JPEG
In
computing,
JPEG is a commonly used standard method of
compressing photographic images. The
file format which employs this compression is commonly also called JPEG; platforms with short file extensions may use .JPG or .JPE to identify this format.
The name stands for
Joint Photographic Experts Group.\nJPEG itself specifies only how an image is transformed into a stream of
bytes,\nbut not how those bytes are encapsulated in any particular storage medium.\nA further standard, created by the
Independent JPEG Group, called
JFIF (JPEG File Interchange Format)\nspecifies how to produce a file suitable for computer storage and transmission\n(such as over the
Internet) from a JPEG stream.\nIn common usage, when one speaks of a "JPEG file" one generally means a JFIF file, or sometimes an
Exif JPEG file.\nThere are, however, other JPEG-based file formats.
JPEG/JFIF is the most common format used for storing and transmitting photographs on the
World Wide Web.\nIt is
not as well suited for line drawings and other textual or iconic graphics because its compression method performs badly on these types of images (the
PNG and
GIF formats are in common use for that purpose;\nGIF, having only 8 bits per
pixel is not well suited for colour photographs,\nbut PNG may have as much or more detail than JPEG).
The
MIME media type for JFIF is
image/jpeg (defined in RFC 1341).
Encoding
\nThere are many options in the standard, but many are little used. Here is a brief desciption of one of the more common ones when applied to an input that has 24 bits per pixel (eight each of red, green, and blue). This particular option is a lossy data compression method.
Color Space Transformation
\nFirst the image is converted from RGB into a different color space called YUV. \nThis is similar to the color space used by NTSC and PAL color television transmission.\nThe Y component represents brightness of a pixel, and the U and V components together represent the hue and saturation.\nThis part is useful because the human eye can see more detail in the Y component than in the others.
Downsampling
\nThis enables the next step which is to reduce the U and V components to half size in both vertical and horizontal directions (called "downsampling" or "chroma subsampling"), thereby reducing the size in bytes of the whole image by a factor of two.\nFor the rest of the compression process, Y, U and V are processed separately and in a very similar manner. The ratios at which the downsampling can be done on JPEG are 4:4:4, 4:2:2 and 4:1:1.
Discrete Cosine Transform
\nNext, each component (Y, U, V) of the image is "tiled" into sections of eight by eight pixels each, then each tile is converted to frequency space using a two-dimensional discrete cosine transform (DCT).
Quantization
\nThe human eye is fairly good at seeing small differences in brightness over a relatively large area, but not so good at distinguishing the exact strength of a high frequency brightness variation. This fact allows you to get away with greatly reducing the amount of information in the high frequency components. This is done by simply dividing each component in the frequency domain by a constant for that component, and then rounding to the nearest integer. This is the main lossy operation in the whole process. As a result of this, it is typically the case that many of the higher frequency components are rounded to zero, and many of the rest become small positive or negative numbers.
Entropy Coding
\nThis is a special form of lossless data compression. Basically a combination of putting the components in a "zigzag" order that groups similar frequencies together, then run length coding zeros, then using Huffman coding on what's left.\nThe standard also allows but does not require the use of Arithmetic coding, which is always superior to Huffman coding, but this feature is rarely used\nbecause it is covered by patents that encumber development of software; because it is much slower to encode and decode than Huffman coding; and because it does not provide much benefit (on the order of 5% smaller files).
Compression ratio and artifacts
\nThe resulting compression ratio can be varied according to need, by being more or less aggressive in the divisors used in the quantization phase. Ten to one compression usually results in an image that can't be distinguished by eye from the original. 100 to one compression is usually possible, but will look distinctly "blocky" and "blurry" compared to the original. The appropriate level of compression depends on the use to which the image will be put.
Those who surf the web may be familiar with the irregularities known as compression artifacts that appear in JPEG digital images. These are due to the quantization step of the JPEG algorithm. They are especially noticeable around eyes in pictures of faces. They can be reduced by choosing a lower level of compression; they may be eliminated by saving an image using a lossless file format, though for photographic images this will usually result in a larger file size.
Decoding
\nDecoding to display the image consists of doing all the above in reverse.
Usage
\nJPEG is at its best on photographs and paintings of realistic scenes with smooth variations of tone and color. In this case it usually performs much better than purely lossless methods while still giving a good looking image (in fact it will produce a much higher quality image than other common methods such as GIF which are lossless for drawings and iconic graphics but require severe quantization for full-color images).
Other lossy encoding formats
\nNewer lossy methods, particularly wavelet compression, perform even better in these cases. However, JPEG is a well established standard with plenty of software available, including free software, so it continues to be heavily used as of 2004. Also, many wavelet algorithms are patented, making it difficult or impossible to use them freely in many software projects.
The JPEG committee has now created its own wavelet-based standard, JPEG 2000, which is intended to eventually supersede the original JPEG standard.
Potential patent issues
\nIn 2002 Forgent Networks asserted that it owns and will enforce patent rights on the JPEG technology, arising from a patent that had been filed in 1986 (US Patent 4,698,672). \nThe announcement has created a furor reminiscent of Unisys' attempts to assert its rights over the GIF image compression standard.
The JPEG committee investigated the patent claims in 2002 and found that they were invalidated by prior art. Nevertheless, between 2002 and 2004 Forgent was able to obtain about $90 million by licensing their patent to some 30 companies. In April 2004 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.
The JPEG committee has as one of its explicit goals that their standards be implementable without payment of license fees, and they have secured appropriate license rights for their upcoming JPEG 2000 standard from over 20 large organizations.
See also
\n* Image compression\n* JPEG 2000
External links
\n* Wotsit.org's entry on the JPEG format\n* ITU T.81 JPEG compression\n* JFIF File Format
\nCategory:Graphics file formats
