Brevard User's Group - Digital Imaging SIG

Brevard Users Group - Digital Imaging Graphics Images Colors

SIG Meets at 7:30 on the 2nd and 4th thursday's,
after the Windows SIG. At the Melbourne Library.

GIF vs. JPG--Which Is Best?

Books used by the Imaging SIG for Photoshop® Elements
	PhotoShop Elements Solutions By: Sybex Softcover 304 pages September 2001 ISBN: 0-7821-2973-0
	50 Fast Digital Photo Techniques By: Gregory Georges Paperback 336 Pages July 2001 ISBN: 0-7645-3578-1
	Photoshop Elements 3 For Dummies By: Deke McClelland, Galen Fott Paperback 384 Pages December 2001 ISBN: 0-7645-1636-1

Graphics Interchange Format

     The GIF (Graphics Interchange Format) format was invented in 1987 by Compuserve to allow images to be displayed. This format allows for 256 colors (which was a lot at the time), compression, interlacing and animation. It is a very powerful format, suitable for many different types of images. Due to the limited number of colors, GIF is primarily useful in images with a distinct separation of colors. A cartoon, for example, is ideal for the GIF format. When you save an image in GIF format, you have the option to specify how many colors will be saved. By doing this you can decrease the size of an image even further. All of the tools which are available to optimize GIF images work by reducing the number of colors to the bare minimum. This can produce astounding results in the size of the finished file.
     Unlike JPEG, GIF uses a non-lossy compression algorithm. This means that images do not loose bits when they are decompressed. In order to accomplish this, GIF uses a proprietary encoding/decoding scheme called LZW (Lempel Zev Welch). LZW is an excellent compression algorithm which typically results in very small files (in comparison to fully expanded BMP files).
     This compression method is actually the cause of a bit of controversy. As it turns out, LZW is owned by UniSys, and over the past few years they have made some attempts to collect licensing fees for products which save in the GIF format. These attempts have had mixed results, and has caused the development of a new non-proprietary standard called PNG. You may indicate that a color in a GIF image is transparent, meaning whatever is below the image will show through. The GIF89a standard included animation, which basically allows multiple images to be included in a single GIF file. In that image you can specify the timing that the frames are shown and whether or not the animation stops or loops forever.
     Interlacing is a cool feature which allows graphics to be "faded in". Initially a very low quality image is displayed, and bits are slowly added as the entire image is received. GIF supports this feature, which is useful for displaying very large images quickly. An interesting albeit little known fact about GIF images is that they can include comments, which take up space and make the graphic larger. There are a number of products which will remove these comments. I found that the freeware program called GIFclean is simple to use and works well.

Joint Photographic Experts Group

     JPEG (Joint Photographic Experts Group) is a very efficient, true-color, compressed image format. It uses lossy compression, which means that bits are removed from the image in order to save space. JPEG files support millions of colors (compared with the 256 for GIF). The JPG format is best for images with gradients, blends, and inconsistent color variations such as photographs or paintings. Images which have well separated tones should be saved in the GIF or PNG format. For example, if you include text in your image, you will notice a definite fuzzing of the characters when you view it.
     You have several options when optimizing JPG images. JPG is a lossy format (which means it throws away bits), so with each generation of the image you save you will loose information, and that will degrade the quality of the image. Any good image editor (Photoshop, Paint Shop Pro and others) will allow you to specify a compression percentage when you save the file. Before you purchase a graphics editor make sure it has a preview pane on the save so you can see what the compressed image looks like when you save it. This is a lot easier than saving a copy, examining it, resaving it and so on.
     JPEG supports a concept similar to interlacing in GIF which is called progressive JPEG. This simply means that a rough image is displayed initially, followed by more and more detail as additional bits of the image are received. This is good for displaying large images. Unfortunately, progressive JPEG is a relatively new standard and is not supported by all browsers.

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Brevard User's Group - Digital Imaging SIG

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