2.7. Intelligent Scissors

2.7. Intelligent Scissors
	2. Selection Tools

Figure 14.30. Intelligent Scissors tool icon in the Toolbox

The Intelligent Scissors tool is an interesting piece of equipment: it has some features in common with the Lasso, some features in common with the Path tool, and some features all its own. It is useful when you are trying to select a region defined by strong color-changes at the edges. To use the Scissors, you click to create a set of "control nodes", also referred to as anchors or control points, at the edges of the region you are trying to select. The tool produces a continuous curve passing through these control nodes, following any high-contrast edges it can find. If you are lucky, the path that the tool finds will correspond to the contour you are trying to select.

Unfortunately, there seem to be some problems with the edge-following logic for this tool, with the result that the selections it creates tend to be pretty crude in a lot of cases. A good way to clean them up is to switch to QuickMask mode, and use paint tools to paint in the problematic parts. On the whole, most people find the Path tool to be more useful than the Scissors, because, even though it does not have the intelligent edge-finding capability, the paths it produces persist until you delete them, and can be altered at any time.

2.7.1. Activating the tool

You can access the Intelligent Scissors Tool in different ways:

From the image menu bar Tools → Selection Tools → Intelligent Scissors,
by clicking on the tool icon in the ToolBox,
by using the keyboard shortcut I.

2.7.2. Key modifiers

The default behavior of the Shift, Ctrl, and Alt keys is described in Section 2.1.1, “Key modifiers (Defaults)” for all selection tools.

There is, however, one key modifier that has a special behavior if you use it while editing a selection, that is after you have added the first node:

Shift

By default, the auto-edge snap feature is enabled: whenever you click and drag the mouse pointer, the Scissors tool finds the point of the maximal gradient (where the color change is maximal) for placing a new control node or moving an existing node.

Holding down this key while clicking and dragging disables this feature, and the control node will be placed at the position of the mouse pointer.

Backspace removes last segment drawn, Escape cancels all selection segments.

2.7.3. Tool handling

Figure 14.31. Using Intelligent Scissors

Each time you left-click with the mouse, you create a new control point, which is connected to the last control point by a curve that tries to follow edges in the image. To finish, click on the first point (the cursor changes to indicate when you are in the right spot). You can adjust the curve by dragging the control nodes, or by clicking to create new control nodes. When you are satisfied, click anywhere inside the curve to convert it into a selection.

As said above when you click with this tool you drop points. The selection boundary is driven by these control points. During creation you can move each one by clicking and dragging, except the first and the last one. The selection is closed when you are clicking the last point over the first one. When the selection is closed the pointer shape changes according to its position: inside , on the boundary , and outside . You can adjust the selection creating new points by clicking on the boundary or by moving each control points (merged first and last point). The selection is validated when you click inside.

You have to notice that you can get only one selection; if you create a second selection, the first one is erased when you validate the second one.

	Warning
Be sure not to click inside the curve until you are completely done adjusting it. Once you have converted it into a selection, undoing takes you back to zero, and you will have to start constructing the curve again from scratch if you need to change it. Also be sure not to switch to a different tool, or again all of your carefully created control nodes will be lost. (But you still can transform your selection into a path and work it with the Path tool.)

Warning

Be sure not to click inside the curve until you are completely done adjusting it. Once you have converted it into a selection, undoing takes you back to zero, and you will have to start constructing the curve again from scratch if you need to change it. Also be sure not to switch to a different tool, or again all of your carefully created control nodes will be lost. (But you still can transform your selection into a path and work it with the Path tool.)

To move the selection, see Moving selections.

2.7.4. Options

Figure 14.32. Tool Options for the Intelligent Scissors

Normally, tool options are displayed in a window attached under the Toolbox as soon as you activate a tool. If they are not, you can access them from the image menu bar through Windows → Dockable Windows → Tool Options which opens the option window of the selected tool.

Modes; Antialiasing; Feather edges

	Note
	See Selection Tools Options for help with options that are common to all these tools. Only options that are specific to this tool are explained here.

Interactive boundary: If this option is enabled, dragging a control node during placement will indicate the path that will be taken by the selection boundary. If it is not enabled, the node will be shown connected to the previous node by a straight line while you are dragging it around, and you won't see the resulting path until you release the pointer button. On slow systems, if your control nodes are far apart, this may give a bit of a speed-up.