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Grokking The Gimp
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4.3.5 Blurring of Masks and Other Effects

Section  3.1.2 discussed an edge-softening technique called antialiasing  and an edge-blending technique called feathering. Effects similar to antialiasing and feathering can be had by applying blur filters to masks.

White pixels in a mask correspond to selected pixels in an image layer, and black pixels in the mask correspond to unselected image pixels. If there are no gray pixels between the black and white zones, this gives the selection edges represented by the mask the harsh edge already seen in the discussion on antialiasing. Blurring a mask softens the sharp edges in the mask by creating a graded zone of gray between the regions of black and white pixels. The width of the gray zone is controlled by the blur radius. The gray zones correspond to partially selected pixels in the image layer, and this is what creates the antialiasing/feathering effect for masks.

Figure  4.30

Figure 4.30: An Image and a Mask
Figure 4.30

is used to illustrate the use of blur filters on masks. Figure  4.30(a) shows an image of a wood duck, and Figure  4.30(b) shows a channel mask representing a selection of it. Figure  4.30(c) shows the associated Channels dialog.

A closer examination of the wood duck mask is shown in Figure  4.31(a).

Figure 4.31: Aliased Mask Edge Softened Using Blur Filter
Figure 4.31

As can be seen, the mask has an unpleasant, hard, and aliased edge. This can be softened by blurring the mask. Figure  4.31(c) shows the result of applying the Gaussian Blur (IIR)  filter to the mask. The filter dialog is displayed in Figure  4.31(b), which shows the choice of blur radius. In general, a small radius produces an antialiasing effect, and a large blur radius creates a feathering effect. A blur radius of 1 was used for this example.

Other interesting and artistic edge effects can be obtained by processing masks with one or more of the GIMP's large collection of filters, found in the Image:Filters menu. Figures  4.32, 4.33, and 4.34 illustrate some examples.

Figures  4.32(a)

Figure 4.32: Applying the Waves Filter to the Wood Duck Mask
Figure 4.32

illustrates the Waves   filter dialog, found in the Image:Filters/Distorts menu. The resulting effect on the wood duck mask is shown in Figures  4.32(b), and the result of applying this mask as a selection to cut away the image's background is shown in Figures  4.32(c). Figure  4.33(a)
Figure 4.33: Applying the Glass Tile Filter to the Wood Duck Mask
Figure 4.33

shows the Glass Tile   filter dialog, found in the Image:Filters/Glass Effects menu. Figures  4.33(b) and (c) show the effect of this filter on the mask and the image. Figure  4.34(a)
Figure 4.34: Applying the Spread Filter to the Wood Duck Mask
Figure 4.34

shows the Spread   filter dialog, found in the Image:Filters/Noise menu. Figures  4.34(b) and (c) show the effect of this filter on the mask and the image.

Many other interesting filter possibilities can be found. Have fun! Experiment!

Grokking The Gimp
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