Detect the angle of a line in an image

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Introduction

Today's image processing technology help us in many areas such as determining tumors, text extraction, checking images,... In this algorithm, we can get the angle of a line in an image. Of course there are several ways to do this and advances algorithms, but this is a simple algorithm.

Features

• Works in real images with several colors.
• Simple to implement.
• Gets the length of the line.
• Very light-weight code.

Source Code

I have commented the code clearly. First we must define a global CImage and then load the image file from hard disk:

CImage i;

After that, we need to draw an image when cleared with the OnPaint() function:

void CDetectangleoflineDlg::OnPaint()
{
  if(!i.IsNull()){//image isn't null
    //get handle device content from this dialog
	CClientDC dc(this);
	i.BitBlt(dc.m_hDC,0,0,SRCCOPY);
	//draw picture in dialog backgroun
  }
}

Next, we load the image:

void CDetectangleoflineDlg::OnBnClickedButton1()//load image
{
  CFileDialog f(1);//to call open file dialog
  TCHAR tc[260];CString s;GetCurrentDirectory(260,tc);s=tc;
  if(f.DoModal()==IDOK){//image selected
	if(!i.IsNull())//image isn't null
		i.Destroy();//destroy image
          i.Load(f.GetFolderPath()+_T("\\")+f.GetFileName());
          //load image to i (CImage)
	OnPaint();//call draw image
	b.EnableWindow(1);
  }
}

Finally, we use the Process algorithm to determine the angle:

void CDetectangleoflineDlg::OnBnClickedButton2()//process
{
	CImage i1;
	i1=i;//copy 'i' to 'i1'
	CDC cdc;//to get device content of 'i1'
	//get device content of i to cdc to draw or getbitmap
	cdc.Attach(i1.GetDC());
	//define pens and choose them in 'cdc'
	CPen p(PS_SOLID,1,RGB(255,255,0)),p1(PS_SOLID,1,RGB(0,0,255));
	cdc.SelectObject(&p);
	//define neccessary variables
	register int x1,x2,y1,y2;
	x1=x2=y1=y2=-1;
	double pi=3.1415926535;
	register long r,g,b,r1,g1,b1;
	register COLORREF c,c1;
	//'tol':toleranc;defference beetwen colors.
	//'d':the most width of line
	register int tol=30,d=20;
	//get first color as background color
	c=i1.GetPixel(0,0);r=GetRValue(c);g=GetGValue(c);b=GetBValue(c);
	for(register int y=0;y<i1.GetHeight();y++)//check all points in image
		for(register int x=0;x<i1.GetWidth();x++){
		    //get current point's color to compare
			c1=i1.GetPixel(x,y);r1=GetRValue(c1);
			g1=GetGValue(c1);b1=GetBValue(c1);

			if((abs(r-r1)>tol)&&(abs(g-g1)>tol)&&(abs(b-b1)>tol)){
			//line detect
				if(x1==-1){//first point of line
					x1=x2=x;y1=y2=y;
				}else if(sqrt(pow(x-x2,2.0)+pow(y-y2,2.0))<=d){
				 //next point
					x2=x;y2=y;
					cdc.SetPixel(x,y,RGB(255,255,0));
					//coloring line with yellow color
				}
			}
		}
    //compute line's length width: r^2=x^2+y^2
	register float radius=sqrt(pow(x1-x2,2.0)+pow(y1-y2,2.0));
	//end:to get angle of line
	float t=atan((float)(y2-y1)/(x2-x1))*180/pi;
	cdc.SelectObject(&p1);//set 'p1' for blue color
	//draw lines around Line
	cdc.MoveTo((x1+x2)/2-radius/2,(y1+y2)/2);
	cdc.LineTo((x1+x2)/2+radius/2,(y1+y2)/2);
	cdc.MoveTo((x1+x2)/2,(y1+y2)/2-radius/2);
	cdc.LineTo((x1+x2)/2,(y1+y2)/2+radius/2);
	CClientDC dc(this);//get device content of dialog to draw 'i1'
	if(t>=-180&&t<=180){//to skip error
	    //convert 't'int to 's'CString
		char a[10]="";itoa(t,a,10);CString s;s=a;
		dc.TextOut(0,300,s+_T(" degree"));
		//print the result
	}
	i1.ReleaseDC();
	cdc.Detach();
	i1.BitBlt(dc.m_hDC,i.GetWidth()+10,0,SRCCOPY);//draw 'i1'
	i1.Destroy();
}

Algorithm

First we need a color for the backcolor, 'c', and a color to get the points of the image, 'c1'. We have to find the first and last points of the line (x1,y1) and (x2,y2). We set -1 as the default value of x1. This means we don't find the first point yet. For detecting the first point, we move and look around the image at (x,y). When c1!=c && x==-1 we find (x1,y1). We imagine a virtual radius like 'd' and with a default value 10. After that we move and look again, and when c!=c && sqrt((x-x2)^2+(y-y2)^2))<=d, this point is related to the previous point and line. We continue this till the end. Finally, we'll find (x1,y2),(x2,y2) and we can get the length of the line and get the angle of the line with arctan.

teta=arttan((y2-y1)/(x2-x1))

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)