When a GIF or PNG-8 image is on a transparent background, you can maintain the transparency by selecting the Transparency check box in the Save for Web dialog or the ImageReady Optimize palette. When this option is not selected, the transparent areas of the image are filled with the matte color. If the Matte pop-up menu is set to None, white is used. The Transparency check box must be active when you map colors to transparency using the color table.
The Transparency Dither option softens the transition from colors to transparency. It works similarly to the standard Dither option (discussed in the section "Dithering," earlier in this chapter), but instead of blending colors, it blends from color to transparent.
As with regular dithering, you're offered the choice of Diffusion, Pattern, and Noise methods for Transparency Dither. In addition, when using the Diffusion method, you can choose the amount of diffusion dithering to apply by using the slider to the right in the Settings area.
If you deselect the Transparency option, the matte color is used to fill transparent areas of the image. The matte color can be used to blend the image with a solid-color web page background. The options are None (no matte), Eyedropper Color (the color most recently selected with the Save for Web dialog's Eyedropper tool), White, Black, and Other (which opens the Color Picker). ImageReady doesn't offer the Eyedropper Color option, but it does display a palette of the 216 web-safe colors from which you can select.
If your image has a transparent background and uses feathering or anti-aliasing, selecting your web page's background color in the Matte pop-up menu can prevent or reduce any halo around your image.
Interlaced GIF and PNG-8 images appear in the web browser as they download. Although it seems that the image is loading faster, interlacing doesn't actually speed up the process. Rather, the viewer gets feedback while the page loads. Interlacing is recommended for images over a few kilobytes in size.