2D and 3D GRAPHICS

Example

The following code sets up a very simple hit detection scheme on a black polygon. When you click the mouse on or near the black polygon, the command line displays the text, "Selected," and when you click the mouse anywhere else in the view, the command line displays, "Not selected."

  Copyright ©  Taligent, Inc. 1995
1  class THelloWorld : public TDocumentComponentView, public MMouseEventHandler
2  {
3      public:
4      THelloWorld( TGUIBundle* b ) :
5          TDocumentComponentView( b ),
6      {
7          CheckForInitialize();
8          SetAllocatedArea( TGRect( 0,0, 1000, 1000 ) );
9  
10          TFillBundle* blackBundle = new TFillBundle( TRGBColor( 0, 0, 0 ) );
11  
12          TGPolygon blackPolygonGeometry;
13          blackPolygonGeometry.Append( TGPoint( 100, 60 ) );
14          blackPolygonGeometry.Append( TGPoint( 140, 115 ) );
15          blackPolygonGeometry.Append( TGPoint( 60, 115 ) );
16          fGraphic = new TPolygon( blackPolygonGeometry, blackBundle );
17      }
18  
19      ~THelloWorld()
20      {
21  
22          CheckForFinalize( );
23          delete fGraphic;
24      }
25      void
26      Initialize()
27      {
28          TDocumentComponentView::Initialize();
29      }
30      void DrawContents( TGrafPort& thePort ) const
31      {
32          TGArea area;
33          GetAllocatedArea( area );
34          TFillBundle bundle( new TRGBColor( 1,1,1 ) );
35          thePort.Draw ( area.GetBounds(), bundle );
36  
37          fGraphic->Draw( thePort );
38      }
39      Boolean
40      MouseDown( TMouseDownEvent& mouseDown )
41      {
42          TGPoint selection( mouseDown.GetEventPosition() );
43          GlobalToLocal( selection  );
44  
45          TGRect bounds( fGraphic->GetGeometricBounds() );
46  
47          if ( bounds.Contains( selection ) )
48              qprintf( "Selected\n" );
49          else
50              qprintf( "not selected\n" );
51  
52          Invalidate();
53  
54          return( TRUE);
55      }
56      VersionDeclarationsMacro( THelloWorld );
57  
58      private:
59          THelloWorld( conts THellowWorld& );
60          TPolygon* fGraphic;

Changes to THelloWorld

Line 1: To implement hit detection, you need to mix THelloWorld with MMouseEventHandler. MMouseEventHandler has a MouseDown function that you implement in lines 40 through 55.

The addition of MMouseEventHandler means that the graphic must be accessible to the MouseDown function.

Lines 10 through 16 and Lines 58 through 60: Instead of creating and drawing the polygon in DrawContents, a private field is added to THelloWorld of type TPolygon. The THelloWorld constructor creates the polygon and stores it in fGraphic.

Line 23: A line is added to the destructor to delete fGraphic.

MouseDown function

Lines 40 through 55: The View system calls the MouseDown function when a mouse down event is detected. The mouseDown parameter contains the TGPoint where the mouse down event occurred in global coordinates. The code gets the TGPoint from mouseDown, converts it to local coordinates, gets the bounds of the polygon and tests to see whether the TGPoint is contained within the bounds of the polygon. Since the bounds is a rectangle and the polygon is not rectangular, there are points close to, but outside of, the polygon that return True because they are still within the polygon bounds.

The Invalidate function redraws the view after changes are made. If you adapted this function to change the color of the fill, the Invalidate function redraws the View with the new fill color.

Changes to DrawContents

Lines 30 through 38: To draw the polygon, you call the Draw function on fGraphic.

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