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Grokking The Gimp
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2.6.5 The Transform Tool

General transformations of a layer can be done using the Transform tool from the Toolbox. This tool can perform rotation, scaling, shearing, and perspective transformations of the active layer. As usual, care must be taken to specify the active layer before applying this tool.

Figure  2.24(a)

  
Figure 2.24: Transform Tool Dialog
Figure 2.24

illustrates an image layer contained in a somewhat larger window. Shown in Figure  2.24(b) is the Tool Options dialog, which is obtained by double-clicking the Transform tool icon in the Toolbox window. The four radio buttons in the Transform area of the dialog are used to select the desired transformation type, which can be Rotation, Scaling, Shearing, or Perspective.

Transforming a layer requires interpolation of pixel values, and this can introduce some jagged-edge artifacts into the result. The toggle button labeled Smoothing diminishes this effect, which is why it is on by default. It does tend to make the image a little less sharp, though (see Section  6.4.1 for a discussion of how to recover some of the lost sharpness).

The Tool paradigm area of the dialog has two options. The default is Traditional, which maps the image to a transformed grid (see Figure  2.26). For those who like to go against the grain, the Corrective option maps the transformed grid to the image, thus producing an inverse transformation. For example, rotating the transform grid in one direction with the Corrective option maps the grid back to the image, making the image itself rotate in the opposite direction. Sound complicated? It isn't. Just try it yourself and you'll see.

     
2.6.5.0.1 Rotation

Figures  2.25

  
Figure 2.25: Initiating the Rotation Transformation
Figure 2.25

and 2.26
  
Figure 2.26: Finalizing the Rotation Transformation
Figure 2.26

illustrate the use of the Rotation option of the Transform tool. Figure  2.25(a) shows the image layer, and Figure  2.25(b) the Rotation Information  dialog that appears when the left mouse button is clicked in the image window. The Rotation Information dialog has entry boxes that can be used to give a specific angle of rotation as well as the position of the point that will be the center of rotation. By default, the center of rotation is the geometric center of the active layer.

For each of the Transform options, Rotation, Scaling, Shearing, or Perspective, the first mouse click on the image brings up a dialog specific to the chosen option. This mouse click also displays a grid  superimposed on the active layer in the image window. The number of grid lines can be controlled from the Tool Options dialog (see Figure  2.24(b)). The grid lines can be seen in Figure  2.25(c). Placing the mouse cursor within the active layer in the image window changes the cursor in accordance with the type of transformation to be performed. The image outline and grid can then be transformed by clicking and dragging. The transformation of the grid can be adjusted as many times as desired because the transformation of the image is not initiated until the option transform button is clicked. For example, the Rotate button in Figure  2.26(b) must be clicked to cause the actual rotation of the image.

As the grid is rotated for the Rotate option of the Transform tool, the angle is interactively reported in the Rotation Information dialog. The rotated outline for an angle of 25o is shown in Figure  2.26(a). As already stated, once the desired angle of rotation is found, the image itself is rotated by clicking on the Rotate button, seen in the dialog shown in Figure  2.26(b). The resulting rotated layer is shown in Figure  2.26(c).

As an alternative to interactively rotating the grid with the mouse, the rotation can also be performed by entering a value into the Angle entry box or by using the slider in the Rotation dialog. Useful values of rotation can be determined with the Measure  tool. An example of this is illustrated in Section  7.2.

     
2.6.5.0.2 Scaling

Figure  2.27

  
Figure 2.27: The Scaling Transformation
Figure 2.27

illustrates the Scaling option of the Transform tool. Like for rotation, scaling can be performed interactively with the mouse or by entering values into the Width and Height entry boxes in the Scaling dialog.

Values can be entered into the Scaling dialog  in a number of units, the default being pixels. It is often convenient to perform scaling as a percentage of the original dimensions (which are shown at the top of the Scaling dialog as the Original Width and Height). This can be done by choosing the % option from the pull-down menu in the dialog. Figure  2.27(b) shows that this choice was used to scale the grid in Figure  2.27(a) by 50% in both dimensions. When the Scale button in the dialog is clicked, this produces the result shown in Figure  2.27(c).

It is useful to be able to constrain the X and Y scale ratio so as to maintain their aspect ratio when scaling a layer with the Transform tool. This is done by pressing both the Control and Alt keys while using the mouse to scale the transform grid. Useful values for scaling can be determined using the Measure  tool. Section  7.5 illustrates an example of this.

     
2.6.5.0.3 Shearing

Figure  2.28

  
Figure 2.28: The Shear Transformation
Figure 2.28

shows the application of the Shearing option of the Transform tool. As with the other options, shearing can be applied interactively with the mouse or by entering values into the Shear Information dialog.  Shearing can be applied either horizontally or vertically, but not in both directions simultaneously. If applied using the mouse, the direction of shear is determined by the mouse's initial direction of movement. Otherwise, it depends on the first entry box used in the dialog.

     
2.6.5.0.4 Perspective

Perspective is perhaps the most intriguing option of the Transform tool. This option is illustrated in Figure  2.29.

  
Figure 2.29: The Perspective Transformation
Figure 2.29

This is the only transform option where values cannot be directly entered into a dialog. As shown in Figure  2.29(b), there is numerical feedback about how the Perspective transform option is applied to the image, but this is not particularly valuable because it is unclear how to reuse the information. The consequence is that the Perspective option may only be applied interactively with the mouse.

Figure  2.29(a) shows that the grid line feature provides very useful feedback for this particular tool. The lines help visualize the perspective warping that will take place once the Transform button is clicked in the Perspective Transform dialog.  The perspective transform allows each of the four corners of the layer to be independently repositioned. The resulting image is warped to a general quadrilateral. Figure  2.29(c) shows the result of applying the perspective transform specified by the grid shown in Figure  2.29(a).

It should be noted that all the transforms can be used in conjunction with the Transform Lock  option of the Paths dialog. This is extremely useful and is discussed in more detail in Section  3.4.1. A relevant example is shown in Section  7.2.




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