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# How Get Polar coordinate system Parameters:(R, theta) from HoughLines

Hi, I used the opencv documentation given by applying the algorithm of hough Libes on an image .

I have a problem, I can not get the value( R,theta) of the intersection between curves of every point in the image??

#include "opencv2/imgproc/imgproc.hpp"

#include <iostream>

using namespace cv;
using namespace std;

void help()
{
cout << "\nThis program demonstrates line finding with the Hough transform.\n"
"Usage:\n"
"./houghlines <image_name>, Default is pic1.jpg\n" << endl;
}

int main(int argc, char** argv)
{
const char* filename = argc >= 2 ? argv[1] : "pic1.jpg";

if(src.empty())
{
help();
cout << "can not open " << filename << endl;
return -1;
}

Mat dst, cdst;
Canny(src, dst, 50, 200, 3);
cvtColor(dst, cdst, CV_GRAY2BGR);

#if 0
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);
}
#else
vector<Vec4i> lines;
HoughLinesP(dst, lines, 1, CV_PI/180, 50, 50, 10 );
for( size_t i = 0; i < lines.size(); i++ )
{
Vec4i l = lines[i];
line( cdst, Point(l[0], l[1]), Point(l[2], l[3]), Scalar(0,0,255), 3, CV_AA);
}
#endif
imshow("source", src);
imshow("detected lines", cdst);

waitKey();

return 0;
}

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Yes I know, I want to get all the array of values of rho and theta !!!

( 2014-05-16 02:27:42 -0500 )edit

sorry abir, i misread it (in the deleted comment), and confused Houghlines and HoughlinesP

( 2014-05-16 02:33:21 -0500 )edit

No problem , I want to get the orientation of the point d'intersection between lines !! For that I want to retrieve the values ​​of theta and rho!! please help me :)

( 2014-05-16 02:38:30 -0500 )edit

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The theta (angle of line) is atan2(X1-X2,Y1-Y2).

The distance from origin rho is a point-line distance, the formula is here (#14): http://mathworld.wolfram.com/Point-LineDistance2-Dimensional.html

The X1,X2,Y1,Y2 are the members of the lines vector.

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