15.3.1 Introduction to Graphics Structures

The graphics functions use pointers, which are of class graphics_handle, in order to address the data structures which control visual display. A graphics handle may point to any one of a number of different base object types and these objects are the graphics data structures themselves. The primitive graphic object types are: figure, axes, line, text, patch, surface, text, image, and light.

Each of these objects has a function by the same name, and, each of these functions returns a graphics handle pointing to an object of the corresponding type. In addition there are several functions which operate on properties of the graphics objects and which also return handles: the functions plot and plot3 return a handle pointing to an object of type line, the function subplot returns a handle pointing to an object of type axes, the function fill returns a handle pointing to an object of type patch, the functions area, bar, barh, contour, contourf, contour3, surf, mesh, surfc, meshc, errorbar, quiver, quiver3, scatter, scatter3, stair, stem, stem3 each return a handle to a complex data structure as documented in Data Sources.

The graphics objects are arranged in a hierarchy:

1. The root object is returned by groot (historically, equivalent to the handle 0). In other words, get (groot) returns the properties of the root object.

2. Below the root are figure objects.

3. Below the figure objects are axes or hggroup objects.

4. Below the axes objects are line, text, patch, surface, image, and light objects.

Graphics handles may be distinguished from function handles (see Function Handles) by means of the function ishghandle. ishghandle returns true if its argument is a handle of a graphics object. In addition, a figure or axes object may be tested using isfigure or isaxes respectively. To test for a specific type of graphics handle, such as a patch or line object, use isgraphics. The more specific test functions return true only if the argument is both a graphics handle and of the correct type (figure, axes, specified object type).

The get and set commands are used to obtain and set the values of properties of graphics objects. In addition, the get command may be used to obtain property names.

For example, the property "type" of the graphics object pointed to by the graphics handle h may be displayed by:

get (h, "type")

The properties and their current values may be obtained in the form of a structure using s = get (h), where h is the handle of a graphics object. If only the names of the properties and the allowed values (for radio properties only) are wanted, one may use set (h).

Thus, for example:

h = figure ();
get (h, "type")
⇒ ans = figure
set (h)
⇒
        alphamap:
        beingdeleted:  [ {off} | on ]
        busyaction:  [ cancel | {queue} ]
        buttondownfcn:
        clipping:  [ off | {on} ]
        closerequestfcn:
        color:
        colormap:
        createfcn:
        currentaxes:
        deletefcn:
        dockcontrols:  [ {off} | on ]
        filename:
        graphicssmoothing:  [ off | {on} ]
        handlevisibility:  [ callback | off | {on} ]
        ...

The uses of get and set are further explained in get, set.

: res = isprop (obj, "prop")

Return true if prop is a property of the object obj.

obj may also be an array of objects in which case res will be a logical array indicating whether each handle has the property prop.

For plotting, obj is a handle to a graphics object.

Programming Note: There is no support in Octave versions 6.X.X for obj to be an instance of a class (i.e., a classdef object). This functionality has already been added on the development branch which will become Octave 7.1.0.

See also: get, set, ismethod, isobject.