**2. Fundamental of compression**

The main goal of all image compression techniques is to minimize the number of bits required to represent a digital image, while preserving an acceptable level of image quality. Image data are amendable to compression due to the spatial redundancies they exhibit and also because they contain information that, from a perceptual point of view, can be considered irrelevant. Many standard and non-standard image compression techniques have been developed to compress digital images. These techniques exploit some or all of these image properties to improve the quality of the decoded images at higher compression ratios. Some of these image coding schemes are tabulated in Table 1.

Image compression techniques can be classified into two main groups, named: lossless and lossy compression techniques. In lossless compression process, the original data and the reconstructed data must be identical for each and every data sample. Lossless compression is demanded in different applications such as: medical imagery, i.e. cardiography, to avoid the loss of data and errors introduced into the imagery. Also, it is applied to the case that is not possible to determine the acceptable loss of data.

In most image processing applications, there is no need for the reconstructed data to be identical in value with its original. Therefore, some amount of loss is permitted in the reconstructed data. This kind of compression techniques, which results in an imperfect reconstruction, is called lossy compression. By using lossy compression, it is possible to represent the image with some loss using fewer bits in comparison to a lossless compression.
