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Optimizing Web Graphics
The Bandwidth Shortage
By Andrew King
If the Web seems slow to you, you're not alone. The network's backbone, which can handle up to 155 Mbps (soon to become 600 Mbps), isn't the problem. Instead, it's what happens at each end that frustrates users. The increasing size of digital media and lack of server bandwidth are the main culprits. More bandwidth won't necessarily solve this problem. What will help is minimizing the amount of data that travels through this bandwidth. Graphics dominate the size of most Web pages today, and the secret of shrinking graphic-file size is to reduce bit depth, resolution, and dimension while preserving image quality. This classic size-versus-quality tradeoff is the key to the art and science of graphics compression. This article will show you how to minimize file size and dithering for Web graphics.
Color Palettes
The two ways to store color raster images are indexed and RGB. Indexed formats are mapped to a 256-color (or less) lookup table (CLUT) or color palette. RGB, or "true-color," formats use eight bits (0ý255) for each Red, Green, and Blue value to form a 24-bit (8+8+8=24) pixel palette that can hold one of 16.7 million (224=16,777,216) colors. Some formats support even higher bit depths, useful for medical imaging or smooth transparencies.
A CLUT is a digital version of a paint store's color-mixing chart: It contains up to 256 entries, each with its own formula for a specific color. Indexed images refer to each color by its position in this palette.
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