2.7. Intelligent Scissors
Figure 13.21. 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 to the Intelligent Scissors Tool in different ways:
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:
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.
Figure 13.22. 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
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
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 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
Figure 13.23. 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
→ → which opens the option window of the selected tool.
Modes; Antialiasing; Feather edges
See Selection Tools
for help with options that are common to all these tools. Only
options that are specific to this tool are explained here.
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.