Blurring with a threshold

The purpose of the Filter ^ Blur ^ Smart Blur is to blur the low-contrast portions of an image while retaining the edges. This way, you can downplay photo grain, blemishes, and artifacts without harming the real edges in the image. (If you're familiar with Filter ^ Pixelate ^ Facet, it may help to know Smart Blur is essentially a customizable version of that filter.)

The two key options inside the Smart Blur dialog box (see Figure 10-30) are the Radius and Threshold slider bars. As with all Radius options, this one expands the number of pixels calculated at a time as you increase the value. Meanwhile, the Threshold value works just like the one in the Unsharp Mask dialog box, specifying how different two neighboring pixels must be to be considered an edge.

Figure 10-30: The Smart Blur filter lets you blur the low-contrast areas of an image without harming the edges.

But the Threshold value has a peculiar and unexpected effect on the Radius. The Radius value actually produces more subtle effects if you raise the value beyond the Threshold. For example, take a look at Figure 10-31. Here we have a grid of images subject to different Radius and Threshold values. (The first value below each image is the radius.) In the top row of the figure, the 5.0 radius actually produces a more pronounced effect than its 20.0 and 60.0 cousins. This is because 5.0 is less than the 10.0 threshold, while 20.0 and 60.0 are more.

5.0, 10.0 20.0, 10.0 60.0, 10.0

Figure 10-31: Combinations of different Radius (first number) and Threshold (second) values. Notice that the most dramatic effects occur when the radius is equal to about half the threshold.

Figure 10-31: Combinations of different Radius (first number) and Threshold (second) values. Notice that the most dramatic effects occur when the radius is equal to about half the threshold.

The Quality settings control the smoothness of the edges. The High setting takes more time than Medium and Low, but it looks smoother as well. (I set the value to High to create all the effects in Figure 10-31.) The two additional Mode options enable you to trace the edges defined by the Threshold value with white lines. Overlay Edge shows image and lines, while Edge Only shows just the traced lines. About the only practical purpose for these options is to monitor the precise effect of the Threshold setting in the preview box. Otherwise, the Edge options are clearly relegated to special effects.

Frankly, I'm not convinced that Smart Blur is quite ready for prime time. You already know what I think of the Threshold option, and it hasn't gotten any better here. Without control over the transitions between focused and unfocused areas, things are going to look pretty strange.

Tip The better way to blur low contrast areas is to create an edge mask, as I explained back in the "Sharpening grainy photographs" section. Just reverse the selection by choosing Select Inverse and apply the Gaussian Blur filter.

Figure 10-32 shows how the masking technique compares with Smart Blur. In the first image, I applied Unsharp Mask with a Threshold of 20. Then I turned around and applied Smart Blur with a Radius of 2.0 and a Threshold of 20.0, matching the Unsharp Mask value. The result makes Ike look like he has dandruff coming out of every pore in his face.

Figure 10-32: The difference between relying on Photoshop's automated Threshold capabilities (left) and sharpening and blurring with the aid of an edge mask (right). Despite the advent of computers, a little manual labor still wins out over automation.

In the second image, I created an edge mask — as explained in the "Creating and Using an Edge Mask" steps — and applied Unsharp Mask with a Threshold of 0. Then I pressed Ctrl+Shift+I to reverse the selection and applied Gaussian Blur with a Radius of 2.0. The result is a smooth image with sharp edges that any president would be proud to hang in the Oval Office.

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