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15.3.2 Graphics Objects

The hierarchy of graphics objects was explained above. See Introduction to Graphics Structures. Here the specific objects are described, and the properties contained in these objects are discussed. Keep in mind that graphics objects are always referenced by handle.

root figure

the top level of the hierarchy and the parent of all figure objects. The handle index of the root figure is 0.

figure

A figure window.

axes

A set of axes. This object is a child of a figure object and may be a parent of line, text, image, patch, or surface objects.

line

A line in two or three dimensions.

text

Text annotations.

image

A bitmap image.

patch

A filled polygon, currently limited to two dimensions.

surface

A three-dimensional surface.

15.3.2.1 Creating Graphics Objects

You can create axes, line, patch, and surface objects directly using the axes, line, patch, fill, and surface functions. These objects become children of the current axes object.

Function File: axes ()
Function File: axes (property, value, …)
Function File: axes (hax)
Function File: h = axes (…)

Create an axes object and return a handle to it, or set the current axes to hax.

Called without any arguments, or with property/value pairs, construct a new axes. For accepted properties and corresponding values, see set.

Called with a single axes handle argument hax, the function makes hax the current axis. It also restacks the axes in the corresponding figure so that hax is the first entry in the list of children. This causes hax to be displayed on top of any other axes objects (Z-order stacking).

See also: gca, set, get.

Function File: line ()
Function File: line (x, y)
Function File: line (x, y, property, value, …)
Function File: line (x, y, z)
Function File: line (x, y, z, property, value, …)
Function File: line (property, value, …)
Function File: line (hax, …)
Function File: h = line (…)

Create line object from x and y (and possibly z) and insert in the current axes.

Multiple property-value pairs may be specified for the line object, but they must appear in pairs.

If the first argument hax is an axes handle, then plot into this axis, rather than the current axes returned by gca.

The optional return value h is a graphics handle (or vector of handles) to the line objects created.

See also: image, patch, rectangle, surface, text.

Function File: patch ()
Function File: patch (x, y, c)
Function File: patch (x, y, z, c)
Function File: patch (fv)
Function File: patch ("Faces", faces, "Vertices", verts, …)
Function File: patch (…, prop, val, …)
Function File: patch (hax, …)
Function File: h = patch (…)

Create patch object in the current axes with vertices at locations (x, y) and of color c.

If the vertices are matrices of size MxN then each polygon patch has M vertices and a total of N polygons will be created. If some polygons do not have M vertices use NaN to represent "no vertex". If the z input is present then 3-D patches will be created.

The color argument c can take many forms. To create polygons which all share a single color use a string value (e.g., "r" for red), a scalar value which is scaled by caxis and indexed into the current colormap, or a 3-element RGB vector with the precise TrueColor.

If c is a vector of length N then the ith polygon will have a color determined by scaling entry c(i) according to caxis and then indexing into the current colormap. More complicated coloring situations require directly manipulating patch property/value pairs.

Instead of specifying polygons by matrices x and y, it is possible to present a unique list of vertices and then a list of polygon faces created from those vertices. In this case the "Vertices" matrix will be an Nx2 (2-D patch) or Nx3 (3-D path). The MxN "Faces" matrix describes M polygons having N vertices—each row describes a single polygon and each column entry is an index into the "Vertices" matrix to identify a vertex. The patch object can be created by directly passing the property/value pairs "Vertices"/verts, "Faces"/faces as inputs.

A third input form is to create a structure fv with the fields "vertices", "faces", and optionally "facevertexcdata".

If the first argument hax is an axes handle, then plot into this axis, rather than the current axes returned by gca.

The optional return value h is a graphics handle to the created patch object.

Implementation Note: Patches are highly configurable objects. To truly customize them requires setting patch properties directly. Useful patch properties are: "cdata", "edgecolor", "facecolor", "faces", "facevertexcdata".

See also: fill, get, set.

Function File: fill (x, y, c)
Function File: fill (x1, y1, c1, x2, y2, c2)
Function File: fill (…, prop, val)
Function File: fill (hax, …)
Function File: h = fill (…)

Create one or more filled 2-D polygons.

The inputs x and y are the coordinates of the polygon vertices. If the inputs are matrices then the rows represent different vertices and each column produces a different polygon. fill will close any open polygons before plotting.

