Line detectionin opencv C++

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If you know the end points of line you can easily find-out the angle of line using atan2() function in C++ and also you can decide whether the line is vertical or horizontal. Suppose you have two end points of line P1 and P2 then using the below code you can calculate the angle

    double Angle = atan2(P2.y - P1.y,P2.x - P1.x) * 180.0 / CV_PI;
    if(Angle<0) Angle=Angle+360;

Regards...

use HoughLines instead of HoughLinesP. This will give you the angle of the line and its perpendicular distance from the origin. This way you can simple filter lines according to their angles without any additional calculations.

check this link for more information:

for hough line code is -: HoughLinesP(dst,lines,1,CV_PI/180,80,30,10); now make a conditional statement like-:

if( theta>CV_PI/180*170 || theta<CV_PI/180*10)
    { Point pt1, pt2;
    ..........
    line( cdst, pt1, pt2, Scalar(0,0,255), 3, CV_AA);
    }

this while restrict the scope of detected lines and then you will be able to show your horizontal lines only.