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Grokking The Gimp
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8.1 The Bucket Fill Tool

Bucket Fill is a very useful tool for rendering. It is found in the Toolbox window and is represented by the bucket icon shown in Figure  8.1(a).

  
Figure 8.1: Using the Bucket Fill Tool
Figure 8.1

The Bucket Fill tool is used for filling regions, in whole layers or selections, with a specified color or image pattern. The Tool Options dialog, shown in Figure  8.1(b), indicates that the choice between color and image pattern is specified by clicking the appropriate radio button. For color fills, either the Active Foreground Color or Active Background Color can be used. The choice of color is specified with the Color Selection dialog shown in Figure  8.1(c). For pattern fills, the image pattern is chosen from the Pattern Selection  dialog, shown in Figure  8.1(d). The Pattern Selection dialog is found in Image:Dialogs/Patterns, and can also be invoked by typing C-S-p in the image or Toolbox windows, or by clicking on the Active Pattern icon in the Toolbox window.

The Bucket Fill tool fills the image with a color or a pattern in a manner similar to the way the Magic Wand makes selections (see Section  3.1.1). Clicking in an image specifies a seed pixel. The color value of the seed is then compared against the color of the seed's neighboring pixels; if the difference between their color values and the seed's is less than a threshold they are filled. This process repeats for the neighbors' neighboring pixels, and so on, until all the contiguous pixels whose color values differ from the seed's by less than the threshold are filled. The Bucket Fill's threshold value is set using the Threshold slider found in the Bucker Fill's Tool Options dialog (see Figure  8.1(b)).

There are several controls found in the Brush Selection dialog that affect the way Bucket Fill applies paint or patterns to an image canvas. In particular, there is the Opacity slider, which controls the alpha channel of the applied color or pattern (see Chapter  4 for more on alpha channels), and there is the Mode menu which selects the blending mode to be used (blending modes are described in Section  5.6).

Figure  8.2

  
Figure 8.2: Using the Threshold and Opacity Options
Figure 8.2

illustrates the effects of the Threshold and Opacity settings. Figure  8.2(a) shows the location of a seed pixel selected in an image containing a radial gradient. The color of the gradient varies gradually from the center of the image to its edge. This allows you to see how the Bucket Fill tool affects a region of the image depending on the placement of the seed and the value of the Threshold. Figure  8.2(b) shows how much of the original gradient is filled with blue when the Threshold is set to 15. Setting the Threshold to 255 would have resulted in completely filling the image with blue. Figure  8.2(c) shows the result when the Opacity slider in the Brush Selection dialog is set to 40%. Here, the applied blue fill mixes with the red gradient behind it in a ratio of 40% to 60%.

Figure  8.3

  
Figure 8.3: Filling with Image Patterns
Figure 8.3

illustrates filling a part of an image with patterns. Figures  8.3(a) and (b) show that the image consists of two layers, a white background and some blue text on an otherwise transparent layer (shown in the Balcony Angels font). Figure  8.3(c) shows the result of filling each letter in the text with a different pattern, and Figure  8.3(d) shows that the Threshold for these fills is set to 255. This is necessary to get a complete fill. A lower value of Threshold would fail at some text edge pixels because the text is antialiased there (see Section  3.1.2).




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