Image Optimization Part 2

Image formats:
It doesn’t take much experience with images on computers to run into a GIF file. GIF is a popular format that constitutes a significant portion of images used on the Web. But GIF is only one of a number of different graphics standards. JPEG, PCX, TIFF, and others are also widely used image file types (not all used on the Web). Is it possible to use any image format in an IMG tag? The short answer is no. Among all the various formats, the browser itself can handle only a handful. Basically there are three (possibly four) that are used in an IMG tag, GIF, JPEG, and XBM (check for PNG support). Other formats can still be displayed on the Web, but not by using the IMG tag and they usually require a special plug-in or application helper. Each of the three formats that can be displayed by an IMG tag has different characteristics. Examining these might make deciding which standard to use easier.

XBM
XBM is an X Window bitmap, and only supports two colors. Most PC and Mac browsers can display these, but if you don't have UNIX, you might have a problem finding the utilities that create them. GIF and JPEG, on the other hand, are standard formats for both the PC and the Mac.

GIF
GIF (Graphics Interchange Format) supports a maximum of 256 colors and is generally preferred when you want crisp lines (such as when text appears in the image, as in some icons), or have images with large blocks of single colors.

JPEG
JPEG (Joint Photographic Experts Group) supports up to 16.7million colors and is very good at compressing photographic and photo-realistic images. You can control the amount of compression, trading size for quality.

Below you will find a summary of the image formats discussed above plus some others not mentioned. Tips and tricks concerning the optimization of these images will be discussed in part 4 of this series.

GIF	CompuServe's Graphics Interchange Format. 256 colors, has special capabilities such as transparency and interlacing. Uses LZW compression. Widely available.
JPG, JPEG	Joint Photographic Experts Group. 16 million colors. Very efficient compression for photographic images. User controllable trade-off in image quality versus file size.
PNG	A format that supports more colors than JPEG, compresses files 10 to 30 percent smaller than GIF, has multiple levels of transparency, and uses a faster interlacing technique for displaying images more quickly. (Check for browser support)
XBM	Bitmap image for X Window. 2 colors.
BMP	Windows bitmap. Native to Windows 3.x and 9.x OS's. 16 million colors. RLE compression. Generally larger than other compressed image formats.
PICT	Macintosh Picture formats. A native format on the Mac, analogous to BMP on Windows. Supports 16 million colors, is based on QuickDraw display language.
TIFF	Tag Image File Format. A popular cross-platform image format for migrating between Mac and PC. 16 million colors. There are several flavors of TIFF, which sometimes leads to compatibility problems across platforms if options are not carefully matched.
PCX	PC Paintbrush formats. 16 million colors.
TGA	TrueVision's Targa Format. 16 million colors. Used in ray-tracing applications on PC and UNIX platforms (like POV).

Go to Optimizing Images for the Web Part III

About the author...
Charles Brown is a freelance writer specializing in Internet and consumer related articles. He is also a content writer for EWM / PSI (Everyone Working Mutually providing Products Services & Ideas). Mr. Brown is also the author of a new series of eBooks that will begin publication in mid February, 2001. "The Art of Complaining © 1998 Charles E. Brown" will be a how-to on writing effective letters of complaint. "The Art of eShopping © 2000 Charles E. Brown" which will be a how-to on safe and efficient internet shopping. Scheduled for publication in July/August, 2001are "The Art of Web Design © 2000 Charles E. Brown" and "The Art of Desktop Publishing © 2000 Charles E. Brown".

http://www.productsservicesideas.com

Optimizing Images for the Web - Part II