HoughLines transform in opencv

Question

I am working on image processing using opencv and Eclipse.

  vector<Vec2f> lines;  
  HoughLines(dst, lines, 1, CV_PI/180, 100, 0, 0 );

  for( size_t i = 0; i < lines.size(); i++ )  
  {  
     float rho = lines[i][0], theta = lines[i][1];  
     Point pt1, pt2;  
     double a = cos(theta), b = sin(theta);  
     double x0 = a*rho, y0 = b*rho;  
     pt1.x = cvRound(x0 + 1000*(-b));  
     pt1.y = cvRound(y0 + 1000*(a));  
     pt2.x = cvRound(x0 - 1000*(-b));  
     pt2.y = cvRound(y0 - 1000*(a));  
     line( cdst, pt1, pt2, Scalar(0,0,255), 3, CV_AA);  
  }  

Can anyone explain that how the points are being defined by this code. We are using

y=(-cos(theta)/sin(theta))x + r/(sin(theta))
rho=xo*cos(theta) + yo*sin(theta)

I am not able to understand why the multiplication of 1000 is being done in the line

pt1.x = cvRound(x0 + 1000*(-b));  

please try to explain this in simple terms. Thanks in advance

Answer 1

The question has already been answered. But since I spend the last fifteen minutes drawing this diagram I might as well post it anyway. Maybe it helps:

So what you have is a Point p0 = (x0,y0) which is on the line. You then compute two other points on the line which are 1000 units away from p0 in each direction.