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:.
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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. 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.
ISBN 13: 9780792375197
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. The software is frequently referenced from the text as an additional resource for understanding complex or subtle aspects of the standard.
Conversely, the software makes frequent reference to the text and has been written to mesh with the terminology and notation employed therein. Part I: Fundamental Concepts. Image Compression Overview. Entropy and Coding Techniques.
JPEG - Wikipedia
Image Transforms. Rate Control Techniques. Filter Banks and Wavelets. Zero-Tree Coding. Highly Scalable Compression.
ISBN 13: 9780792375197
Why pay to see a low-quality pirated version when you can see the real movie? Copying a movie from the original cassette will be just about impossible because the movies will be distributed on magnetic tapes that can only be opened with a key that the movie theaters will receive by e-mail.
And that key will only decode the movie in the projection equipment at that particular theater. Even if someone gets the key and the cassette, they won't be able to open the movie because they'll need the equipment in a specific movie theater. So this is a very, very secure system. Conversion to Digital Complete by By the end of this year, 1, theaters are expected to have digital projection equipment.
That number will jump to 5, theaters in and all the major chains will be completely converted by , Marcellin said. While the movies are the most glamorous application for JPEG, it's also being used in many other places, such as MapQuest on the Internet, medical imaging systems, and software, such as Adobe Photoshop. The University of Arizona. Apply Visit Give. July 7, JPEG will revolutionize the quality and distribution of major motion pictures.
What is JPEG? It's a "just enough" method for compressing and displaying digital images. JPEG works just as well for movies. The studios expect that digital movies will save a billion dollars a year in distribution costs. Piracy Will be More Difficult Finally, piracy will become more difficult.
Topics :. Science and Technology. February 19,