
Under the "Options" part of this window, you'll see an option called Dither. Dithering is the process of having two similar colors mesh in a pattern in order to give the illusion of a color (or colors) in between. Turn this to "none" if your image has no smooth gradients (such as line art, or an unshaded cartoon), and your solid colors will remain solid. Otherwise, turn this to "Diffusion" and your gradients will be interpolated with a random pattern. There is another setting, called "Pattern", which can dither based on an image in the pattern buffer, but that's a very specialized option.
Underneath Dither, always keep color matching to Best rather than Faster, unless you have a dreadfully slow computer. Also, the "Preserve Exact Colors" option provides a more accurate automatic palette than Photoshop version 4 had.
Above, you'll see the "Palette" part of the window. This is usually set to "Adaptive", unless your image has a very small number of colors, in which case it sets itself to "Exact". When in Adaptive mode, you can limit how many colors are used. GIFs with 256 colors tend to be very large, so I recommend experimenting with these options to see what the least number of colors you can have is with the image still looking good.
To the right, you'll see a close-up of a 64-color version of our Wolf. See that patchy
posterization in the shading? That's dither. This exaggerated dither is why I don't
recommend GIFs for high-color graphics.
Here you see a grayscale version of the same image, with the same number of colors in
the limited palette. Since the palette colors are shared between all the nulled hues now,
the gradient is a lot smoother.
Once you have your palette set right, hit OK on the Indexed Color window. Go to File >
Save As, and save your file. You'll get one more option box where you can select "Normal"
or "Interlaced". You've probably seen Interlaced images load on the Internet. They start
loading rough and gain detail as it loads. This option doesn't affect filesize much, so
it's pretty much a judgment call.
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This is from a JPG saved at quality 5. It's 20 KB, and has visible artifacts (artifacts are the strange errors near the lines caused by lossy data compression). |
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This is from a 256-color GIF. It's very crisp, has no artifacts, and is 17KB. |
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This is from a 16-color GIF. The edges show an almost unnoticeable amount of banding, but it has no artifacts and is a mere 9KB. |
GIF animation is not directly supported by Photoshop. There is a new program that is shipped with Photoshop 5.5 (or sold separatley) called ImageReady which takes Photoshop PSD files and can create GIF animations by turning the available layers on and off, editing their options, or moving them, but that's getting into an entirley new territory.