How to get Hough transform matrix ,theta and rho value of hough transformation in opencv cpp

Question

I have matlab code to get Hough transform matrix, theta and rho values.

[H,T,R] = hough(EdgeImage);

How to get H, T, R values in OpenCV?

Answer 1

In OpenCV, you call the HT as:

vector<Vec2f> lines;
HoughLines(edges, lines, 1, CV_PI/180.0, 100);

where edge is your binary input image, and lines is a std::vector of Vec2f, i.e. a vector of 2 float values: the first value is rho, the second one is theta.

OpenCV doesn't output the H parameter space, if you need also that you need to write some code yourself and adapt HoughLines to output also the H value. However, this is rarely needed in practice.

This is a simple example on how to use standard Hough Transform, adapted from OpenCV tutorials:

#include <opencv2\opencv.hpp>
#include <vector>
using namespace std;
using namespace cv;

int main()
{
    // Load image
    Mat3b img = imread("path_to_image");
    Mat3b res = img.clone();

    // Convert to grayscale
    Mat1b gray;
    cvtColor(img, gray, COLOR_BGR2GRAY);

    // Compute edges
    Mat1b edges;
    Canny(gray, edges, 100, 400);

    vector<Vec2f> lines;
    HoughLines(edges, lines, 1, CV_PI/180.0, 100);

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

        // Draw the line
        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(res, pt1, pt2, Scalar(0, 0, 255), 2);
    }

    imshow("Input", img);
    imshow("Output", res);
    waitKey();

    return 0;
}

Input:

Output: