How to calculate slope, length and distance between hough lines?

edit

really, this is not so much an opencv problem, but a maths one ..

houghlinesP already gives you the endpoints of the line,

Vec4i line = lines[i];
Point a(line[0], line[1]);
Point b(line[2], line[3]);

so

• the slope is dy/dx , or

  float(b.y - a.y) / float(b.x - a.x);

• the length is the euclidean distance, or:

  norm(a,b);

no idea, what you mean with the distance, try to explain ?