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

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.

     
3.2.2 Subtracting

Subtracting a new selection from an existing one means that the result is the first selection less the pixels contained in the intersection of the two. Thus, if A is the set of pixels defined by the first selection and B is the set for the second, $A-(A\cap B)$ is the result of subtracting the second from the first, where $\cap$ is the set intersection operator.

A subtraction is made by pressing the Control key before beginning the new selection. Once the new selection is begun, the Control key should be released. The result of subtracting an ellipse from the rectangular selection in Figure 3.15(a) is shown in Figure 3.15(c). Note that it is the new selection that is subtracted from the old selection, not vice versa.

It is useful to note that pressing the Control key while the mouse is in the image window has the effect of changing the mouse cursor to an arrow with a minus sign below it. This indicates that the next selection will be subtracted from any selection already existing in the image window. Because I always forget which key controls addition and which controls subtraction, I find the mouse cursor indicators are particularly useful.

     
3.2.3 Intersecting

Intersecting two selections means that the resulting selection is, you guessed it, the intersection of the two. Thus, if A is the set of pixels defined by the first selection and B the set for the second, $A\cap B$ is the result of intersecting the two. The intersection is made by pressing the Shift and Control keys together before pressing the left mouse button to begin the new selection. The Control and Shift keys should be released before releasing the mouse button. The result of intersecting an ellipse with the rectangular selection in Figure 3.15(a) is shown in Figure 3.15(d).

It is useful to note that pressing the Shift and Control keys together while the mouse is in the image window has the effect of changing the mouse cursor to an arrow with a $\cap$ sign below it. This indicates that the next selection will be intersected with any selection already existing in the image window.

     
3.2.4 Using Add, Subtract, and Intersect with the Rectangle and Ellipse Selection Tool Features

As described in Section 3.1.1, the Shift key can be used to create perfectly square or circular selections, and the Control key can be used to perform selections centered at the position of the mouse click. In addition, Section 3.2 describes how the Shift key can be used for adding selections, the Control key for subtracting them, and the Shift and Control keys together for intersecting them.

The question you may be asking, then, is how these features can be used together. How can a perfect square be subtracted from a prior selection or a center-initiated ellipse added to one? The answer is quite simple. The trick is to remember that adding, subtracting, and intersecting selections is signaled by pressing the Shift key, the Control key, or both before clicking to initiate the selection. The sequence for creating a perfectly square or circular selection or for center-initiated selections is by pressing the Shift key, Control key, or both after clicking to initiate the selection. These features can be combined by applying the following steps:

1.

Begin by pressing the appropriate combination of Shift and Control to specify whether the selection is to be added to, subtracted from, or intersected with an already existing selection.

2.

While the keys are pressed, click and drag in the image window to initiate the new selection.

3.

While the left mouse button is pressed, release the Shift and/or Control keys.

4.

Apply the appropriate combination of Shift and Control keys to specify a perfectly square, circular, and/or center-initiated selection.

5.

While keeping the last combination of Shift and Control keys pressed, release the mouse button, thus completing the selection.

       
3.2.5 Moving a Selection Boundary

Occasionally it is useful to be able to move a selection after it has been made. That is, it is useful moving the selection itself, not the selection contents. This can be done by Alt-clicking and dragging the selection boundary.

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