GDI Topics:  Lines

 

Lines

A line is a junction of two points. This means that a line has a beginning and an end:

Line

The beginning and the end are two distinct points that can be either POINT, CPoint, or a mix of a POINT and a CPoint values. In real life, before drawing, you should define where you would start. To help with this, the CDC class provides the MoveTo() method. It comes in two versions that are:

CPoint MoveTo(int x, int y);
CPoint MoveTo(POINT point);

The origin of a drawing is specified as a CPoint value. You can provide its horizontal and vertical measures as x and y or you can pass it as a POINT or a CPoint value.

To end the line, you use the CDC::LineTo() method. It comes it two versions declared as follows:

BOOL LineTo(int x, int y);
BOOL LineTo(POINT point);

The end of a line can be defined by its horizontal (x) and its vertical measures (y). It can also be provided as a POINT or a CPoint value. The last point of a line is not part of the line. The line ends just before reaching that point.

Here is an example that draws a line starting at a point defined as (10, 22) coordinates and ending at (155, 64):

void CExoView::OnDraw(CDC* pDC)
{
    pDC->MoveTo(10, 22);
    pDC->LineTo(155, 64);
}
Line Drawing

We have mentioned that the CDC::MoveTo() method is used to set the starting position of a line. When using LineTo(), the line would start from the MoveTo() point to the LineTo() end. As long as you do not call MoveTo(), any subsequent call to LineTo() would draw a line from the previous LineTo() to the new LineTo() point. You can use this property of the LineTo() method to draw various lines. Here is an example:

void CExoView::OnDraw(CDC* pDC)
{
	pDC->MoveTo(60, 20);
	pDC->LineTo(60, 122);
	pDC->LineTo(264, 122);
	pDC->LineTo(60, 20);
}
Multiple lines
 

Practical Learning: Drawing Lines

  1. Start Microsoft Visual C++ and create a Single Document application called GDI1 and based on the CView class
  2. Access the OnPaint event of your view class and implement it 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) };
    
    	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]);
    	// Do not call CView::OnPaint() for painting messages
    }
  3. Test the application
 

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