Image compression is an effective technique for reducing the size of a graphical file without degrading the quality of the image and reduces the storage requirement area. It additionally reduces the time required for images to send over the Internet or downloaded from Web pages.
There are many ways in which image files can be compressed. In 1987, Fractal compression invented by Barnsley and Alan Sloan. It is a very helpful technique in the field of image compression. It is an asymmetrical compression method, That means it takes more time/effort to compress an image rather than decompressing it. This technique depends on the existence of self-symmetry in an image. It has the benefit of faster decompression speed while giving equal or better compression ratio. However, the lengthy compression stepped remains the main drawback of this technique, which will preclude it from being used in applications that need to send compressed images with minimal delay.
In 1992, JPEG standard created by Joint Photographic Experts Group based on the discrete cosine transform(DCT), It is a symmetrical compression method, That means the compression and the decompression processes take approximately the same amount of time/effort.
The JPEG compression algorithm utilized for the web, it works better on images with smooth variations of color. It divides an image into 8 × 8 blocks, then it converts the intensity data of the pixels into their frequency space equivalents. The result is a set of 64 cosine functions with various amplitudes. It is compressed by discarding higher frequency cosine terms.
The major drawback of DCT, While inputs from reprocessed 8 × 8 blocks are integer-valued, the output values are usually real-valued. So, we need a
quantization step to make some decisions about the values in each DCT block and produce output that is an integer-valued.
This technique is not appropriate to line drawings and other textual or iconic graphics, where the sharp contrasts between adjacent pixels can cause noticeable artifacts.