Python openCV detect parallel lines

Question

I have an image and it has some shapes in it. I detected lines with using hough lines. How can I detect which lines are parallel?

Answer 1

Equation of a line in Cartesian coordinates:

y = k * x + b

Two lines y = k1 * x + b1, y = k2 * x + b2 are parallel, if k1 = k2.

So you need to calculate coefficient k for each detected line.

In order to uniquely identify the equation of a line you need to know the coordinates of two points that belong to line.

After having found lines with HoughLines (С++):

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

you have the vector lines, which stores the parameters (r,theta) of the detected lines in polar coordinates. You need to transfer them in Cartesian coordinates:

Here example in C++:

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)); //the first point
  pt1.y = cvRound(y0 + 1000*(a)); //the first point
  pt2.x = cvRound(x0 - 1000*(-b)); //the second point
  pt2.y = cvRound(y0 - 1000*(a)); //the second point
}

After having got these two points of a line you can calculate its equation.