You are staring at the your monitor waiting for the image to download. It finally appears but it has blurry edges. You go to the next page but can't read the text because of the dark image in the background. The next page has animated images, that don't seem to stop. The spinning globes keep spinning. The last page has a large graphic on it, which is a link. You click on it for more information but it goes nowhere. You leave the site in frustration.

Images are an essential ingredient for Web Site design. You want visitors to have an aesthetically pleasing experience. Properly preparing your images is necessary toenhance the appearance of your web site. In Part I of this article I will explain:

• When to use images for your web site?
• What image file formats should you use on the Web?

When to use images for your web site?

Navigation. Graphical buttons can link to other pages or resources.

• Image Maps

  This is a graphic that contains several links on it. It has several "hot spots" or invisible buttons, you can click on. For example you could have a photograph of your family and put a hot spot on each person's face that links to that person's web site.

• Logos and Trademarks

  Your business or organization's trademark are crucial for name recognition and branding.

• Thumbnails

  This is a small, "thumbnail-sized version of an image. When you click on it, you jump to another page with a larger version of the same image. The visitor can see many different, small images on the first page without having to wait for larger files of the larger images to load.

What are the different image file formats?

Web graphics can be categorized into two file formats: bitmap and vector.

• Bitmap

  These are composed of individual values for each color displayed. The larger the dimensions of the image, the larger the associated file size will be for the same graphic. When viewed with magnification, a bitmap resembles a series of little squares, each of which has a color value that contributes to the overall shape. Bitmaps have a very rough appearance when viewed closely but form images when viewed from a distance.

  Bitmaps are best suited for photos, drop-shadow effects and soft, glowing or blurry edges.

• Vector

  These store information about the image in mathematical instructions that are then interpreted and displayed.

  Vector graphics are best suited for line art, shapes and illustrations.

Image File Formats

Graphic file formats used on the Web are GIF, JPEG and PNG

• Graphics Interchange Format (GIF)

  GIF is a platform-independent file format that is limited to a display of 256 colors. GIF has been adopted by most developers because of its small file size.

  GIF is considered a "lossless" format. This means that as the image is compressed, no information is lost.

  Types of GIFs

  • Animated GIF (89A)

    This 89a version of GIF allows storage and playback of a sequence of still images to create the illusion of animation. Animated GIF files consist of sequential frames that reload from a browser's cache and replay in an infinite or predetermined loop to simulate motion.

  • Transparent GIF

    An advantage that a GIF has over a JPEG image is that the designer can designate a color of the GIF image to be transparent. For example, you can create a circular logo in a square image by making the background color transparent. The image appears circular, when, in fact, it is square with information to appear transparent.

  • Interlaced GIF

    Graphic interlacing (the progressive rendering of images) is unique to GIFS and is the preferred method for display of large graphic files. Many people find the "fuzzy-to-sharp" animated effect of interlacing visually appealing, but the most important benefit of interlacing is that it gives the reader a preview of the full area of the picture, while the picture downloads into the browser.

  When to use a GIF

  Buttons, bullets and navigational tools that accent your Web pages. Interlacing is best for larger GIF images such as illustrations and photographs.

• Joint Photographic Experts Group (JPEG)

  Graphics in the JPEG format are capable of much greater color depth than GIFs, but usually require more time to download. JPEG can contain up to 24 bits of color information (16.7 million colors). Remember though, that most users are only capable of displaying 8-bit color.

  When to use a JPEG

  JPEG enables you to use brilliant colors and provides support for complex images and digitized photographs but it is not designed for use with simple images.

  JPEG compression is not as effective as GIF compression and may distort images with few colors or large areas of the same color. JPEG compression is therefore not designed for low-resolution images.

• Portable Network Graphics (PNG)

  The PNG file format is emerging as the new format for Web graphics. PNG files are lossless and support transparency like GIFs, yet also support compression and high bit depth like JPEGs. In addition, PNG bit depth can be adjusted, unlike GIFs or JPEGs, which must be 8-bit and 24-bit depth.

  When to use a PNG

  PNGs behave similarly to GIFs and work best with flat-color, sharp-edged art. PNGs compress both horizontally and vertically, so solid blocks of color generally compress best. They also support  better interlacing than interlaced GIFs.

Knowing what types of graphics to use and when to use them for your web site will help you avoid the many pitfalls of bad web design.

Part 2of this article will discuss how to optimize graphics for your web site and factors that affect optimization.

Copyright 2002 Herman Drost