Mastering Perl/Tk

Creating Items in a Canvas

The whole point of having a Canvas is to put items in it. You can create arcs, bitmaps, images, lines, rectangles, ovals (circles), polygons, text, and widgets. Each has an associated create XXX method, where the type of item you want to create replaces the XXX. Each of the create methods returns a unique ID, which can be used to refer to the item later. When you see a method that takes a tag or an ID as an argument, the ID is the one returned from the create method.

The Arc Item

When you create an arc, you specify a bounding rectangle with two sets of x and y coordinates. The arc is drawn within the confines of the bounding box. The basic createArc statement is as follows:

$id = $canvas->createArc(x1, y1, x2, y2);

Any additional options used with createArc are specified after the coordinates:

$id = $canvas->createArc(x1, y1, x2, y2, option => value);

Each option for the arc item can be used later with the itemcget and itemconfigure Canvas methods. The options are:

-extent => degrees

The length of the arc is specified in degrees by using the -extent option. The default -extent (or length) is 90 degrees. The arc is drawn from the starting point (see -start option) counterclockwise within the rectangle defined by (x1, y1) and (x2, y2). The degrees value should be between -360 and 360. If it is more or less, the value used is the specified number of degrees modulo 360.

Here are some examples of the -extent option:

# This draws half of an oval $canvas->createArc(0,0,100,150, ...