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The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves compression with little perceptible loss in image quality. JPEG compression is used in a number of image file formats. The JPEG specification cites patents from several companies. The following patents provided the basis for its arithmetic coding algorithm.
Klenke in October The patent describes a DCT-based image compression algorithm, and would later be a cause of controversy in see Patent controversy below. Founded in , the group developed the JPEG standard during the late s.
The group published the JPEG standard in The JPEG standard specifies the codec , which defines how an image is compressed into a stream of bytes and decompressed back into an image, but not the file format used to contain that stream. JPEG standards are formally named as Information technology — Digital compression and coding of continuous-tone still images. In , Forgent Networks asserted that it owned and would enforce patent rights on the JPEG technology, arising from a patent that had been filed on October 27, , and granted on October 6, U.
This led to Forgent acquiring ownership over the patent. The JPEG committee investigated the patent claims in and were of the opinion that they were invalidated by prior art , [25] a view shared by various experts. In April , Forgent sued 31 other companies to enforce further license payments.
In July of the same year, a consortium of 21 large computer companies filed a countersuit, with the goal of invalidating the patent. In addition, Microsoft launched a separate lawsuit against Forgent in April This makes any appeal to reinstate the patent highly unlikely to succeed.
Forgent also possesses a similar patent granted by the European Patent Office in , though it is unclear how enforceable it is. As of October 27, , the U. The JPEG committee has as one of its explicit goals that their standards in particular their baseline methods be implementable without payment of license fees, and they have secured appropriate license rights for their JPEG standard from over 20 large organizations.
Patent 5,, issued in , is infringed by the downloading of JPEG images on either a website or through e-mail. If not invalidated, this patent could apply to any website that displays JPEG images. The patent was under reexamination by the U. Patent and Trademark Office from to ; in July , the Patent Office revoked all of the original claims of the patent but found that an additional claim proposed by Global Patent Holdings claim 17 was valid.
A fifth lawsuit was filed against Global Patent Holdings in Nevada. That lawsuit was filed by Zappos. Global Patent Holdings had also used the ' patent to sue or threaten outspoken critics of broad software patents, including Gregory Aharonian [33] and the anonymous operator of a website blog known as the " Patent Troll Tracker. Patent and Trademark Office to reexamine the sole remaining claim of the ' patent on the basis of new prior art. On March 5, , the U.
Patent and Trademark Office agreed to reexamine the ' patent, finding that the new prior art raised substantial new questions regarding the patent's validity. Patent and Trademark Office's review of the ' patent. On April 23, , a judge presiding over the two lawsuits in Chicago, Illinois granted the motions in those cases.
Beginning in and continuing as of early , an entity known as Princeton Digital Image Corporation, [39] based in Eastern Texas, began suing large numbers of companies for alleged infringement of U. Patent 4,, Princeton claims that the JPEG image compression standard infringes the ' patent and has sued large numbers of websites, retailers, camera and device manufacturers and resellers.
The patent was originally owned and assigned to General Electric. The patent expired in December , but Princeton has sued large numbers of companies for "past infringement" of this patent.
Under U. As of March , Princeton had suits pending in New York and Delaware against more than 55 companies. General Electric's involvement in the suit is unknown, although court records indicate that it assigned the patent to Princeton in and retains certain rights in the patent. The JPEG compression algorithm operates at its best on photographs and paintings of realistic scenes with smooth variations of tone and color. For web usage, where reducing the amount of data used for an image is important for responsive presentation, JPEG's compression benefits make JPEG popular.
However, JPEG is not well suited for line drawings and other textual or iconic graphics, where the sharp contrasts between adjacent pixels can cause noticeable artifacts.
The JPEG standard includes a lossless coding mode, but that mode is not supported in most products. As the typical use of JPEG is a lossy compression method, which reduces the image fidelity, it is inappropriate for exact reproduction of imaging data such as some scientific and medical imaging applications and certain technical image processing work. JPEG is also not well suited to files that will undergo multiple edits, as some image quality is lost each time the image is recompressed, particularly if the image is cropped or shifted, or if encoding parameters are changed — see digital generation loss for details.
To prevent image information loss during sequential and repetitive editing, the first edit can be saved in a lossless format, subsequently edited in that format, then finally published as JPEG for distribution.
A perceptual model based loosely on the human psychovisual system discards high-frequency information, i. In the transform domain, the process of reducing information is called quantization.
