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Grokking The Gimp
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3.2 Combining Selections

Selections can be combined in various ways. In particular, the GIMP provides the capability to add, subtract, and intersect selections.

3.2.1 Adding

Adding a selection to an existing one means that the resulting selection is the union of the pixels from the two. Thus, if A is the set of pixels defined by the first selection and B the set for the second, then $A\cup B$ is the result of adding the second to the first, where $\cup$ is the set union operator.

If a selection already exists in the image window, the addition is made by pressing the Shift key before clicking to begin the new selection. Once the mouse button has been pressed, the Shift key should be released. Figure  3.15(a) shows an initial rectangular selection, and Figure  3.15(b) shows the result of adding an ellipse to this.

Figure 3.15: Combining Selections
Figure 3.15

It is useful to note that pressing the Shift key while the mouse is in the image window has the effect of changing the mouse cursor to an arrow with a plus sign below it. This indicates that the next selection will be added to any selection already existing in the image window.

Grokking The Gimp
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