How can I determine the angle a line found by HoughLines function using OpenCV?

Question

Using the HoughLines function in OpenCV, is it possible to determine the angle of a resulting line relative to the base of the image?

Answer 1

If you use HoughLines function, it will provide you lines already defined by two parameters: theta and rho, as

vector lines;
// detect lines
HoughLines(image, lines, 1, CV_PI/180, 150, 0, 0 );

// get lines
for( size_t i = 0; i < lines.size(); i++ )
{
    float rho = lines[i][0], theta = lines[i][1];
   ....
}

Or if you apply HoughLinesP function, you will get lines defined by two points, you just need to calculate the angle of line between two points with regard to the image, as:

vector lines;
// detect the lines
HoughLinesP(image, lines, 1, CV_PI/180, 50, 50, 10 );
for( size_t i = 0; i < lines.size(); i++ )
{
    Vec4i l = lines[i];
    // draw the lines

    Point p1, p2;
    p1=Point(l[0], l[1]);
    p2=Point(l[2], l[3]);
    //calculate angle in radian,  if you need it in degrees just do angle * 180 / PI
    float angle = atan2(p1.y - p2.y, p1.x - p2.x);
  .......
}