fitLine java

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I was wondering how do I properly setup the fitLine function and then extract the individual values in the output line matrix for java. I know that the input for fitLine has to be a MatOfPoint but I'm not sure how to convert a black/white image to an MatOfPoint either.

void fit(Mat img){
  Imgproc.threshold(img, img, 225, 255, Imgproc.THRESH_BINARY);
  MatOfPoint mPoints = new MatOfPoint(img);
  Mat line = new Mat();
  Imgproc.fitLine(mPoints, line, Imgproc.CV_DIST_L2, 0, 0.01, 0.01);
  float vx = line.get(0, 0);
}

I know there is error in the code above but it highlight what I'm trying to do. I'm not exactly sure about the syntax regarding the matrix operations.