System information (version)

• OpenCV => 3.4.3
• Operating System / Platform => Windows 10 1903
• Compiler => MSVC 1916

Detailed description

Fitting a line passing through almost-horizontal points via cv::fitLine results in a wrong line parameters vector.

Steps to reproduce

#include <vector>
#include <iostream>
#include <opencv2/imgproc.hpp>

int main()
{
    std::vector<cv::Point> points = { 
        {432,654},
        {370,656},
        {390,656},
        {410,656},
        {348,658},
    };

    cv::Vec4f lineParam;
    cv::fitLine(points, lineParam, cv::DIST_L1, 0, 0.01, 0.01);
    std::cout << lineParam << std::endl;
}

Running the code above prints [0.701592, -0.712579, 390, 656]. The first two values, vx and vy are for a unit vector pointing approximately 45° down from the 0, while the expected result would have it closer to 0 (the points are almost perfectly aligned horizontally).
Values for reps and aeps are taken from https://docs.opencv.org/2.4/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html#fitline

Update:

If I use cv::DIST_L2 instead of cv::DIST_L1 for the distance function, the line is correctly estimated:

[0.999247, -0.038801, 390, 656]

It would seem, then that the issue is related to using the L1 distance.