Adobe 23101335 User Guide - Page 334

324 CHAPTER 12 Optimizing Images for the Web 7 To modify the color reduction using an alpha channel, click the channel button ( ) next to the Color Reduction Algorithm text box. In the Modify Color Reduction dialog box, choose an alpha channel from the pop-up menu, and click OK. (See "Using channels to modify color reduction" on page 329.) 8 To specify the maximum number of colors in the color palette, select a number from the Colors pop-up menu, enter a value in the text box, or use the arrows to change the number of colors. If the image contains fewer colors than the number specified, the color table will contain only the number of colors in the image. You can choose the Auto option when working with a fixed color palette. This option determines the number of colors in the color table based on the frequency of colors in the image. Choose Auto if you want Photoshop or ImageReady to determine the optimal number of colors in the color table. 9 Select a Dither option and amount. For information on dither, see "Previewing and controlling dithering" on page 341. 10 To vary the Dither setting across the selected slices using an alpha channel, click the channel button ( ) next to the Dither text box. In the Modify Dither Setting dialog box, choose an alpha channel from the pop-up menu, set a dither range, and click OK. (See "Using channels to modify dithering" on page 329.) 11 If the image contains transparency, choose an option for preserving or filling transparent pixels: • Select Transparency (the default setting) to preserve fully transparent pixels as transparent. (Partially transparent pixels are filled with the Matte color or converted to fully transparent or fully opaque pixels, depending on the Matte option you choose.) • Deselect Transparency to fill fully and partially transparent pixels with the Matte color. For more information on setting transparency and Matte color, see "Making transparent and matted images" on page 339. 12 Select Interlaced to create an image that displays as low-resolution versions in a browser while the full image file is downloading. Interlacing can make downloading time seem shorter and assures viewers that downloading is in progress. 13 To automatically shift colors to the closest Web palette equivalents (and prevent the colors from dithering in a browser), drag the Web Snap slider or enter a value to specify a tolerance level for colors to be shifted. A higher value shifts more colors. See "Previewing and controlling dithering" on page 341. 14 To apply a unified color table across all rollover states, select Use Unified Color Table.