Edge detection of very small lines using canny/hough - opencv

Question

I'm new to opencv. I want to detect very small lines in an image and store them as an array of lines. There will not be more than 10 lines (alignment markers) in my image. I've used a opencv sample tutorial to detect lines using canny and Hough.
For testing, my input image has a set of lines that vary in size: 15, 30, 50, 75 and 100 pixles
The app does not detect the 15 and 30 pixel lines.
It detects 2 of the 4 50 pixel lines.
The 75 and 100 pixel lines are detected fine.
What can I do to detect the 15 and 30 pixel lines?
This is my
Input image
This is my
output image
This is my source
/*
* https://github.com/opencv/opencv/blob/master/samples/cpp/tutorial_code/ImgTrans/HoughLines_Demo.cpp
*/
/**
* #file HoughLines_Demo.cpp
* #brief Demo code for Hough Transform
* #author OpenCV team
*/
#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui.hpp"
#include "opencv2/imgproc.hpp"
#include <iostream>
using namespace cv;
using namespace std;
/// Global variables
/** General variables */
Mat src, edges;
Mat src_gray;
Mat standard_hough, probabilistic_hough;
int min_threshold = 50;
int max_trackbar = 150;
const char* standard_name = "Standard Hough Lines Demo";
const char* probabilistic_name = "Probabilistic Hough Lines Demo";
int s_trackbar = max_trackbar;
int p_trackbar = max_trackbar;
/// Function Headers
void help();
void Standard_Hough( int, void* );
void Probabilistic_Hough( int, void* );
/**
* #function main
*/
int main( int argc, char** argv )
{
// Read the image
String imageName("../data/building.jpg"); // by default
if (argc > 1)
{
imageName = argv[1];
}
src = imread( imageName, IMREAD_COLOR );
if( src.empty() )
{ help();
return -1;
}
/// Pass the image to gray
cvtColor( src, src_gray, COLOR_RGB2GRAY );
/// Apply Canny edge detector
Canny( src_gray, edges, 50, 200, 3 );
/// Create Trackbars for Thresholds
char thresh_label[50];
sprintf( thresh_label, "Thres: %d + input", min_threshold );
namedWindow( standard_name, WINDOW_AUTOSIZE );
createTrackbar( thresh_label, standard_name, &s_trackbar, max_trackbar, Standard_Hough);
namedWindow( probabilistic_name, WINDOW_AUTOSIZE );
createTrackbar( thresh_label, probabilistic_name, &p_trackbar, max_trackbar, Probabilistic_Hough);
/// Initialize
Standard_Hough(0, 0);
Probabilistic_Hough(0, 0);
waitKey(0);
return 0;
}
/**
* #function help
* #brief Indications of how to run this program and why is it for
*/
void help()
{
printf("\t Hough Transform to detect lines \n ");
printf("\t---------------------------------\n ");
printf(" Usage: ./HoughLines_Demo <image_name> \n");
}
/**
* #function Standard_Hough
*/
void Standard_Hough( int, void* )
{
vector<Vec2f> s_lines;
cvtColor( edges, standard_hough, COLOR_GRAY2BGR );
/// 1. Use Standard Hough Transform
HoughLines( edges, s_lines, 1, CV_PI/180, min_threshold + s_trackbar, 0, 0 );
/// Show the result
for( size_t i = 0; i < s_lines.size(); i++ )
{
float r = s_lines[i][0], t = s_lines[i][1];
double cos_t = cos(t), sin_t = sin(t);
double x0 = r*cos_t, y0 = r*sin_t;
double alpha = 1000;
Point pt1( cvRound(x0 + alpha*(-sin_t)), cvRound(y0 + alpha*cos_t) );
Point pt2( cvRound(x0 - alpha*(-sin_t)), cvRound(y0 - alpha*cos_t) );
line( standard_hough, pt1, pt2, Scalar(255,0,0), 3, LINE_AA);
}
imshow( standard_name, standard_hough );
}
/**
* #function Probabilistic_Hough
*/
void Probabilistic_Hough( int, void* )
{
vector<Vec4i> p_lines;
cvtColor( edges, probabilistic_hough, COLOR_GRAY2BGR );
/// 2. Use Probabilistic Hough Transform
HoughLinesP( edges, p_lines, 1, CV_PI/180, min_threshold + p_trackbar, 30, 10 );
/// Show the result
for( size_t i = 0; i < p_lines.size(); i++ )
{
Vec4i l = p_lines[i];
line( probabilistic_hough, Point(l[0], l[1]), Point(l[2], l[3]), Scalar(255,0,0), 3, LINE_AA);
}
imshow( probabilistic_name, probabilistic_hough );
}
TIA

Answer 1

The variable min_threshold in your code has been set to 50, so it will not consider any line segment candidates smaller than that. With a bit of noise, it is easy for the 50 pixel line segments to get skipped.
You can set this variable to a value less than 15, so that all line segments can be considered.