octave: Introduction to Graphics Structures
15.3.1 Introduction to Graphics Structures
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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 XREFdatasources.
The graphics objects are arranged in a hierarchy:
1. The root is at 0. In other words, ‘get (0)’ returns the
properties of the root object.
2. Below the root are ‘figure’ objects.
3. Below the ‘figure’ objects are ‘axes’ objects.
4. Below the ‘axes’ objects are ‘line’, ‘text’, ‘patch’, ‘surface’,
‘image’, and ‘light’ objects.
Graphics handles may be distinguished from function handles (
Function Handles) by means of the function ‘ishandle’. ‘ishandle’
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. The test functions return true only if the
argument is both a handle and of the correct type (figure or axes).
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:
__enhanced__ hittest resize
__graphics_toolkit__ integerhandle resizefcn
__guidata__ interruptible selected
__modified__ inverthardcopy selectionhighlight
__myhandle__ keypressfcn selectiontype
__plot_stream__ keyreleasefcn tag
alphamap menubar toolbar
beingdeleted mincolormap type
busyaction name uicontextmenu
buttondownfcn nextplot units
children numbertitle userdata
clipping outerposition visible
closerequestfcn paperorientation windowbuttondownfcn
color paperposition windowbuttonmotionfcn
colormap paperpositionmode windowbuttonupfcn
createfcn papersize windowkeypressfcn
currentaxes papertype windowkeyreleasefcn
currentcharacter paperunits windowscrollwheelfcn
currentobject parent windowstyle
currentpoint pointer wvisual
deletefcn pointershapecdata wvisualmode
dockcontrols pointershapehotspot xdisplay
doublebuffer position xvisual
filename renderer xvisualmode
handlevisibility renderermode
The root figure has index 0. Its properties may be displayed by:
‘get (0, "")’.
The uses of ‘get’ and ‘set’ are further explained in get
XREFget, set XREFset.
-- : 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.
DONTPRINTYET See also: get XREFget, set XREFset, *noteismethod:
DONTPRINTYET See also: get XREFget, set XREFset, ismethod
XREFismethod, isobject XREFisobject.
Info Catalog
octave: Graphics Data Structures
octave: Graphics Objects