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java is somewhat clumsy here, but it's just a few lines:

mask = Mat.zeros(400,400,CvType.CV_8U); // all black

// your RotatedRect will come from minAreaRect(), this is just demo data !
RotatedRect rc = new RotatedRect(Point(200,200), new Size(200,100), 23);

// retrieve corner points:
Point[] p = Point[4];
rc.points(p);

// draw white, filled poly:
MatOfPoint mp = new MatOfPoint(p);
Imgproc.fillConvexPoly(mask, mp, new Scalar(255));

java is somewhat clumsy here, but it's just a few lines:

Mat mask = Mat.zeros(400,400,CvType.CV_8U); // all black

// your RotatedRect will come from minAreaRect(), this is just demo data !
RotatedRect rc = new RotatedRect(Point(200,200), new Size(200,100), 23);

// retrieve corner points:
Point[] p = Point[4];
rc.points(p);

// draw white, filled poly:
MatOfPoint mp = new MatOfPoint(p);
Imgproc.fillConvexPoly(mask, mp, new Scalar(255));

java is somewhat clumsy here, but it's just a few lines:

Mat mask = Mat.zeros(400,400,CvType.CV_8U); // all black

// your RotatedRect will come from minAreaRect(), this is just demo data !
RotatedRect rc = new RotatedRect(Point(200,200), new Size(200,100), 23);

// retrieve corner points:
Point[] p = Point[4];
rc.points(p);

// draw white, filled poly:
MatOfPoint mp = new MatOfPoint(p);
Imgproc.fillConvexPoly(mask, mp, new Scalar(255));

java is somewhat clumsy here, but it's just a few lines:

Mat mask = Mat.zeros(400,400,CvType.CV_8U); // all black

// your RotatedRect will come from minAreaRect(), this is just demo data !
RotatedRect rc = new RotatedRect(Point(200,200), new Size(200,100), 23);

// retrieve corner points:
Point[] p = Point[4];
rc.points(p);

// draw white, filled poly:
MatOfPoint mp = new MatOfPoint(p);
Imgproc.fillConvexPoly(mask, mp, new Scalar(255));

[edit]

• usually you need a contour as input for minAreaRect(), see tutorial and some conversion code (sorry, untested):

  List<MatOfPoint> contours = new ArrayList<>();
  Imgproc.findContours(cannyOutput, contours, ...);
  Point [] pts = contours.get(i).toArray();
  RotatedRect rc = Imgproc.minAreaRect(new MatOfPoint2f(pts));
  

• maybe you can use the points from findNonZero(), too, but to use that you need MatOfPoint (not 2f)

  MatOfPoint Points = new MatOfPoint();
  Core.findNonZero(paper_mat,Points);
  RotatedRect rc = Imgproc.minAreaRect(new MatOfPoint2f(Points.toArray()));
  

• sorry, i was just trying out latest 4.0, where all drawing stuff was moved to Imgproc for consistency, but yes, it's likely in Core with your version