With the Gaussian Blur filter, you can control the amount of blurring, but the only way to specify exactly what area you want to blur is to create a selection outline. With the Smart Blur filter, you can limit the blur to areas that don't contain significant edges -- that is, you can blur just low-contrast regions and leave high-contrast areas untouched. To put it another way, the filter looks for strong color boundaries and then blurs pixels within those boundaries but not along the boundaries themselves.

This can be helpful at times, for example, when a photograph has been subjected to too much JPEG compression, a file-saving option that can cause smooth areas to look jagged or result in random, unnatural specks of color.

To apply the Smart Blur filter, select the area that you want to blur and choose Layer, New, Layer via Copy or press Ctrl+J. Then choose Filter, Blur, Smart Blur. Although the dialog box controls look intimidating at first, you really need to worry about only the top two options, Radius and Threshold. But just for good measure, here's what all four options do:

• Radius controls the distance of the blur, similar to the Radius option in the Unsharp Mask filter. A higher value results in a more pronounced blur effect.
• Threshold tells the program how much contrast must exist between two areas for the area to be considered an edge. As you increase the Threshold value, more and more pixels receive the blur.
• Quality affects the way that the program processes the image preview and the filter application. A higher setting produces smoother results, but requires more time for the program to produce. While you're playing with the settings in the dialog box, you can set the option to Low to get quicker previews, but before you actually apply the filter, always set the option to High.
• Mode is an odd duck. For standard image blurring, use the Normal setting. Edge Only and Overlay Edge create special effects. With Edge Only, you get a black-and-white, line-drawing image. With Overlay Edge, you get a bizarre blurred and brightening effect that you'd have to see to believe.