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It was developed from to by a Joint Photographic Experts Group committee chaired by Touradj Ebrahimi later the JPEG president , [1] with the intention of superseding their original JPEG standard created in , which is based on a discrete cosine transform DCT , with a newly designed, wavelet -based method.
The standardized filename extension is. JPEG code streams are regions of interest that offer several mechanisms to support spatial random access or region of interest access at varying degrees of granularity. It is possible to store different parts of the same picture using different quality.
The standard could be adapted for motion imaging video compression with the Motion JPEG extension. JPEG technology was selected as the video coding standard for digital cinema in The codestream obtained after compression of an image with JPEG is scalable in nature, meaning that it can be decoded in a number of ways; for instance, by truncating the codestream at any point, one may obtain a representation of the image at a lower resolution, or signal-to-noise ratio — see scalable compression.
By ordering the codestream in various ways, applications can achieve significant performance increases. However, as a consequence of this flexibility, JPEG requires codecs that are complex and computationally demanding.
JPEG decomposes the image into a multiple resolution representation in the course of its compression process. This pyramid representation can be put to use for other image presentation purposes beyond compression. These features are more commonly known as progressive decoding and signal-to-noise ratio SNR scalability.
JPEG provides efficient code-stream organizations which are progressive by pixel accuracy and by image resolution or by image size. This way, after a smaller part of the whole file has been received, the viewer can see a lower quality version of the final picture. The quality then improves progressively through downloading more data bits from the source. Lossless compression is provided by the use of a reversible integer wavelet transform in JPEG Like JPEG , JPEG is robust to bit errors introduced by noisy communication channels, due to the coding of data in relatively small independent blocks.
JPEG supports bit depths of 1 to 38 bits per component. The aim of JPEG is not only improving compression performance over JPEG but also adding or improving features such as scalability and editability. JPEG 's improvement in compression performance relative to the original JPEG standard is actually rather modest and should not ordinarily be the primary consideration for evaluating the design.
Very low and very high compression rates are supported in JPEG The ability of the design to handle a very large range of effective bit rates is one of the strengths of JPEG For example, to reduce the number of bits for a picture below a certain amount, the advisable thing to do with the first JPEG standard is to reduce the resolution of the input image before encoding it.
The following sections describe the algorithm of JPEG According to the Royal Library of the Netherlands , "the current JP2 format specification leaves room for multiple interpretations when it comes to the support of ICC profiles, and the handling of grid resolution information".
Initially images have to be transformed from the RGB color space to another color space, leading to three components that are handled separately. There are two possible choices:. If R, G, and B are normalized to the same precision, then numeric precision of C B and C R is one bit greater than the precision of the original components.
This increase in precision is necessary to ensure reversibility. The chrominance components can be, but do not necessarily have to be, downscaled in resolution; in fact, since the wavelet transformation already separates images into scales, downsampling is more effectively handled by dropping the finest wavelet scale. This step is called multiple component transformation in the JPEG language since its usage is not restricted to the RGB color model.
After color transformation, the image is split into so-called tiles , rectangular regions of the image that are transformed and encoded separately. Tiles can be any size, and it is also possible to consider the whole image as one single tile. Once the size is chosen, all the tiles will have the same size except optionally those on the right and bottom borders.
Dividing the image into tiles is advantageous in that the decoder will need less memory to decode the image and it can opt to decode only selected tiles to achieve a partial decoding of the image. The disadvantage of this approach is that the quality of the picture decreases due to a lower peak signal-to-noise ratio.
Using many tiles can create a blocking effect similar to the older JPEG standard. JPEG uses two different wavelet transforms:. The wavelet transforms are implemented by the lifting scheme or by convolution.
After the wavelet transform, the coefficients are scalar- quantized to reduce the number of bits to represent them, at the expense of quality. The output is a set of integer numbers which have to be encoded bit-by-bit. The parameter that can be changed to set the final quality is the quantization step: the greater the step, the greater is the compression and the loss of quality. With a quantization step that equals 1, no quantization is performed it is used in lossless compression.
The result of the previous process is a collection of sub-bands which represent several approximation scales. A sub-band is a set of coefficients — real numbers which represent aspects of the image associated with a certain frequency range as well as a spatial area of the image. The quantized sub-bands are split further into precincts , rectangular regions in the wavelet domain.
They are typically sized so that they provide an efficient way to access only part of the reconstructed image, though this is not a requirement.
Precincts are split further into code blocks. Code blocks are in a single sub-band and have equal sizes—except those located at the edges of the image. The encoder has to encode the bits of all quantized coefficients of a code block, starting with the most significant bits and progressing to less significant bits by a process called the EBCOT scheme.
