Sams Teach Yourself HTML 4 in 24 Hours

By Dick Oliver

• Table of Contents
  • Copyright
  • About the Author
  • Acknowledgments
  • Tell Us What You Think!
  • Put Your HTML Page Online Today
  • I. Your First Web Page
  • Hour 1. Understanding HTML and XML
  • Hour 2. Create a Web Page Right Now
  • Hour 3. Linking to Other Web Pages
  • Hour 4. Publishing Your HTML Pages
  • II. Web Page Text
  • Hour 5. Text Alignment and Lists
  • Hour 6. Text Formatting and Font Control
  • Hour 7. Email Links and Links Within a Page
  • Hour 8. Creating HTML Forms
  • III. Web Page Graphics
  • Hour 9. Creating Your Own Web Page Graphics
    • Choosing Graphics Software
    • Graphics Basics
    • Preparing Photographic Images
    • Controlling JPEG Compression
    • Creating Banners and Buttons
    • Reducing the Number of Colors
    • Interlaced GIFs and Progressive JPEGs
    • Summary
    • Q&A
    • Workshop
  • Hour 10. Putting Graphics on a Web Page
  • Hour 11. Custom Backgrounds and Colors
  • Hour 12. Creating Animated Graphics
  • IV. Web Page Design
  • Hour 13. Page Design and Layout
  • Hour 14. Graphical Links and Imagemaps
  • Hour 15. Advanced Layout with Tables
  • Hour 16. Using Style Sheets
  • V. Dynamic Web Pages
  • Hour 17. Embedding Multimedia in Web Pages
  • Hour 18. Interactive Pages with Applets and ActiveX
  • Hour 19. Web Page Scripting for Non-Programmers
  • Hour 20. Setting Pages in Motion with Dynamic HTML
  • VI. Building a Web Site
  • Hour 21. Multipage Layout with Frames
  • Hour 22. Organizing and Managing a Web Site
  • Hour 23. Helping People Find Your Web Pages
  • Hour 24. Planning for the Future of HTML
  • VII. Appendixes
  • A. Readers' Most Frequently Asked Questions
  • B. HTML Learning Resources on the Internet
  • C. Complete HTML 4 Quick Reference
  • D. HTML Character Entities

Interlaced GIFs and Progressive JPEGs

Both the GIF and JPEG image file formats offer a nifty feature that makes images appear faster than they otherwise could. An image can be stored in such a way that a "rough draft" of the image appears quickly, and the details are filled in as the download finishes. This has a profound psychological effect, because it gives people something to look at instead of drumming their fingers, waiting for a large image to pour slowly onto the screen.

A file stored with this feature is called an interlaced GIF or progressive JPEG. Despite the different names, the visual results are similar.

An interlaced GIF file is an image that appears blocky at first, and then more and more detailed as it finishes downloading. Similarly, a progressive JPEG file appears blurry at first and then gradually comes into focus.

Most graphics programs that can handle GIF files enable you to choose whether to save them interlaced or noninterlaced. In Paint Shop Pro, for example, you can choose Version 89a and Interlaced by clicking the Options button in the Save As dialog box just before you save a GIF file (see Figure 9.10).

Figure 9.10 Paint Shop Pro lets you save interlaced GIF images, which appear to display faster when loading.

To save a progressive JPEG file, select Save As (or Save Copy As), choose the JPG-JPEG-JFIF Compliant image type, click the Options button, and select Progressive Encoding. The progressive JPEG standard is quite new and is only supported by Netscape Navigator version 2 or later and Microsoft Internet Explorer version 3 or later.

Browsers that don't support progressive JPEG will not display the file as if it were just a regular JPEG—they will display either nothing at all or a message saying the file isn't recognizable. Interlaced GIFs, on the other hand, appear correctly even in older browsers that don't support two-stage display.

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