Have you ever wondered why the Images in facebook appear blurred at first ? For those who don’t know here is the answer.

We all know data is stored in different formats like mp3, jpeg, doc etc, and Images are stored as jpeg, gif, bmp, png and many other such formats. Of these Jpeg is the most commonly used format for storing images.

The name “JPEG” stands for Joint Photographic Experts Group, the name of the committee that created the JPEG standard, It is a commonly used method of lossy compression for images. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality

A simple or “baseline” JPEG file is stored as one top-to-bottom scan of the image.  Progressive JPEG divides the file into a series of scans.  The first scan shows the image at the equivalent of a very low quality setting, and therefore it takes very little space.  Following scans gradually improve the quality.  Each scan adds to the data already provided, so that the total storage requirement is roughly the same as for a baseline JPEG image of the same quality as the final scan.

The advantage of progressive JPEG is that if an image is being viewed on-the-fly as it is transmitted, one can see an approximation to the whole image very quickly, with gradual improvement of quality as one waits longer; this is much nicer than a slow top-to-bottom display of the image.  It’s all a matter of how much you want to see, and how quickly. Or rather, how much the image creator wants you to see, and how quickly. The disadvantage is that each scan takes about the same amount of computation to display as a whole baseline JPEG file would.  So progressive JPEG only makes sense if one has a decoder that’s fast compared to the communication link. (If the data arrives quickly, a progressive-JPEG decoder can adapt by skipping some display passes.  Hence, those of you fortunate enough to have faster net links may not see any difference between progressive and regular JPEG; but on a modem-speed link, progressive JPEG is great.)Except for the ability to provide progressive display, progressive JPEG and baseline JPEG are basically identical, and they work well on the same kinds of images.

Up until recently, there weren’t many applications in which progressive JPEG looked attractive, so it hasn’t been widely implemented.  But with the popularity of World Wide Web browsers running over slow modem links, and with the ever-increasing horsepower of personal computers, progressive JPEG has become a win for web use.

{Refered from different sources in the internet}