Adobe 13101332 User Guide - Page 421

Adobe Photoshop Help Using Help | Contents | Index Preparing Graphics for the Web Back 421 Dithering Dithering refers to the method of simulating colors not available in the color display system of your computer. Images with primarily solid colors may work well with no dither. Images with continuous-tone color (especially color gradients) may require dithering to prevent color banding. GIF image with 0% dither, and with 100% dither Choose a option from the Dither Algorithm menu: • No Dither applies no dither to the image. • Diffusion applies a random pattern that is usually less noticeable than Pattern dither. The dither effects are extended across adjacent pixels. If you select this algorithm, specify a Dither percentage to control the amount of dithering that is applied to the image. A higher dithering percentage creates the appearance of more colors and more detail in an image, but can also increase the file size. You can use masks from type layers, shape layers, and alpha channels to vary the Dither percentage across an image. This technique produces higher-quality results in critical image areas without sacrificing file size. (See "Using weighted optimization" on page 428.) Note: Diffusion dither may cause detectable seams to appear across slice boundaries. Linking slices diffuses the dither pattern across all linked slices, and eliminates the seams. • Pattern applies a halftone-like square pattern to simulate any colors not in the color table. • Noise applies a random pattern similar to the Diffusion dither algorithm, but without diffusing the pattern across adjacent pixels. No seams appear with the Noise algorithm. Transparency and Matte Select a combination of Transparency and Matte options to specify how transparent pixels in the image are optimized: • To make fully transparent pixels transparent and blend partially transparent pixels with a color, select Transparency and select a matte color. • To make all pixels with greater than 50% transparency fully transparent and all pixels with 50% or less transparency fully opaque, select Transparency and choose None from the Matte menu. Using Help | Contents | Index Back 421