Modify Graphics Objects

This example shows how to create, display, and modify graphics objects in MATLAB®.

Graphics Objects

When MATLAB creates a plot, it creates a series of graphics objects. Figures, axes, lines, patches, and text are examples of graphics objects. The figure below has three graphics objects -- an axes, a line, and a text object. Use an optional output argument to store the graphics object that is created.

x = -pi:pi/20:pi;
y = sin(x);

f = figure;
p = plot(x,y);
txt1 = text(0.2,0,'sin(x)');

All graphics objects have properties that you can view and modify. These properties have default values. The display of the line object, p, shows the most commonly used line properties, such as Color, LineStyle, and LineWidth.

p = 
  Line with properties:

              Color: [0 0.4470 0.7410]
          LineStyle: '-'
          LineWidth: 0.5000
             Marker: 'none'
         MarkerSize: 6
    MarkerFaceColor: 'none'
              XData: [1x41 double]
              YData: [1x41 double]
              ZData: [1x0 double]

  Show all properties

MATLAB shows the same display if the semicolon is missing from the command that creates the object.

txt2 = text(x(end), y(end), 'pi')

txt2 = 
  Text (pi) with properties:

                 String: 'pi'
               FontSize: 10
             FontWeight: 'normal'
               FontName: 'Helvetica'
                  Color: [0 0 0]
    HorizontalAlignment: 'left'
               Position: [3.1416 1.2246e-16 0]
                  Units: 'data'

  Show all properties

Get Graphics Object Properties

To access individual properties of a graphics object, use dot notation syntax object.PropertyName. For example, return the LineWidth property for the line object.

pcol = p.LineWidth

pcol = 0.5000

Change the line color to red by setting its Color property.

p.Color = 'red';

Parents and Children

MATLAB arranges graphics objects in a hierarchy. The top of the hierarchy is a special object called the graphics root. To access the graphics root, use the groot function.

groot

ans = 
  Graphics Root with properties:

          CurrentFigure: [1x1 Figure]
    ScreenPixelsPerInch: 75
             ScreenSize: [1 1 1280 1024]
       MonitorPositions: [1 1 1280 1024]
                  Units: 'pixels'

  Show all properties

All graphics objects (except the root) have a parent. For example, the parent of an axes is a figure.

ax = gca;
ax.Parent

ans = 
  Figure (1) with properties:

      Number: 1
        Name: ''
       Color: [0.9400 0.9400 0.9400]
    Position: [360 502 560 420]
       Units: 'pixels'

  Show all properties

Many objects also have children. This axes has three children - the two text objects and the line object.

ax.Children

ans = 
  3x1 graphics array:

  Text    (pi)
  Text    (sin(x))
  Line

Since the axes has multiple children, the value of the Children property is an array of graphics objects. To access an individual child of the axes, index into the array. You can then set properties of the child object.

t = ax.Children(2);       % get the 'sin(x)' text object
t.FontWeight = 'bold';    % set the font to bold

Preallocate Graphics Objects Array

It is a best practice in MATLAB to preallocate an array before using it. Use the gobjects command to preallocate an array of graphics objects. You can then add graphics objects to the array.

objarray = gobjects(1,5);
objarray(1) = f;
objarray(2) = ax;
objarray(3) = p;
objarray(4) = txt1;
objarray(5) = txt2;
objarray

objarray = 
  1x5 graphics array:

    Figure    Axes      Line      Text      Text

Get All Object Properties

Graphics objects in MATLAB have many properties. To see all the properties of an object, use the get command.

get(f)

                 Alphamap: [1x64 double]
             BeingDeleted: off
               BusyAction: 'queue'
            ButtonDownFcn: ''
                 Children: [1x1 Axes]
                 Clipping: on
          CloseRequestFcn: 'closereq'
                    Color: [0.9400 0.9400 0.9400]
                 Colormap: [256x3 double]
              ContextMenu: [0x0 GraphicsPlaceholder]
                CreateFcn: ''
              CurrentAxes: [1x1 Axes]
         CurrentCharacter: ''
            CurrentObject: [0x0 GraphicsPlaceholder]
             CurrentPoint: [0 0]
                DeleteFcn: ''
             DockControls: on
                 FileName: ''
        GraphicsSmoothing: on
         HandleVisibility: 'on'
            InnerPosition: [360 502 560 420]
            IntegerHandle: on
            Interruptible: on
           InvertHardcopy: on
              KeyPressFcn: ''
            KeyReleaseFcn: ''
                  MenuBar: 'none'
                     Name: ''
                 NextPlot: 'add'
                   Number: 1
              NumberTitle: on
            OuterPosition: [356 498 568 447]
         PaperOrientation: 'portrait'
            PaperPosition: [0.5167 2.7000 7.4667 5.6000]
        PaperPositionMode: 'auto'
                PaperSize: [8.5000 11]
                PaperType: 'usletter'
               PaperUnits: 'inches'
                   Parent: [1x1 Root]
                  Pointer: 'arrow'
        PointerShapeCData: [16x16 double]
      PointerShapeHotSpot: [1 1]
                 Position: [360 502 560 420]
                 Renderer: 'opengl'
             RendererMode: 'auto'
                   Resize: on
               Scrollable: off
            SelectionType: 'normal'
           SizeChangedFcn: ''
                      Tag: ''
                  ToolBar: 'none'
                     Type: 'figure'
                    Units: 'pixels'
                 UserData: []
                  Visible: off
      WindowButtonDownFcn: ''
    WindowButtonMotionFcn: ''
        WindowButtonUpFcn: ''
        WindowKeyPressFcn: ''
      WindowKeyReleaseFcn: ''
     WindowScrollWheelFcn: ''
              WindowState: 'normal'
              WindowStyle: 'normal'
                 XDisplay: ':0'

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