GDI Topics: Polygons

 

Introduction

The polylines we have used so far were drawn by defining the starting point of the first line and the end point of the last line and there was no relationship or connection between these two extreme points. A polygon is a closed polyline. In other words, it is a polyline defined so that the end point of the last line is connected to the start point of the first line.

To draw a polygon, you can use the CDC::Polygon() method. Its syntax is:

BOOL Polygon(LPPOINT lpPoints, int nCount);

This member function uses the same types of arguments as the Polyline() method. The only difference is on the drawing of the line combination. Here is an example:

void CExoView::OnDraw(CDC* pDC)
{
	CPoint Pt[7];
	Pt[0] = CPoint(20, 50);
	Pt[1] = CPoint(180, 50);
	Pt[2] = CPoint(180, 20);
	Pt[3] = CPoint(230, 70);
	Pt[4] = CPoint(180, 120);
	Pt[5] = CPoint(180, 90);
	Pt[6] = CPoint(20, 90);

	pDC->Polygon(Pt, 7);
}
Polygon

Practical Learning:  Drawing Polygons

  1. To draw some polygons, change the program as follows:
     
    void CView1View::OnPaint() 
    {
    	CPaintDC dc(this); // device context for painting
    	
    	// TODO: Add your message handler code here
    	CPoint PtLine[] = { CPoint( 50,  50), CPoint(670,  50),
    			  CPoint(670, 310), CPoint(490, 310),
    			  CPoint(490, 390), CPoint(220, 390),
    			  CPoint(220, 310), CPoint( 50, 310), 
    			  CPoint( 50,  50) };
    
    	CPoint Bedroom1[] = { CPoint( 55,  55), CPoint(175,  55),
    			    CPoint(175, 145), CPoint( 55, 145)};
    	CPoint Closets[]  = { CPoint( 55, 150), CPoint(145, 150),
    			    CPoint(145, 205), CPoint( 55, 205) };
    	CPoint Bedroom2[] = { CPoint(55, 210), CPoint(160, 210),
    			    CPoint(160, 305), CPoint(55, 305) };
    
    	dc.MoveTo(PtLine[0]);
    	dc.LineTo(PtLine[1]);
    	dc.LineTo(PtLine[2]);
    	dc.LineTo(PtLine[3]);
    	dc.LineTo(PtLine[4]);
    	dc.LineTo(PtLine[5]);
    	dc.LineTo(PtLine[6]);
    	dc.LineTo(PtLine[7]);
    	dc.LineTo(PtLine[8]);
    
    	dc.Polygon(Bedroom1, 4);
    	dc.Polygon(Closets, 4);
    	dc.Polygon(Bedroom2, 4);
    	// Do not call CView::OnPaint() for painting messages
    }
  2. Test the application and return to MSVC

Multiple Polygons

If you want to draw multiple polygons, you can use the CDC::PolyPolygon() method whose syntax is:

BOOL PolyPolygon(LPPOINT lpPoints, LPINT lpPolyCounts, int nCount);

Like the Polygon() method, the lpPoints argument is an array of POINT or CPoint values. The PolyPolygon() method needs to know the number of polygons you would be drawing.  Each polygon uses the points of the lpPoints values but when creating the array of points, the values must be incremental. This means that PolyPolygon() will not randomly access the values of lpPoints. Each polygon has its own set of points. 

Unlike Polygon(), the nCount argument of PolyPolygon() is the number of polygons you want to draw and not the number of points.

The lpPolyCounts argument is an array or integers. Each member of this array specifies the number of vertices (lines) that its polygon will have..

Here is an example:

void CExoView::OnDraw(CDC* pDC)
{
	CPoint Pt[12];
	int lpPts[] = { 3, 3, 3, 3 };

	// Top Triangle
	Pt[0] = CPoint(125,  10);
	Pt[1] = CPoint( 95,  70);
	Pt[2] = CPoint(155,  70);

	// Left Triangle
	Pt[3] = CPoint( 80,  80);
	Pt[4] = CPoint( 20, 110);
	Pt[5] = CPoint( 80, 140);

	// Bottom Triangle
	Pt[6] = CPoint( 95, 155);
	Pt[7] = CPoint(125, 215);
	Pt[8] = CPoint(155, 155);
	
	// Right Triangle
	Pt[9] = CPoint(170,  80);
	Pt[10] = CPoint(170, 140);
	Pt[11] = CPoint(230, 110);

	pDC->PolyPolygon(Pt, lpPts, 4);
}
PolyPolygon
 

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