Tutorial: What’s Inside a JPEG File?

Source: The Society for Imaging Science and Technology
http://www.imaging.org/resources/web_tutorials/inside_jpeg/inside_jpeg.cfm

As common as these files are in everyone’s life, very few users actually know what goes on inside a JPEG file and still fewer understand the many pitfalls associated with using this choice when storing their digital photographs. This essay will describe the inner working of this file format that we so commonly use, highlighting its many merits and its many flaws. Like any tool that is available to us, there are correct ways to use the JPEG file format and there are times when it is a poor choice.
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