The input c determines the color of the polygon. The simplest form is a single color specification such as a plot format or an RGB-triple. In this case the polygon(s) will have one unique color. If c is a vector or matrix then the color data is first scaled using caxis and then indexed into the current colormap. A row vector will color each polygon (a column from matrices x and y) with a single computed color. A matrix c of the same size as x and y will compute the color of each vertex and then interpolate the face color between the vertices.

Multiple property/value pairs for the underlying patch object may be specified, but they must appear in pairs.

If the first argument hax is an axes handle, then plot into this axis, rather than the current axes returned by gca.

The optional return value h is a vector of graphics handles to the created patch objects.

Example: red square

vertices = [0 0
            1 0
            1 1
            0 1];
fill (vertices(:,1), vertices(:,2), "r");
axis ([-0.5 1.5, -0.5 1.5])
axis equal

See also: patch, caxis, colormap.

Function File: surface (x, y, z, c)
Function File: surface (x, y, z)
Function File: surface (z, c)
Function File: surface (z)
Function File: surface (…, prop, val, …)
Function File: surface (hax, …)
Function File: h = surface (…)

Create a surface graphic object given matrices x and y from meshgrid and a matrix of values z corresponding to the x and y coordinates of the surface.

If x and y are vectors, then a typical vertex is (x(j), y(i), z(i,j)). Thus, columns of z correspond to different x values and rows of z correspond to different y values. If only a single input z is given then x is taken to be 1:rows (z) and y is 1:columns (z).

Any property/value input pairs are assigned to the surface object.

If the first argument hax is an axes handle, then plot into this axis, rather than the current axes returned by gca.

The optional return value h is a graphics handle to the created surface object.

See also: surf, mesh, patch, line.

15.3.2.2 Handle Functions

To determine whether a variable is a graphics object index, or an index to an axes or figure, use the functions ishandle, isaxes, and isfigure.

Built-in Function: ishandle (h)

Return true if h is a graphics handle and false otherwise.

h may also be a matrix of handles in which case a logical array is returned that is true where the elements of h are graphics handles and false where they are not.

See also: isaxes, isfigure.

Function File: ishghandle (h)

Return true if h is a graphics handle and false otherwise.

This function is equivalent to ishandle and is provided for compatibility with MATLAB.

See also: ishandle.

Function File: isaxes (h)

Return true if h is an axes graphics handle and false otherwise.

If h is a matrix then return a logical array which is true where the elements of h are axes graphics handles and false where they are not.

See also: isaxes, ishandle.

Function File: isfigure (h)

Return true if h is a figure graphics handle and false otherwise.

If h is a matrix then return a logical array which is true where the elements of h are figure graphics handles and false where they are not.

See also: isaxes, ishandle.

The function gcf returns an index to the current figure object, or creates one if none exists. Similarly, gca returns the current axes object, or creates one (and its parent figure object) if none exists.

Function File: h = gcf ()

Return a handle to the current figure.

The current figure is the default target for graphics output. If multiple figures exist, gcf returns the last created figure or the last figure that was clicked on with the mouse.

If a current figure does not exist, create one and return its handle. The handle may then be used to examine or set properties of the figure. For example,

fplot (@sin, [-10, 10]);
fig = gcf ();
set (fig, "numbertitle", "off", "name", "sin plot")

plots a sine wave, finds the handle of the current figure, and then renames the figure window to describe the contents.

Note: To find the current figure without creating a new one if it does not exist, query the "CurrentFigure" property of the root graphics object.

get (0, "currentfigure");

See also: gca, gco, gcbf, gcbo, get, set.

Function File: h = gca ()

Return a handle to the current axis object.

The current axis is the default target for graphics output. In the case of a figure with multiple axes, gca returns the last created axes or the last axes that was clicked on with the mouse.

If no current axes object exists, create one and return its handle. The handle may then be used to examine or set properties of the axes. For example,

ax = gca ();
set (ax, "position", [0.5, 0.5, 0.5, 0.5]);

creates an empty axes object and then changes its location and size in the figure window.

Note: To find the current axis without creating a new axes object if it does not exist, query the "CurrentAxes" property of a figure.

get (gcf, "currentaxes");

See also: gcf, gco, gcbf, gcbo, get, set.

Function File: h = gco ()
Function File: h = gco (fig)

Return a handle to the current object of the current figure, or a handle to the current object of the figure with handle fig.

The current object of a figure is the object that was last clicked on. It is stored in the "CurrentObject" property of the target figure.

