| Geant4 User's Guide For Application Developers Visualization

8.9 Polylines, Markers and Text

Polylines, markers and text are defined in the `graphics_reps` category, and are used only for visualization. Here we explain their definitions and usages.

#### 8.9.1 Polylines

A polyline is a set of successive line segments. It is defined with a class G4Polyline defined in the `graphics_reps` category. A polyline is used to visualize tracking steps, particle trajectories, coordinate axes, and any other user-defined objects made of line segments.

G4Polyline is defined as a list of G4Point3D objects, i.e., vertex positions. The vertex positions are set to a G4Polyline object with the `push_back()` method.

For example, an x-axis with length 5 cm and with red color is defined in Source listing 8.9.1.

     //----- C++ source codes: An example of defining a line segment
    // Instantiate an emply polyline object
    G4Polyline  x_axis;

    // Set red line colour
    G4Colour         red(1.0, 0.0, 0.0);
    G4VisAttributes  att(red);
    x_axis.SetVisAttributes(&att);

    // Set vertex positions
    x_axis.push_back( G4Point3D(0., 0., 0.) );
    x_axis.push_back( G4Point3D(5.*cm, 0., 0.) );

     //----- end of C++ source codes
Source listing 8.9.1 Defining an x-axis with length 5 cm and with colour red.

#### 8.9.2 Markers

Here we explain how to use 3D markers in Geant4 Visualization.

What are Markers?

Markers set marks at arbitrary positions in the 3D space. They are often used to visualize hits of particles at detector components. A marker is a 2-dimensional primitive with shape (square, circle, etc), color, and special properties (a) of always facing the camera and (b) of having the possibility of a size defined in screen units (pixels). Here "size" means "overall size", e.g., diameter of circle and side of square (but diameter and radius access functions are defined to avoid ambiguity).

So the user who constructs a marker should decide whether or not it should be visualized to a given size in world coordinates by setting the world size. Alternatively, the user can set the screen size and the marker is visualized to its screen size. Finally, the user may decide not to set any size; in that case, it is drawn according to the sizes specified in the default marker specified in the class G4ViewParameters.

By default, "square" and "circle" are supported in Geant4 Visualization. The former is described with class G4Square, and the latter with class G4Circle:

Marker Type | Class Name
circle | G4Circle
right square | G4Square

These classes are inherited from class G4VMarker. They have constructors as follows:

          //----- Constructors of G4Circle and G4Square
          G4Circle::G4Circle (const G4Point3D& pos );
          G4Square::G4Square (const G4Point3D& pos);

Access functions of class G4VMarker are summarized below.

Access functions of markers

Source listing 8.9.2 shows the access functions inherited from the base class G4VMarker.

     //----- Set functions of G4VMarker
     void G4VMarker::SetPosition( const G4Point3D& );
     void G4VMarker::SetWorldSize( G4double );
     void G4VMarker::SetWorldDiameter( G4double );
     void G4VMarker::SetWorldRadius( G4double );
     void G4VMarker::SetScreenSize( G4double );
     void G4VMarker::SetScreenDiameter( G4double );
     void G4VMarker::SetScreenRadius( G4double );
     void G4VMarker::SetFillStyle( FillStyle );
     // Note: enum G4VMarker::FillStyle {noFill, hashed, filled};

     //----- Get functions of G4VMarker
     G4Point3D G4VMarker::GetPosition () const;
     G4double G4VMarker::GetWorldSize () const;
     G4double G4VMarker::GetWorldDiameter () const;
     G4double G4VMarker::GetWorldRadius () const;
     G4double G4VMarker::GetScreenSize () const;
     G4double G4VMarker::GetScreenDiameter () const;
     G4double G4VMarker::GetScreenRadius () const;
     FillStyle G4VMarker::GetFillStyle () const;
     // Note: enum G4VMarker::FillStyle {noFill, hashed, filled};
Source listing 8.9.2 The access functions inherited from the base class G4VMarker.

Source listing 8.9.3 shows sample C++ source code to define a very small red circle, i.e., a dot with diameter 1.0 pixel. Such a dot is often used to visualize a hit.

     //----- C++ source codes: An example of defining a red small maker
     G4Circle circle(position); // Instantiate a circle with its 3D
                                // position. The argument "position"
                                // is defined as G4Point3D instance
     circle.SetScreenDiameter (1.0); // Should be circle.SetScreenDiameter
                                     //  (1.0 * pixels) - to be implemented
     circle.SetFillStyle (G4Circle::filled); // Make it a filled circle
     G4Colour colour(1.,0.,0.);              // Define red color
     G4VisAttributes attribs(colour);        // Define a red visualization attribute
     circle.SetVisAttributes(attribs);       // Assign the red attribute to the circle
     //----- end of C++ source codes
Source listing 8.9.3 Sample C++ source code to define a very small red circle.

#### 8.9.3 Text

Text, i.e., a character string, is used to visualize various kinds of description, particle name, energy, coordinate names etc. Text is described by the class G4Text . The following constructors are supported:

         //----- Constructors of G4Text
         G4Text (const G4String& text);
         G4Text (const G4String& text, const G4Point3D& pos);

where the argument `text` is the text (string) to be visualized, and `pos` is the 3D position at which the text is visualized.

Note that class G4Text also inherits G4VMarker. Size of text is recognized as "font size", i.e., height of the text. All the access functions defined for class G4VMarker mentioned above are available. In addition, the following access functions are available, too:

         //----- Set functions of G4Text
         void G4Text::SetText ( const G4String& text ) ;
         void G4Text::SetOffset ( double dx, double dy ) ;

         //----- Get functions of G4Text
         G4String G4Text::GetText () const;
         G4double G4Text::GetXOffset () const;
         G4double G4Text::GetYOffset () const;

Method `SetText()` defines text to be visualized, and `GetText()` returns the defined text. Method `SetOffset()` defines x (horizontal) and y (vertical) offsets in the screen coordinates. By default, both offsets are zero, and the text starts from the 3D position given to the constructor or to the method `G4VMarker:SetPosition()`. Offsets should be given with the same units as the one adopted for the size, i.e., world-size or screen-size units.

Source listing 8.9.4 shows sample C++ source code to define text with the following properties:

- Text: "Welcome to Geant4 Visualization"
- Position: (0.,0.,0.) in the world coordinates
- Horizontal offset: 10 pixels
- Vertical offset: -20 pixels
- Colour: blue (default)

     //----- C++ source codes: An example of defining a visualizable text

     //----- Instantiation
     G4Text text ;
     text.SetText ( "Welcome to Geant4 Visualization");
     text.SetPosition ( G4Point3D(0.,0.,0.) );
     // These three lines are equivalent to:
     //  G4Text text ( "Welcome to Geant4 Visualization",
     //                 G4Point3D(0.,0.,0.) );

     //----- Size (font size in units of pixels)
     G4double fontsize = 24.; // Should be 24. * pixels - to be implemented.
     text.SetScreenSize ( fontsize );

     //----- Offsets
     G4double x_offset = 10.; // Should be 10. * pixels - to be implemented.
     G4double y_offset = -20.; // Should be -20. * pixels - to be implemented.
     text.SetOffset( x_offset, y_offset );

     //----- Color (Blue is the default setting, and so the codes below are omissible)
     G4Colour blue( 0., 0., 1. );
     G4VisAttributes att ( blue );
     text.SetVisAttributes ( att );

     //----- end of C++ source codes
Source listing 8.9.4 An example of defining text.

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