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Grokking The Gimp
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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.

Grokking The Gimp
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