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Grokking The Gimp
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4.3.3 Gradients

When used in layer and channel masks, gradients are very useful blending tools. Figure  4.20

Figure 4.20: Two Images
Figure 4.20

is used to illustrate how two images can be blended together using a gradient in a layer mask. To begin, a new transparent layer is created in the lunar module image (Figure  4.20(a)), and the space shuttle image (Figure  4.20(b)) is copied and pasted into it. The paste actually creates a floating selection that is subsequently positioned and anchored to the transparent layer. The result of the paste is illustrated in Figure  4.21(a),
Figure 4.21: Pasting and Positioning the Shuttle Over the Lunar Module
Figure 4.21

and the image's Layers dialog is shown in Figure  4.21(b). It can be seen from the Layers dialog that the space shuttle is in the upper layer and that, in preparation for the next step, a layer mask has been created for it.

The blending of the two layers is performed by constructing a gradient  in the layer mask. The following steps are used to accomplish this:

The layer mask is made active by clicking on its thumbnail in the Layers dialog.
The Gradient tool is chosen from the Toolbox, and the Active Foreground Color and the Active Background Color are set to black and white, respectively.
The Tool Options dialog  is opened by double-clicking on the Gradient icon in the Toolbox, and the Blend option is set to FG to BG (RGB).
The gradient is applied by clicking and dragging in the image window starting at the leftmost edge of the space shuttle layer, and releasing slightly to the left of the left shuttle rocket.
The resulting gradient can be seen in the layer mask's thumbnail, shown in Figure  4.22(b).
Figure 4.22: Two Images Blended Using a Gradient in a Layer Mask
Figure 4.22

Several points are worth noting here. First, the gradient is created in the layer mask by clicking and dragging in the image window. The gradient is applied to the layer mask because it was made active in the first step of the preceding procedure. Second, as seen in Figure  4.22(a), the gradient in the layer mask blends the upper layer with the lower one by creating a gradual transition from black to white in the mask. The black pixels of the mask make the upper layer completely transparent. The trend from the dark gray to light gray pixels in the mask gradually blends the upper layer into the lower until the mask is totally white, at which point the upper layer is totally opaque. The width of the blend is controlled by the width of the gradient.

The blend made in Figure  4.22 produces a straight, horizontally varying gradient, but what if a more complicated blending interface is desired? This can be solved using the Mode  menu found in the Brush Selection  dialog. Figure  4.23(a)

Figure 4.23: The Brush Selection Dialog and the Mode Menu
Figure 4.23

shows the Brush Selection dialog, and Figure  4.23(b) shows the mode menu's choices.

The Mode menu controls how the paint of the gradient combines with what is already active in the image window (whether that be an image layer, a channel mask, or a layer mask). The Normal mode is the default, and this mode just replaces anything that was in the layer with the paint from the gradient. The other modes combine the paint from the gradient in various ways, which are described in depth in Chapter  5. For the moment, however, let's turn our attention to the Multiply and Screen modes. These modes will permit us to create gradients with tailor-made interfaces. To illustrate this, Figure  4.24(a)

Figure 4.24: Using Blending Modes to Combine Gradients
Figure 4.24

shows a horizontally varying gradient created by clicking and dragging with the mouse in the image window, beginning at the tail of the red arrow and releasing at its tip. (Note that the red arrow is just for illustrative purposes and is not part of the image or the gradient.)

Choosing the Multiply  mode from the Brush Selection dialog's Mode menu and applying a second gradient to the first is shown in Figure  4.24(b). The direction of the applied gradient is indicated by the red arrow. If the normal combining mode had been used, the second gradient would have replaced the first. However, setting the blending mode to Multiply has produced a completely different effect. The two gradients have been multiplied together. What does it mean to multiply two gradients? Assigning a value of 0 to black, a value of 1 to white, and proportional values for grays provides us with a definition. This numerical correspondence is used to create a gradient that is the product of the first two. More precise definitions for the blending modes are given in Chapter  5.

Figure  4.24(c) shows the result of using the Screen  mode, applied as shown by the red arrow in the figure. This mode works in a manner similar to Multiply mode. It performs a multiplication, except that it is white that is assigned the value of 0 and black the value of 1. As can be seen in Figure  4.24, the Multiply and Screen blending modes can be used to create custom gradient interfaces. The Multiply mode can be used to make a black pivot around the point of application, and the Screen mode to make a white pivot.

Figure  4.22 shows an example of using a gradient in a layer mask to blend two layers. In the upper part of this figure, the blend seems too abrupt, because the black space of the lunar module image contrasts strongly with the gray-blue sky of the space shuttle launch. This can be softened by making the gradient interface curve around the space shuttle image, letting more of the lunar module's black show through from below. Using the Multiply and Screen blending modes to do this produces the effect shown in Figure  4.25(a).

Figure 4.25: Custom Blending in the Upper Portion of the Image by Combining Additional Gradients Using the Multiply and Screen Modes
Figure 4.25

The corresponding curved gradient interface can be seen in the layer mask thumbnail shown in Figure  4.25(b).

Effects similar to what were achieved with Multiply and Screen can be had using two other blending modes: Darken Only and Lighten Only.  The results are a little different from those obtained with Multiply and Screen because these two modes provide more angular, predictable results at the corners of intersection. The result is more like a mitred picture frame than a fluid, smooth transition. Figures  4.26(a), (b), and (c)

Figure 4.26: Compare Darken Only and Lighten Only with Multiply and Screen in Figure  4.24
Figure 4.26

illustrates the application of the Darken Only and Lighten Only modes. Compare them with the application of the Multiply and Screen modes, shown in Figure  4.24.

Blending modes are discussed in more detail in Section  5.6. A sophisticated use of gradients and blending modes in layer masks is illustrated in Section  7.3.

Grokking The Gimp
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