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Grokking The Gimp
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5.6 The Blending Modes

Usually layers are opaque, which means that upper layers cover and visually block lower ones. Blending modes change this behavior and allow some color features of a layer to be combined with the colors of layers below it. Blending modes can also be used to affect how color from a painting tool combines with the layer the paint is applied to.

Figure  5.11(a)

Figure 5.11: The Blending Mode Menus
Figure 5.11

illustrates the blending mode menu for layers, which is found in the Layers dialog. Figure  5.11(b) shows the blending mode menu for painting tools , which is found in the Brush Selection  dialog.5.1 The menu in the Brush Selection dialog applies the selected blending mode to all the paint tools: the Pencil, Paintbrush, Airbrush, Ink Pen, and Xinput Airbrush tools. In addition, this menu controls the way the Bucket Fill, Gradient, and Clone tools apply their paint.

The different blending modes are described in this section, and their practical uses and applications are described in the next. The GIMP has 16 different blending modes. They are listed in the following five logical groups:

  • Normal, Dissolve, and Behind (available in paint mode only)
  • Difference, Addition, and Subtract
  • Multiply (Burn), Divide (Dodge), Screen, and Overlay
  • Darken Only and Lighten Only
  • Hue, Saturation, Color, and Value

In the following descriptions of the blending modes, the pixels of the upper layer (or of the applied paint) are referred to as the foreground pixels and those of the lower layer or layers as the background pixels. The notations F and B are used to represent their respective values. Blending the foreground pixel value F with the background value B yields the resultant pixel value R.

5.6.1 The Normal, Dissolve, and Behind Blending Modes

Normal, Dissolve, and Behind are pseudo-blending modes because they don't really combine the foreground and background pixel values of the image.

Normal mode is the default GIMP behavior where the foreground pixels are visible and the background pixels are not. Of course, this can be changed by adjusting the opacity slider in the Layers dialog (more on opacity and transparency is discussed in Section  5.7).

Dissolve mode works by allowing a percentage of background pixels to be seen through the foreground. It does this by making some parts of the foreground partially transparent and the rest fully transparent. These two sets are intermingled in a random way. For the Dissolve mode to have an effect, the foreground layer must have an alpha channel with values less than 255. The alpha channel for a layer can be modified with a layer mask. The details of working with layer masks are discussed in Section  4.2.

Figure  5.12

Figure 5.12: The Dissolve Blending Mode
Figure 5.12

illustrates the use of Dissolve. Figure  5.12(a) shows the Layers dialog which illustrates how this example is constructed. The image consists of two layers: a red background and a white foreground. The foreground has a uniform alpha channel set to a value of 191 (about 75% opaque). The Mode menu in Figure  5.12(a) shows that the Dissolve mode has been chosen for the foreground.

Figure  5.12(b) shows the result of the Dissolve blending mode. Due to the value of the foreground's alpha channel the result is that 75% of the white pixels are 75% opaque and 25% are fully opaque. The details of the effect can be more clearly seen in the small region framed by the black box shown in Figure  5.12(b). This region is zoomed 900% and redisplayed in Figure  5.12(c), which makes the relationship of the red and white pixels more apparent.

Unlike all the other blend modes in the GIMP, the Behind mode only works with painting tools. It is not available as a blending mode for layers. To understand how it works, imagine a pane of glass that has something painted on the front surface but there are some parts of the pane that are bare, or have only a partially transparent paint on it. Painting on the back surface of the pane lets the color from this new paint show through to the front wherever the front is not fully opaque. Figure  5.13

Figure 5.13: The Behind Blending Mode
Figure 5.13

illustrates this effect.

In Figure  5.13(a), a single-layer image with a centered red circle is displayed. The rest of the layer is transparent. The Brush Selection dialog, shown in Figure  5.13(b), has been used to choose a large, hard brush and to set the blending mode to Behind. The figure shows the result of painting a bright green stripe, using the Paintbrush tool, through the red circle. In Behind mode, however, the green is only seen through the transparent parts of the layer. This mode only works for layers with alpha channels.

Grokking The Gimp
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