17-09-2014, 04:26 PM
Coding techniques are crucial for the effective transmission or storage of data-intensive visual information. In fact, a single uncompressed colour image or video frame with a medium resolution of 500 x 500 pixels would require 100 s for transmission over an ISDN (Integrated Services Digital Network) link having a capacity of 64,000 bit (64 Kbps).The resulting delay is intolerably large, considering that a delay as small as 1-2 s is needed to conduct an interactive “slide show,” and a much smaller delay (of the order of 0.1 s) is required for video transmission or playback. As a result compression is essential for the storage and real-time transmission of digital audiovisual information, where large amounts of data must be handled by devices having a limited bandwidth and storage capacity.Image compression is minimizing the size in bytes of a graphics file without degrading the quality of the image to an unacceptable level. The reduction in file size allows more images to be stored in a given amount of disk or memory space. It also reduces the time required for images to be sent over the intense or download from web pages.There are several different ways in which image file s can be compressed. For internet use, the two most common compressed graphic image formats are the jpeg format and the gif format .the jpeg method is more often used for photographic .while the gif method is commonly used for line art and other images in which geometric shapes are relatively simple.