In simpler terms, quantization is a method for optimally reducing a large number scale with different occurrences of each number into a smaller one, and the transform-domain is a convenient representation of the image because the high-frequency coefficients, which contribute less to the overall picture than other coefficients, are characteristically small-values with high compressibility.
The quantized coefficients are then sequenced and losslessly packed into the output bitstream. Nearly all software implementations of JPEG permit user control over the compression ratio as well as other optional parameters , allowing the user to trade off picture-quality for smaller file size.
In embedded applications such as miniDV, which uses a similar DCT-compression scheme , the parameters are pre-selected and fixed for the application. The compression method is usually lossy , meaning that some original image information is lost and cannot be restored, possibly affecting image quality. There is an optional lossless mode defined in the JPEG standard. However, this mode is not widely supported in products. 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, support for progressive JPEGs is not universal. When progressive JPEGs are received by programs that do not support them such as versions of Internet Explorer before Windows 7 [41] the software displays the image only after it has been completely downloaded. There are also many medical imaging, traffic and camera applications that create and process bit JPEG images both grayscale and color.
The libjpeg codec supports bit JPEG and there even exists a high-performance version. Several 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 of 1 MCU block Minimum Coded Unit usually 16 pixels in both directions, for chroma subsampling.
Utilities that implement this include:. Blocks can be rotated in 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. This limits the possible lossless crop operations, and also prevents flips and rotations of an image whose bottom or right edge does not lie on a block boundary for all channels because the edge would end up on top or left, where — as aforementioned — a block boundary is obligatory.
Rotations where the image is not a multiple of 8 or 16, which value depends upon the chroma subsampling, are not lossless. Rotating such an image causes the blocks to be recomputed which results in loss of quality. 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.
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. Furthermore, several JPEG images can be losslessly joined, as long as they were saved with the same quality and the edges coincide with block boundaries.
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:.
Several additional standards have evolved to address these issues. Within these segments of the file that were left for future use in the JIF standard and are not read by it, these standards add specific metadata. Thus, in some ways, JFIF is a cut-down version of the JIF standard in that it specifies certain constraints such as not allowing all the different encoding modes , while in other ways, it is an extension of JIF due to the added metadata.
The documentation for the original JFIF standard states: [44]. Nor should it, for the only purpose of this simplified format is to allow the exchange of JPEG compressed images. Most image capture devices such as digital cameras that output JPEG are actually creating files in the Exif format, the format that the camera industry has standardized on for metadata interchange. This allows older readers to correctly handle the older format JFIF segment, while newer readers also decode the following Exif segment, being less strict about requiring it to appear first.
The most common filename extensions for files employing JPEG compression are. Because these color spaces use a non-linear transformation, the dynamic range of an 8-bit JPEG file is about 11 stops ; see gamma curve. If the image doesn't specify color profile information untagged , the color space is assumed to be sRGB for the purposes of display on webpages. A JPEG image consists of a sequence of segments , each beginning with a marker , each of which begins with a 0xFF byte, followed by a byte indicating what kind of marker it is.
Some markers consist of just those two bytes; others are followed by two bytes high then low , indicating the length of marker-specific payload data that follows. The length includes the two bytes for the length, but not the two bytes for the marker. Some markers are followed by entropy-coded data; the length of such a marker does not include the entropy-coded data.
Note that consecutive 0xFF bytes are used as fill bytes for padding purposes, although this fill byte padding should only ever take place for markers immediately following entropy-coded scan data see JPEG specification section B. Within the entropy-coded data, after any 0xFF byte, a 0x00 byte is inserted by the encoder before the next byte, so that there does not appear to be a marker where none is intended, preventing framing errors.
Decoders must skip this 0x00 byte. Note however that entropy-coded data has a few markers of its own; specifically the Reset markers 0xD0 through 0xD7 , which are used to isolate independent chunks of entropy-coded data to allow parallel decoding, and encoders are free to insert these Reset markers at regular intervals although not all encoders do this. Since several vendors might use the same APP n marker type, application-specific markers often begin with a standard or vendor name e.
At a restart marker, block-to-block predictor variables are reset, and the bitstream is synchronized to a byte boundary. Restart markers provide means for recovery after bitstream error, such as transmission over an unreliable network or file corruption. Since the runs of macroblocks between restart markers may be independently decoded, these runs may be decoded in parallel. The encoding process consists of several steps:.
The decoding process reverses these steps, except the quantization because it is irreversible. In the remainder of this section, the encoding and decoding processes are described in more detail.
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