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Grokking The Gimp
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2.6.2.2 Image Resizing

At first blush it would seem that the Canvas Size  function found in the Image:Image menu should work in a similar fashion to Scale Image. However, there is an important difference between resizing an image and scaling it. Because the image contents do not change with the image boundaries, there is a non-unique choice in how they are positioned within the resized image window. Thus, when resizing to a smaller image window, the position of the image within the new window depends on the values for the new width, height, and X and Y offsets.

Figure  2.16

  
Figure 2.16: Image Resizing (Original Becomes Smaller)
Figure 2.16

illustrates how using Canvas Size can give rise to an inconvenient problem. Figure  2.16(a) shows the original image, and Figure  2.16(b) shows the Set Canvas Size dialog. In the dialog the X and Y ratios have been set to 50% of the original image size. The result is shown in Figure  2.16(c). As can be seen, the image is poorly positioned within the new window. This can be compensated for by using the Move tool to reposition the image inside the window. The result of doing this is shown in Figure  2.16(d). However, the result still leaves the image subject, the wolf's head, poorly framed. The problem is that there is no easy way to enter numbers into the Set Canvas Size  dialog to obtain an aesthetically pleasing result.

A better solution is to resize the image interactively using the Crop   tool. Figure  2.17

  
Figure 2.17: Resizing Using the Crop Tool
Figure 2.17

illustrates how the original image can be resized smaller this way. The tool is applied by clicking on the Crop tool icon in the Toolbox and then clicking and dragging in the image window to create the crop rectangle. Once drawn, the dimensions of the crop rectangle can be adjusted by clicking and dragging either the upper-left or lower-right corners of the rectangle. The rectangle can also be repositioned by clicking and dragging the upper-right or lower-left corners.

The result of using the Crop tool to nicely frame the wolf's head is shown in Figure  2.17(a). When using this tool, the Crop dialog appears, as shown in Figure  2.17(b). It is possible to crop the image by clicking on the Crop button in the dialog or by simply clicking inside the crop rectangle in the image window. This makes the image smaller and simultaneously discards the image parts outside of the window. Alternatively, the image can be resized by clicking on the Resize button in the Crop dialog. This makes the image window smaller without discarding the image parts outside the resulting window.

Figure  2.17(c) shows the result of cropping the image to the crop rectangle seen in Figure  2.17(a). From the above discussion, it should be clear that there is really no reason to use Canvas Size to make an image window smaller. It is simply more convenient to do it with the Crop tool.

Although Canvas Size is not optimum for making an image smaller, it is quite useful for making it larger. This is especially valuable when compositing several raw images. Typically you discover, after positioning various imported layers, that the resulting image window is not large enough to adequately frame the composition. When this happens, Canvas Size is the tool that fixes the problem. Figure  2.18

  
Figure 2.18: Image Resizing (Original Becomes Larger)
Figure 2.18

illustrates the result of resizing an image.

Figure  2.18(a) shows the original image, and Figure  2.18(b) shows the Set Canvas Size dialog box. This time the X and Y ratios are scaled to 125% of the original. The result is shown in Figure  2.18(c). Once the image window has been resized it is possible to reposition the image layer or layers using the Move tool.

Grokking The Gimp
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