The High Pass filter is especially useful as a precursor to Image ^ Adjust ^ Threshold, which converts all pixels in an image to black and white (again, covered in Chapter 17). As illustrated in Figure 10-18, the Threshold command produces entirely different effects on images before and after you alter them with the High Pass filter. In fact, applying the High Pass filter with a low Radius value and then issuing the Threshold command converts your image into a line drawing.
In the second row of examples in the figure, I followed Threshold with Filter ^ Blur ^ Gaussian Blur (the subject of the next section). I set the Gaussian Blur Radius value to 1.0. Like the Threshold option in the Unsharp Mask dialog box, the Threshold command results in harsh transitions; Gaussian Blur softens them to produce a more natural effect.
Why change your image to a bunch of slightly different gray values and then apply a command such as Threshold? One reason is to create a mask, as discussed at length in the "Building a Mask from an Image" section of Chapter 9. (In Chapter 9, I used Levels instead of Threshold, but both are variations on the same theme.)
You might also want to bolster the edges in an image. For example, to achieve the last row of examples in Figure 10-18, I layered the images prior to applying High Pass, Threshold, and Gaussian Blur. Then I monkeyed around with the Opacity setting and the blend mode to achieve an edge tracing effect.
Note I should mention that Photoshop provides several automated edge tracing filters —
including Find Edges, Trace Contour, and the Gallery Effects acquisition, Glowing Edges. But High Pass affords more control than any of these commands and permits you to explore a wider range of alternatives. Also worth noting, several Gallery Effects filters — most obviously Filter ^ Sketch ^ Photocopy — lift much of their code directly from High Pass. Although it may seem at first glance a strange effect, High Pass is one of the seminal filters in Photoshop.
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