Core_Computer Studies: JPEG

JPEG was developed by the Joint Photographic Experts Group the original name of the committee that wrote the standard. Yes, we are dealing with something that works that was designed by committee! JPEG, (pronounced "jay-peg") like GIFs, are image files that Web browsers like Netscape Navigator and Internet Explorer can read and display. GIFs are limited to use on-screen because of their index color mode. JPEGs are bitmap images that can be used for more. The files are recognizable by their file extension, which is .jpg or .jpeg.

JPEG compresses either full-color or gray-scale images and used mainly for photographic and other continuous-tone images because of their 24-bit color range. This is not surprising as they were developed for that purpose.

You can save a JPEG at whatever resolution is required in either RGB or CMYK color mode. In other words unlike the GIF format, JPEG retains all the color information in the RGB image. JPEGs also have an image compression option which can prove helpful for transporting them over a network or via other storage and transport media such as CD-ROMs and Zip disks. Opening a JPEG image automatically decompresses it.

However when we use image compression we must be aware that you will lose a certain amount of data contained in the image file is discarded. JPEG compression is a 'lossy' form of compression which means that, when it is used, the data you lose is gone you can't get it back. In other words the decompressed image isn't quite the same as the one you started with. This compression method tricks the eye because people perceive small color changes less accurately than small changes in brightness. The key thing to remember when you compress an image using JPEG is that you are changing the image.

I always save a file in the format that I have created it in and archive it to CD that way if ever I want the original I have it.

An advantage however is that you can decide, when saving the image, how much you want the file compressed. With a large amount of compression, you have smaller file size, ideal for the web but lose quite a bit of data from the image. With small amounts of compression, one will have a larger sized file but very little data will be lost from the image. So when working with JPEG image compression, aim to find the acceptable balance for a particular image and its intended uses.

These notes are written for students who undertake the Core Computer course at School of Art, Australian National University. Students of the course should feel free to contact any member of staff with comments questions and general suggestions..

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