May 24, 2004
At lower bit-depths, color quantization can reduce smooth color gradients into discernible bands of color (see Figure 1). Dithering is the process of changing pixels in these color transition zones to minimize banding. This feathering process strategically places patterns of available colors to emulate colors eliminated in the color quantization process. The resulting image appears smoother, but dithering makes file sizes larger (see Figure 2).
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US broadband penetration grew by 1.9 percentage points in April, with 47.87% of active Internet users enjoying a high-speed connection at home. This jump of nearly 2 points is 2.5 times the average rate of broadband growth. 52.13% of US home users dial into the Internet with "narrowband" connections of 56Kbps or less. Meanwhile, broadband growth in other countries suggests a plateau at 75% saturation.
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May 17, 2004
Histogram influencing improves GIFs or PNGs by weighting the color palette that your graphics program creates. When reducing colors in an image, by default Photoshop uses the entire image to create the color palette. You can optimize this palette by creating selections over troublesome areas. In many cases, histogram influencing allows lower bit-depths, resulting in smaller file sizes.
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May 10, 2004
For palette-based formats like GIF and PNG, file size is directly related to the size of the color palette, or the number of colors in the image. As the number of colors in an image crosses a power of two, the file size jumps. A 33-color image must use a six-bit palette, while a 32-color image can use a 5-bit palette. Smaller palettes mean smaller codes (representing pixel patterns), which makes for smaller files. So minimizing the number of colors in index-color images like GIFs and PNGs will minimize file size.
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May 3, 2004
Substituting CSS-based techniques for old-style graphic rollovers and DHTML menus is one of the most effective web site optimization techniques available. Graphic rollovers use "on" and "off" graphics to create rollover effects, with and without JavaScript. DHTML menus create nested menu structures with JavaScript, some of it complex. With the widespread adoption of CSS2-aware browsers, there is a better choice: CSS-based techniques. Substituting list-based markup, CSS2 to style, and an optional dash of JavaScript makes for fast, small, and accessible CSS menus.
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