In this encoding process, each bit plane of the code block gets encoded in three so-called coding passes , first encoding bits and signs of insignificant coefficients with significant neighbors i. The three passes are called Significance Propagation , Magnitude Refinement and Cleanup pass, respectively. The bits selected by these coding passes then get encoded by a context-driven binary arithmetic coder , namely the binary MQ-coder as also employed by JBIG2.
The context of a coefficient is formed by the state of its eight neighbors in the code block. The result is a bit-stream that is split into packets where a packet groups selected passes of all code blocks from a precinct into one indivisible unit. Packets are the key to quality scalability i. Packets from all sub-bands are then collected in so-called layers. The way the packets are built up from the code-block coding passes, and thus which packets a layer will contain, is not defined by the JPEG standard, but in general a codec will try to build layers in such a way that the image quality will increase monotonically with each layer, and the image distortion will shrink from layer to layer.
Thus, layers define the progression by image quality within the code stream. The problem is now to find the optimal packet length for all code blocks which minimizes the overall distortion in a way that the generated target bitrate equals the demanded bit rate. While the standard does not define a procedure as to how to perform this form of rate—distortion optimization , the general outline is given in one of its many appendices: For each bit encoded by the EBCOT coder, the improvement in image quality, defined as mean square error, gets measured; this can be implemented by an easy table-lookup algorithm.
Furthermore, the length of the resulting code stream gets measured. This forms for each code block a graph in the rate—distortion plane, giving image quality over bitstream length. The optimal selection for the truncation points, thus for the packet-build-up points is then given by defining critical slopes of these curves, and picking all those coding passes whose curve in the rate—distortion graph is steeper than the given critical slope. This method can be seen as a special application of the method of Lagrange multiplier which is used for optimization problems under constraints.
Packets can be reordered almost arbitrarily in the JPEG bit-stream; this gives the encoder as well as image servers a high degree of freedom. Already encoded images can be sent over networks with arbitrary bit rates by using a layer-progressive encoding order.
On the other hand, color components can be moved back in the bit-stream; lower resolutions corresponding to low-frequency sub-bands could be sent first for image previewing. All these operations do not require any re-encoding but only byte-wise copy operations. Higher-resolution images tend to benefit more, where JPEG 's spatial-redundancy prediction can contribute more to the compression process. In very low-bitrate applications, studies have shown JPEG to be outperformed [30] by the intra-frame coding mode of H.
Good applications for JPEG are large images, images with low-contrast edges — e. Tiling, color component transform, discrete wavelet transform, and quantization could be done pretty fast, though entropy codec is time-consuming and quite complicated.
Although the JPEG format supports lossless encoding, it is not intended to completely supersede today's dominant lossless image file formats. Whereas JPEG entirely describes the image samples, JPEG-1 includes additional meta-information such as the resolution of the image or the color space that has been used to encode the image. The part-2 extension to JPEG , i.
Images in this extended file-format use the. There is no standardized extension for code-stream data because code-stream data is not to be considered to be stored in files in the first place, though when done for testing purposes, the extension. For traditional JPEG, additional metadata , e.
ISO is covered by patents, but the contributing companies and organizations agreed that licenses for its first part—the core coding system—can be obtained free of charge from all contributors.
It has always been a strong goal of the JPEG committee that its standards should be implementable in their baseline form without payment of royalty and license fees The up and coming JPEG standard has been prepared along these lines, and agreement reached with over 20 large organizations holding many patents in this area to allow use of their intellectual property in connection with the standard without payment of license fees or royalties.
However, the JPEG committee acknowledged in that undeclared submarine patents may present a hazard:. It is of course still possible that other organizations or individuals may claim intellectual property rights that affect implementation of the standard, and any implementers are urged to carry out their own searches and investigations in this area. Attention is drawn to the possibility that some of the elements of this Recommendation International Standard may be the subject of patent rights other than those identified in the above mentioned databases.
The analysis of this ISO patent declaration database shows that 3 companies finalized their patent process, Telcordia Technologies Inc. Bell Labs US patent number 4,,, whose licensing declaration is not documented, Mitsubishi Electric Corporation, with 2 Japan patents and , that have been expired since , respectively source Mitsubishi Electric Corporation, Corporate Licensing Division , and IBM N. The Telcordia Technologies Inc.
Its title is "Sub-band coding of images with low computational complexity", and it seems that its relation with JPEG is "distant", as the technique described and claimed is widely used not only by JPEG This provides an updated context of JPEG legal status in , showing that since , though ISO and IEC deny any responsibility in any hidden patent rights other than those identified in the above mentioned ISO databases, the risk of such a patent claim on ISO and its discrete wavelet transform algorithm appears to be low.
Instead, each frame is an independent entity encoded by either a lossy or lossless variant of JPEG Its physical structure does not depend on time ordering, but it does employ a separate profile to complement the data.
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