The following is a Plus Edition article written by and copyright by Dick Eastman.
This is an update to a Plus Edition article I published nearly a year ago. The article has been expanded significantly with additional information, so I am publishing it again.
Genealogists and millions of others have saved hundreds of millions of digital photographs on their hard drives as well as on CD-ROM disks. Perhaps the most popular file format for digital photographs is JPG (or JPEG), a commonly used method of compression for photographic images. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10 to 1 compression with little perceivable loss in image quality.
JPEG is the most common image format used by digital cameras and other photographic image capture devices, such as scanners. It is also the most common format for storing and transmitting photographic images on the World Wide Web.
NOTE #1: The abbreviation "JPEG" stands for Joint Photographic Experts Group, the name of the committee that created the standard. The group was organized in 1986 and issued a standard in 1992, which was approved in 1994 as ISO 10918-1. The JPEG standard specifies both the codec, which defines how an image is compressed into a stream of bytes and decompressed back into an image, and the file format used to contain that stream.
NOTE #2: Several compression methods have been developed for graphics images, including JPEG, GIF, and PNG. However, JPG remains the most popular for storage of photographs. I will focus only on JPG. Most of the statements in this article apply equally to all the other compressed formats.
Note #3: Lossless JPEG was developed in 1993, using a completely different technique from the lossy JPEG standard. Even though both are called JPEG (or JPG), the two are completely different standards. Lossless JPEG has some popularity in medical imaging and is used in DNG and some high-end digital cameras to compress raw images, but otherwise has never been widely adopted. You will rarely encounter lossless JPG images on the World Wide Web or in consumer-grade digital cameras or scanners. You can read more about lossless JPEG at http://en.wikipedia.org/wiki/Lossless_JPEG. For the remainder of this article, I will focus on normal JPEG compression.
JPG has become very popular in the past eighteen years and for very good reasons. Storing images in JPG format consumes much less disk space than many other file formats. That was very important some years ago. However, as disk prices have plunged, the requirements for squeezing as much out of each kilobyte as possible have decreased.
As good as JPG is, we do need to keep in mind that it also has significant drawbacks.
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