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Grokking The Gimp
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6.4.3 A Representative Example of Applying
the Unsharp Mask

Figure  6.34

  
Figure 6.34: An Example of Applying the Unsharp Mask
Figure 6.34

illustrates the application of the Unsharp Mask to a more representative image. Figure  6.34(a) shows the original and Figure  6.34(b) the result of applying the mask. The parameters chosen to perform the sharpening are shown in the Unsharp Mask dialog illustrated in Figure  6.34(c).

The choice of parameters used to obtain the results in Figure  6.34 was obtained by trial and error. Using Undo (C-z) in conjunction with the keyboard shortcut that recalls the last applied filter (A-S-f ), it was possible to apply the Unsharp Mask repeatedly, experimenting with values. In this way, it was possible to obtain just the desired effect.

What parameter values should be used with the Unsharp Mask? The answer depends on the image, of course. However, one rule of thumb is the Radius of the mask should be small--2 to 4 pixels wide. This is because the halo and shadow created by the Unsharp Mask should be subtle. On the other hand, the Amount should be as much as you can get away with without overly accentuating noise. This is typically in the range of 75% to 150% depending on the pixel values in the image.

Grokking The Gimp
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  Published under the terms of the Open Publication License Design by Interspire