowing a reasonable preview after receiving only a portion of the data. However, progressive JPEGs are not as widely supported, and even some software which does support them (such as some versions of Internet Explorer) only displays the image once it has been completely downloaded. There are also many medical imaging systems that create and process 12-bit JPEG images. The 12-bit JPEG format has been part of the JPEG specification for some time, but again, this format is not as widely supported. Lossless editing. A number of alterations to a JPEG image can be performed losslessly (that is, without recompression and the associated quality loss) as long as the image size is a multiple 1 MCU block (Minimum Coded Unit) (usually 16 pixels in both directions, for 4:2:0). Blocks can be rotated in 90 degree increments, flipped in the horizontal, vertical and diagonal axes and moved about in the image. Not all blocks from the original image need to be used in the modified one. The top and left of a JPEG image must lie on a block boundary, but the bottom and right need not do so. This limits the possible lossless crop operations, and also what flips and rotates can be performed on an image whose edges do not lie on a block boundary for all channels.When using lossless cropping, if the bottom or right side of the crop region is not on a block boundary then the rest of the data from the partially used blocks will still be present in the cropped file and can be recovered relatively easily by anyone with a hex editor and an understanding of the format. It is also possible to transform between baseline and progressive formats without any loss of quality, since the only difference is the order in which the coefficients are placed in the file. JPEG files. The file format is known as 'JPEG Interchange Format' (JIF), as specified in Annex B of the standard. However, this "pure" file format is rarely used, primarily because of the difficulty of programming encoders and decoders that fully implement all aspects of the standard and because of certain shortcomings of the standard: * Colour space definition * Component sub-sampling registration * Pixel aspect ratio definition Several additional standards have evolved to address these issues. The first of these, released in 1992, was JPEG File Interchange Format (or JFIF), followed in recent years by Exchangeable image file format (Exif) and ICC colour profiles. There is some confusion between the original 'JPEG Interchange Format' (JIF) and the similarly titled 'JPEG File Interchange Format' (JFIF). In some ways JFIF is a cutdown version of the JIF standard in that it specifies certain constraints (such as standard colour space), while in other ways it is an extension of JIF due to the standard Application Segment header. The documentation for the original JFIF standard states: JPEG File Interchange Format is a minimal file format which enables JPEG bitstreams to be exchanged between a wide variety of platforms and applications. This minimal format does not include any of the advanced features found in the TIFF JPEG specification or any application specific file format. Nor should it, for the only purpose of this simplified format is to allow the exchange of JPEG compressed images. Image files that employ JPEG compression are commonly called "JPEG files". Most image capture devices (such as digital cameras) and most image editing software programs that write to a "JPEG file" are actually creating a file in the JFIF and/or Exif format.Strictly speaking, the JFIF and Exif standards are incompatible because they each specify that their header appears first. In practice, most JPEG files in Exif format contain a small JFIF header that precedes the Exif header. This allows older readers to correctly handle the older format JFIF header, while newer readers also decode the following Exif header. JPEG file extensions The most common filename extensions for files employing JPEG compression are .jpg and .jpeg, though .jpe, .jfif and .jif are also used. It is also possible for JPEG data to be embedded in other file types - TIFF encoded files often embed a JPEG image as a thumbnail of the main image. Colour profile. Many JPEG files embed an ICC colour profile (colour space). Commonly used colour profiles include sRGB and Adobe RGB. Because these colour spaces use a non-linear transformation, the dynamic range of an 8-bit JPEG file is about 11 stops. However, many applications are not able to deal with JPEG colour profiles and simply ignore them.
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