Often, using a selection tool will either select less than everything you want or more than everything you want. By default, using a selection tool again, normally, will deselect the previously selected area and replace it with a new selection. In general, it's possible to control this behavior so we can add or subtract from the current selected area.
For example, if we want to select two separate rectangular areas, we would select the first rectangle normally, then add to this selection the second rectangle.
On the other hand, if we want to select a rectangular border, for example, we would select an outer rectangle that covers the entire border, and the subtract from it an inner rectangle that isn't part of the border.
Operations like this are sometimes called Boolean operations. In the tool options of every selection tool in Krita, we can find a set of buttons to switch between five possible actions:
- Replace: the default, which replaces the old selection with new.
- Intersect: which makes the selection smaller by intersecting it with the new selection.
- Add: which combines both selections.
- Subtract: which removes from the old the new.
- Symmetric difference: this is the opposite of intersect. I honestly don't know why would anyone use it.
In general, it's possible to use modifier keys to change the behavior before clicking with a selection tool.
In Krita, if you hold Shift before using a selection tool, the mouse cursor will gain a plus ("+") symbol, and the selection will be added to the previous selection. If you hold Alt, it will gain a minus ("-") symbol, and the selection will be subtracted.
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