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Grokking The Gimp
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6.4.2 A Simplified but Illustrative Example

So much for the theory of the Unsharp Mask. A simple example brings out the practical effects of this filter. Unsharp Mask can be found in the Image:Filters/Enhance menu. Figure  6.33(a)

Figure 6.33: Illustrating the Effect of Applying the Unsharp Mask
Figure 6.33

shows a simple image with just two grayscale regions. Thus, the values along a row of the image resemble the graph of Figure  6.32. Figure  6.33(b) shows the result of applying the Unsharp Mask using the parameters in the filter's dialog shown in Figure  6.33(c). Here a maximum value for the Radius has been chosen to accentuate the effect.

Note that in the sharpened result (see Figure  6.33(b)), there is a light halo on the right side of the edge and a deep shadow on the dark side of the edge. This corresponds to the discussion of the red curve in the lower graph of Figure  6.32. The effect of the Unsharp Mask is very pronounced but was chosen to clearly illustrate the effect. In general, the dialog parameters are chosen to produce more subtle results.

The Unsharp Mask dialog has three parameters. Looking at the dialog in Figure  6.33(c), there is Radius, which specifies the relative width of the halo/shadow created by the mask. The parameter named Amount controls the relative magnitude of the dip and the peak created on each side of the edge. Finally, Threshold specifies the difference in pixel values that must exist across the edge for the Unsharp Mask to be applied. Thus, if Threshold is set to 25, adjacent pixels whose difference in pixel value is less than 25 are not sharpened. An important fact is that the Unsharp Mask is applied individually to each color channel, R, G, and B; the results are then combined.

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