The canvas command creates a new window (given
by the pathName argument) and makes it into a canvas widget.
Additional options, described above, may be specified on the
command line or in the option database
to configure aspects of the canvas such as its colors and 3-D relief.
The canvas command returns its
pathName argument. At the time this command is invoked,
there must not exist a window named pathName, but
pathName's parent must exist.
Canvas widgets implement structured graphics.
A canvas displays any number of items, which may be things like
rectangles, circles, lines, and text.
Items may be manipulated (e.g. moved or re-colored) and commands may
be associated with items in much the same way that the bind
command allows commands to be bound to widgets. For example,
a particular command may be associated with the <Button-1> event
so that the command is invoked whenever button 1 is pressed with
the mouse cursor over an item.
This means that items in a canvas can have behaviors defined by
the Tcl scripts bound to them.