OpenCV Error : Assertion failed when using fitLine - c++

Question

I want to use the fitLine function to come up with a line to draw on my source image src_crop. I load the frame in my main() and call the drawLine().
But the code aborts with the following error :
Code:
#include "stdafx.h"
#include <fstream>
#include <iostream>
#include <vector>
#include <stdlib.h>
#include <stdio.h>
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"
using namespace std;
using namespace cv;
/// Global variables
Mat src_gray;
Mat src_crop;
Mat dst, detected_edges;
int edgeThresh = 1;
int lowThreshold = 27;
int const max_lowThreshold = 100;
int ratio = 3;
int kernel_size = 3;
char* window_name = "Edge Map";
int i,j;
void drawLine(int, void*)
{
vector<Vec4f> outline;
vector<Point2f> ssline;
int flag2 = 0;
/// Reduce noise with a kernel 3x3
blur(src_gray, detected_edges, Size(3, 3));
/// Canny detector
Canny(detected_edges, detected_edges, lowThreshold, lowThreshold*ratio, kernel_size);
/// Using Canny's output as a mask, we display our result
dst.create(detected_edges.size(), detected_edges.type());
dst = Scalar::all(0);
src_crop.copyTo(dst, detected_edges);
//namedWindow("Detected Edges", CV_WINDOW_AUTOSIZE);
//imshow("Detected Edges", detected_edges);
cvtColor(dst, dst, CV_BGR2GRAY);
for (j = 0; j < dst.cols; j++)
{
for (i = 0; i < dst.rows; i++)
{
if (Scalar(dst.at<uchar>(i,j)).val[0] >= 90)
{
//cout << "Hi";
flag2 = 1;
break;
}
}
if (flag2 == 1)
break;
}
int k = j;
int l = i;
for (j = k; j < dst.cols; j++)
{
Point2f ss = Point2f(l,j);
ssline.push_back(ss);
}
fitLine(ssline, outline, CV_DIST_L1, 0, 0.01, 0.01);
//imshow("Result", src_crop);
}
int main(int argc, char** argv)
{
/// Load an image
src = imread(s);
if (!src.data)
{
return -1;
}
/// Create a matrix of the same type and size as src (for dst)
//dst.create(src.size(), src.type());
src_crop = src;
/// Convert the image to grayscale
cvtColor(src_crop, src_gray, CV_BGR2GRAY);
/// Create a window
namedWindow(window_name, CV_WINDOW_AUTOSIZE);
/// Create a Trackbar for user to enter threshold
createTrackbar("Min Threshold:", window_name, &lowThreshold, max_lowThreshold, drawLine);
/// Show the image
drawLine(0, 0);
if (waitKey(30) >= 0) break;
return 0;
}
The code stops working at the point fitLine() is called. This I found by testing the code with printf statements.
Can anyone kindly help me solve the issue?

Answer 1

Aside the fact the your code won't compile, the issue is that you're passing to fitLine the parameter outline as a vector<Vec4f>, while it should be a Vec4f.
Change outline declaration as:
Vec4f outline;