If the last mouse click did not occur on any child object of the figure, then the current object is the figure itself.

If no mouse click occurred in the target figure, this function returns an empty matrix.

Programming Note: The value returned by this function is not necessarily the same as the one returned by gcbo during callback execution. An executing callback can be interrupted by another callback and the current object may be changed.

See also: gcbo, gca, gcf, gcbf, get, set.

The get and set functions may be used to examine and set properties for graphics objects. For example,

get (0)
    ⇒ ans =
       {
         type = root
         currentfigure = [](0x0)
         children = [](0x0)
         visible = on
         …
       }

returns a structure containing all the properties of the root figure. As with all functions in Octave, the structure is returned by value, so modifying it will not modify the internal root figure plot object. To do that, you must use the set function. Also, note that in this case, the currentfigure property is empty, which indicates that there is no current figure window.

The get function may also be used to find the value of a single property. For example,

get (gca (), "xlim")
    ⇒ [ 0 1 ]

returns the range of the x-axis for the current axes object in the current figure.

To set graphics object properties, use the set function. For example,

set (gca (), "xlim", [-10, 10]);

sets the range of the x-axis for the current axes object in the current figure to ‘[-10, 10]’. Additionally, calling set with a graphics object index as the only argument returns a structure containing the default values for all the properties for the given object type. For example,

set (gca ())

returns a structure containing the default property values for axes objects.

Built-in Function: val = get (h)
Built-in Function: val = get (h, p)

Return the value of the named property p from the graphics handle h. If p is omitted, return the complete property list for h. If h is a vector, return a cell array including the property values or lists respectively.

See also: set.

Built-in Function: set (h, property, value, …)
Built-in Function: set (h, properties, values)
Built-in Function: set (h, pv)

Set named property values for the graphics handle (or vector of graphics handles) h. There are three ways how to give the property names and values:

See also: get.

Function File: parent = ancestor (h, type)
Function File: parent = ancestor (h, type, "toplevel")

Return the first ancestor of handle object h whose type matches type, where type is a character string. If type is a cell array of strings, return the first parent whose type matches any of the given type strings.

If the handle object h itself is of type type, return h.

If "toplevel" is given as a third argument, return the highest parent in the object hierarchy that matches the condition, instead of the first (nearest) one.

See also: findobj, findall, allchild.

Function File: h = allchild (handles)

Find all children, including hidden children, of a graphics object.

This function is similar to get (h, "children"), but also returns hidden objects (HandleVisibility = "off"). If handles is a scalar, h will be a vector. Otherwise, h will be a cell matrix of the same size as handles and each cell will contain a vector of handles.

See also: findall, findobj, get, set.

Function File: findfigs ()

Find all visible figures that are currently off the screen and move them onto the screen.

See also: allchild, figure, get, set.

Figures can be printed or saved in many graphics formats with print and saveas. Occasionally, however, it may be useful to save the original Octave handle graphic directly so that further modifications can be made such as modifying a title or legend.

This can be accomplished with the following functions by

fig_struct = hdl2struct (gcf);
save myplot.fig -struct fig_struct;
…
fig_struct = load ("myplot.fig");
struct2hdl (fig_struct);
Function File: s = hdl2struct (h)

Return a structure, s, whose fields describe the properties of the object, and its children, associated with the handle, h.

The fields of the structure s are "type", "handle", "properties", "children", and "special".

See also: struct2hdl, findobj.

Function File: h = struct2hdl (s)
Function File: h = struct2hdl (s, p)
Function File: h = struct2hdl (s, p, hilev)

Construct a graphics handle object h from the structure s.

The structure must contain the fields "handle", "type", "children", "properties", and "special". If the handle of an existing figure or axes is specified, p, the new object will be created as a child of that object. If no parent handle is provided then a new figure and the necessary children will be constructed using the default values from the root figure.

A third boolean argument hilev can be passed to specify whether the function should preserve listeners/callbacks, e.g., for legends or hggroups. The default is false.

See also: hdl2struct, findobj.

Function File: hnew = copyobj (horig)
Function File: hnew = copyobj (horig, hparent)

Construct a copy of the graphic object associated with handle horig and return a handle hnew to the new object.

If a parent handle hparent (root, figure, axes, or hggroup) is specified, the copied object will be created as a child of hparent.

See also: struct2hdl, hdl2struct, findobj.


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