Introduction to Graphics Structures (GNU Octave)

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#### 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 `whos` function can be used to show the object type of each currently defined graphics handle. (Note: this is not true today, but it is, I hope, considered an error in whos. It may be better to have whos just show graphics_handle as the class, and provide a new function which, given a graphics handle, returns its object type. This could generalize the ishandle() functions and, in fact, replace them.)

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 are returned by `get (h)` where h is a handle of a graphics object. If only the names of the allowed properties are wanted they may be displayed by: `get (h, "")`.

Thus, for example:

    h = figure ();
    get (h, "type")
    ans = figure
    get (h, "");
    error: get: ambiguous figure property name ; possible matches:

    __gl_extensions__      dockcontrols           renderer
    __gl_renderer__        doublebuffer           renderermode
    __gl_vendor__          filename               resize
    __gl_version__         graphicssmoothing      resizefcn
    __graphics_toolkit__   handlevisibility       selected
    __guidata__            hittest                selectionhighlight
    __modified__           integerhandle          selectiontype
    __mouse_mode__         interruptible          sizechangedfcn
    __myhandle__           inverthardcopy         tag
    __pan_mode__           keypressfcn            toolbar
    __plot_stream__        keyreleasefcn          type
    __rotate_mode__        menubar                uicontextmenu
    __zoom_mode__          mincolormap            units
    alphamap               name                   userdata
    beingdeleted           nextplot               visible
    busyaction             numbertitle            windowbuttondownfcn
    buttondownfcn          outerposition          windowbuttonmotionfcn
    children               paperorientation       windowbuttonupfcn
    clipping               paperposition          windowkeypressfcn
    closerequestfcn        paperpositionmode      windowkeyreleasefcn
    color                  papersize              windowscrollwheelfcn
    colormap               papertype              windowstyle
    createfcn              paperunits             wvisual
    currentaxes            parent                 wvisualmode
    currentcharacter       pointer                xdisplay
    currentobject          pointershapecdata      xvisual
    currentpoint           pointershapehotspot    xvisualmode
    deletefcn              position

The properties of the root figure may be displayed by: `get (groot, "")`.

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. Otherwise, obj should be an instance of a class.

  See also: get, set, ismethod, isobject.

Next: Graphics Objects, Up: Graphics Data Structures [Contents][Index]