Data Compression Lec#2 PowerPoint

Compression System Model
The compression system model consists of two parts: the compressor(Encoding) and the decompressor(Decoding). The compressor consists of a preprocessing stage and encoding stage, whereas the decompressor consists of a decoding stage followed by a post processing stage Figure(1.1). Before encoding, preprocessing is performed to prepare the image for the encoding process, and consists of a number of operations that are application specific. After the compressed file has been decoded, post processing can be performed to eliminate some of the potentially undesirable artifacts brought about by the compression process. Often, many practical compression algorithms are a combination of a number of different individual compression techniques.
The compressor can be further broken down into stages as illustrated in Figure (1.2). The first stage in preprocessing is data reduction. Here, the image data can be reduced by gray-level and/or spatial quantization, or they can undergo any desired image enhancement (for example, noise removal) process. The second step in preprocessing is the mapping process, which maps the original image data into another mathematical space where it is easier to compress the data. Next, as part of the encoding process, is the quantization stage, which takes the potentially continuous data from the mapping stage and puts it in discrete form. The final stage of encoding involves coding the resulting data, which maps the discrete data from the quantizer onto a code in an optimal manner. A compression algorithm may consist of all the stages, or it may consist of only one or two of the stages.
The decomposer can be further broken down into the stages shown in Figure (1.3). Here the decoding process is divided into two stages. The first, the decoding stage, takes the compressed file and reverses the original coding by mapping the codes to the original, quantized values. Next, these values are processed by a stage that performs an inverse mapping to reverse