220.127.116.11 Bezier Paths
The Bezier Path selection tool is very powerful. It is the only
selection tool that allows the interactive adjustment of the
selections it makes. The Bezier Path tool works by placing control points on an image. These are
initially connected by straight-line path segments; however,
using the control handles hidden inside each control point, the path segments can be made
to curve in practically arbitrary manners. By using a Bezier path, an
initial selection can be interactively corrected until the final
result fits the desired selection like a glove.
Using the Bezier Control Points and Handles: (a) A Bezier
path, (b) Simultaneously manipulating two control handles of a control
point, (c) Manipulating an individual control handle, and (d) Moving a
illustrates the basic operations of a Bezier path. The initial Bezier
path shown in Figure
(a) was created
by clicking in the image window five times. The upper-left corner was
the first point, and the lower-left, lower-right, and upper-right
corners were the second, third, and fourth points added to the path.
The final click of the mouse was made on the initial point, thus
closing the path. As each point was added, the straight-line segments
seen in the figure appeared between the control points.
While constructing a Bezier path, the mouse cursor appears as an arrow,
with a filled circle just below and slightly to the arrow's right.
This circle indicates that the path is not closed and that the
next mouse click will create a new control point. Note that Bezier
paths need not be closed. This is discussed in more detail in
3.4, which describes the Paths dialog.
The path segments between control points can be made to curve by
manipulating the points' control handles. For a closed Bezier path,
the control handles are made visible by clicking on a control point
3.6(b) shows how the control
handles for the upper-left corner of the Bezier path have been pulled
out of that control point. The dragging action of the mouse positions
one of the control handles while the other moves in tandem with, but
diametrically opposite, the first. As can be seen in
3.6(b), the two path segments
attached to the control point are made to curve by the action of the
Note that the control handles do not disappear once the mouse button
has been released and that the handles can be readjusted at any time
by clicking the mouse on either handle and dragging. However, only
one set of handles may be visible at once. Clicking on another
control point displays its handles while toggling off the visibility
of the handles for any other point. Also note that when the mouse
cursor is close enough to a control point or control handle to
manipulate it, the cursor changes from an arrow with a filled circle
below it to an arrow with the outline of square. Because control
handles resemble squares, this special cursor is a useful way to
indicate that the mouse cursor is close enough to the control point
for the handles to be active.
The two control handles for a point can be moved independently of each
other by using the Shift key. Pressing the Shift key while dragging
a handle with the mouse makes that handle move while the other remains
stationary. Moving a single control handle in this way allows the
curvature of a single path segment to be manipulated.
3.6(c) shows the result of using
the Shift key to move one of the control handles independently of the
other. Note how this changes the curve of the upper segment of the
square while leaving the curve of the left segment as it was.
It is also possible to reposition a control point. This is done by
pressing the Control key before clicking on a control point.
Dragging the mouse while the mouse button and Control key are
pressed moves the control point. The result of repositioning a
control point is shown in Figure
After creating a Bezier path with all the control points properly
positioned and all the path segments appropriately curved, it may then
be converted to a selection. This is done by clicking inside the
closed path. Note that when the mouse cursor is moved inside the
closed path, the cursor changes to an arrow with the outline of a
dashed rectangle below it. The dashed rectangle resembles the
Marching Ants, which will appear when the Bezier path is converted to a
show the application of the Bezier Path tool in a practical
3.7 shows that a closed
Bezier path has
Creating a Bezier Path
been created by placing seven control points around the perimeter of a
sea turtle's shell. Although the points are all positioned on the
edge of the shell, the straight-line segments between points do not
marry well to the shell's shape.
As just described, the Bezier path can be adjusted to the shell's
shape by manipulating the control handles. The first pair of handles
are shown in Figure
Manipulating the Control Handles of a Bezier Control Point
The two handles had to be adjusted independently of each other to
properly match the curve of the shell on either side of the control
point. This can be seen in the figure by the different lengths of the
control handles and by the slight angle between them.
After adjusting the control handles for each point in the Bezier path
it is converted to a selection. The result, showing the Marching
Ants, is illustrated in Figure
Selection Created from the Bezier Path
What is particularly interesting about this example is that the
selection has been made with only a few control points. This is a
strength of this tool. However, there are limitations to what can be
accomplished with a Bezier path. A very irregular shape will need
many control points and will require a lot of work to adjust the path
to the shape.
A practical question is where should the control points be placed?
Another is how many control points are necessary to adequately select
a shape? For the placement of control points here are some rules of
- A control point must always be placed at a corner (like the one
found at the rear-most point of the sea turtle's shell in
- A straight-edge region can only be created by placing a
control point at each end of the edge, as shown in
- An inflection point on a curve can be created between a pair
of control points, as shown in the bottom path segment of
3.10(b). It can also be created
with a single control point if this is placed exactly at the point of
The Required Number of Bezier Control Points?
The number of control points needed to adequately adjust a Bezier path
to a shape is a more difficult question.
3.10(c) shows that a pair of
control points are all that is needed to create a circle. Thus, a
single control point can cover up to 180 degrees of curve. However,
this doesn't account for all curve characteristics. I personally like
to place control points as the shape of my selection passes through 90
degrees of curve. This personal rule is borne out in the sea turtle
selection shown in Figures
In any case, the issue of the number of points required for a shape is
not critical because it can be resolved using the Add Point and Delete
Point functions available in the Paths dialog. These are discussed in
detail in Section