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JPEG
JPEG is an acronym for "Joint Photographic Experts Group". The JPEG standard is an international standard for colour image compression. JPEG is particularly important for multimedia applications as it is used, in part, for the encoding of moving images using the mpeg standard.
Multimedia applications place a phenomenal strain on technology. The transfer of high quality images around a local area network or their storage on CD-ROM would be impossible without some kind of efficient storage algorithm. Until JPEG was created the massive storage requirement for high quality images was a stumbling block for the widespread use of images. JPEG provided a solution to the
problem that althought here were many algorithms available for compression, there was no international standard.
The goals of the Group were:-
"To be at or near the state of the art with regard to compression rate and accompanying image fidleity"
"Be applicable to practically any kind of continuou-tone digital source image."
"Have tractable computational complexity, to make feasible software implementations with viable performance on a range of CPU's."
To have the following modes of operation:- Sequential Encoding
Progressive Encoding Lossless Encoding Hierarchical Encoding
"JPEG is concerned primarily with images that have two spatial dimensions, contain greyscale or colour information, and possess no temporal dependence."
Implementation
The JPEG standard does not specify the exact behaviour of compressors and decompressors which allows some room for creative implementation. For example, it is possible in your implementation to have a trade-off between the speed of compression and image quality - slower but more accurate or faster but less accurate. This depends on the requirements of the user and the applications.
Four Modes of Operation
In this next section, I shall discuss the four modes of operation which JPEG provides
Sequantial DCT-based Encoding
Operating in this mode, each of the image components are compressed either individually or in groups.
A single "scan" coding part of an image is done so in a single left-to-right, top-to-bottom motion.
Of the four modes of operation, Sequential Encoding has the most interesting features and, as such, I shall discuss this further in later sections.
Progressive DCT-based Encoding
The progressive DCT-based mode is achieved by a sequence of "scans" each of which codes a part of the DCT information. This is discussed in greater depth in later sections. This is useful if data
transmission time is long. The dat is sent firstly with low precision and then refined in later scans.
Sequential Lossless Mode
Once JPEG had decided on pursuing a DCT-based method they realised that a DCT-based lossless mode was difficult to define. They discovered that the restraints which would have to placed upon the
encoders and decoders would not allow them to be defined independently and would thus be
impractical. But, in order to satisfy the requirements which they set themselves, it was essential that a lossless mode of operation was defined.
The method which was chosen is completely independent of the DCT-based methods which form the other three approaches. The method employed is a predictive mode. The approach has not undergone the same rigorous testing a have the DCT-based approches, but have produced results which are very close to those regarded as state-of-the-art for lossless continuous-tone compression. The downside of having lossless mode of operation is that a low compression ratio is offered in comparison to the lossy modes, although this may be essential for a particular application.
Hierarchical Encoding
This mode of operation provides for progressive encoding with increasing spatial resolution between progressive stages. At the first stage, the lowest resolution is coded using one of the sequential or progressive JPEG modes. The output at each hierarchical stage is then upsampled if necessary (upsampled to increase the spatial resolution) and becomes the prediction for the next stage. By encoding the image at different resolutions in this manner, lower resolution versions may be accessed without first having to decompress the image at its full resolution. This can increase speed at which an image is available to the viewer.
Quality of Compression
As mentioned above, I shall concentrate on the Sequential DCT-Based encoding method. This is a lossy method whereby the image which you get out of the decompressor does not have the same quality as the image which you put into the encoder.
It can be seen that the lossy modes of JPEG are intended to produce images which are to be looked at by the human eye. If one requires a JPEG compressed image to be machine analysed then the lossless mode of operation is obviously recommended.
JPEG can make image files smaller. It can easliy provide compression of around 20:1. Being able to compress a 2Mb file down to 100Kb is certainly advantageous. A concern with the lossy mode of operation discussed earlier is that with repeated compression and decompression, the loss accumulates with every repetition of the process. The effect of this can be limited by using the same quality setting to decompress and recompress an image. The quality setting is included as part of any particular
implementation and allows the user to specify the quality of compression they want. In most systems the higher the quality, the larger the image. Even the advent of special graphics cards cannot limit the effect of this.
Figure 1. DCT-Based Encoder Processing Steps
Figure 2. DCT-Based Decoder Processing Steps
The above images are reproduced from Comms of the ACM April 1991/vol. 34, no 4.
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Run Length Encoding | Statistical Encoding Huffman Coding | Lempel Ziv
Source Encoding Differential Encoding| JPEG | JPEG Page 2 | JPEG Page 3 | JPEG Page 4 |
MPEG Derek Sansom
Last modified: Fri Mar 20 13:32:38 GMT 1998
