Basic Drawing

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Goals

In this tutorial you will learn how to:

• Draw a line by using the OpenCV function line()
• Draw an ellipse by using the OpenCV function ellipse()
• Draw a rectangle by using the OpenCV function rectangle()
• Draw a circle by using the OpenCV function circle()
• Draw a filled polygon by using the OpenCV function fillPoly()

C++

OpenCV Theory

For this tutorial, we will heavily use two structures: cv::Point and cv::Scalar :

Point

It represents a 2D point, specified by its image coordinates \(x\) and \(y\). We can define it as:

Point pt;
pt.x = 10;
pt.y = 8;

or

Point pt =  Point(10, 8);

Scalar

• Represents a 4-element vector. The type Scalar is widely used in OpenCV for passing pixel values.
• In this tutorial, we will use it extensively to represent BGR color values (3 parameters). It is not necessary to define the last argument if it is not going to be used.
• Let's see an example, if we are asked for a color argument and we give:

  Scalar( a, b, c )

  We would be defining a BGR color such as: Blue = a, Green = b and Red = c

Java

OpenCV Theory

For this tutorial, we will heavily use two structures: cv::Point and cv::Scalar :

Point

It represents a 2D point, specified by its image coordinates \(x\) and \(y\). We can define it as:

Point pt = new Point();
pt.x = 10;
pt.y = 8;

or

Point pt = new Point(10, 8);

Scalar

• Represents a 4-element vector. The type Scalar is widely used in OpenCV for passing pixel values.
• In this tutorial, we will use it extensively to represent BGR color values (3 parameters). It is not necessary to define the last argument if it is not going to be used.
• Let's see an example, if we are asked for a color argument and we give:

  Scalar( a, b, c )

  We would be defining a BGR color such as: Blue = a, Green = b and Red = c

Code

C++

• This code is in your OpenCV sample folder. Otherwise you can grab it from here

  
  #include <opencv2/core.hpp>
  #include <opencv2/imgproc.hpp>
  #include <opencv2/highgui.hpp>
  
  #define w 400
  
  using namespace cv;
  
  void MyEllipse( Mat img, double angle );
  void MyFilledCircle( Mat img, Point center );
  void MyPolygon( Mat img );
  void MyLine( Mat img, Point start, Point end );
  
  int main( void ){
  
    char atom_window[] = "Drawing 1: Atom";
    char rook_window[] = "Drawing 2: Rook";
  
    Mat atom_image = Mat::zeros( w, w, CV_8UC3 );
    Mat rook_image = Mat::zeros( w, w, CV_8UC3 );
  
  
    MyEllipse( atom_image, 90 );
    MyEllipse( atom_image, 0 );
    MyEllipse( atom_image, 45 );
    MyEllipse( atom_image, -45 );
  
    MyFilledCircle( atom_image, Point( w/2, w/2) );
  
  
    MyPolygon( rook_image );
  
    rectangle( rook_image,
           Point( 0, 7*w/8 ),
           Point( w, w),
           Scalar( 0, 255, 255 ),
           FILLED,
           LINE_8 );
  
    MyLine( rook_image, Point( 0, 15*w/16 ), Point( w, 15*w/16 ) );
    MyLine( rook_image, Point( w/4, 7*w/8 ), Point( w/4, w ) );
    MyLine( rook_image, Point( w/2, 7*w/8 ), Point( w/2, w ) );
    MyLine( rook_image, Point( 3*w/4, 7*w/8 ), Point( 3*w/4, w ) );
  
    imshow( atom_window, atom_image );
    moveWindow( atom_window, 0, 200 );
    imshow( rook_window, rook_image );
    moveWindow( rook_window, w, 200 );
  
    waitKey( 0 );
    return(0);
  }
  
  
  void MyEllipse( Mat img, double angle )
  {
    int thickness = 2;
    int lineType = 8;
  
    ellipse( img,
         Point( w/2, w/2 ),
         Size( w/4, w/16 ),
         angle,
         0,
         360,
         Scalar( 255, 0, 0 ),
         thickness,
         lineType );
  }
  
  void MyFilledCircle( Mat img, Point center )
  {
    circle( img,
        center,
        w/32,
        Scalar( 0, 0, 255 ),
        FILLED,
        LINE_8 );
  }
  
  void MyPolygon( Mat img )
  {
    int lineType = LINE_8;
  
    Point rook_points[1][20];
    rook_points[0][0]  = Point(    w/4,   7*w/8 );
    rook_points[0][1]  = Point(  3*w/4,   7*w/8 );
    rook_points[0][2]  = Point(  3*w/4,  13*w/16 );
    rook_points[0][3]  = Point( 11*w/16, 13*w/16 );
    rook_points[0][4]  = Point( 19*w/32,  3*w/8 );
    rook_points[0][5]  = Point(  3*w/4,   3*w/8 );
    rook_points[0][6]  = Point(  3*w/4,     w/8 );
    rook_points[0][7]  = Point( 26*w/40,    w/8 );
    rook_points[0][8]  = Point( 26*w/40,    w/4 );
    rook_points[0][9]  = Point( 22*w/40,    w/4 );
    rook_points[0][10] = Point( 22*w/40,    w/8 );
    rook_points[0][11] = Point( 18*w/40,    w/8 );
    rook_points[0][12] = Point( 18*w/40,    w/4 );
    rook_points[0][13] = Point( 14*w/40,    w/4 );
    rook_points[0][14] = Point( 14*w/40,    w/8 );
    rook_points[0][15] = Point(    w/4,     w/8 );
    rook_points[0][16] = Point(    w/4,   3*w/8 );
    rook_points[0][17] = Point( 13*w/32,  3*w/8 );
    rook_points[0][18] = Point(  5*w/16, 13*w/16 );
    rook_points[0][19] = Point(    w/4,  13*w/16 );
  
    const Point* ppt[1] = { rook_points[0] };
    int npt[] = { 20 };
  
    fillPoly( img,
          ppt,
          npt,
          1,
          Scalar( 255, 255, 255 ),
          lineType );
  }
  
  void MyLine( Mat img, Point start, Point end )
  {
    int thickness = 2;
    int lineType = LINE_8;
  
    line( img,
      start,
      end,
      Scalar( 0, 0, 0 ),
      thickness,
      lineType );
  }

Java

• This code is in your OpenCV sample folder. Otherwise you can grab it from here

  import org.opencv.core.*;
  import org.opencv.core.Point;
  import org.opencv.highgui.HighGui;
  import org.opencv.imgproc.Imgproc;
  
  import java.util.*;
  import java.util.List;
  
  class GeometricDrawingRun{
  
      private static final int W = 400;
  
      public void run(){
          String atom_window = "Drawing 1: Atom";
          String rook_window = "Drawing 2: Rook";
  
          Mat atom_image = Mat.zeros( W, W, CvType.CV_8UC3 );
          Mat rook_image = Mat.zeros( W, W, CvType.CV_8UC3 );
  
          MyEllipse( atom_image, 90.0 );
          MyEllipse( atom_image, 0.0 );
          MyEllipse( atom_image, 45.0 );
          MyEllipse( atom_image, -45.0 );
  
          MyFilledCircle( atom_image, new Point( W/2, W/2) );
  
          MyPolygon( rook_image );
  
          Imgproc.rectangle( rook_image,
                  new Point( 0, 7*W/8 ),
                  new Point( W, W),
                  new Scalar( 0, 255, 255 ),
                  -1,
                  8,
                  0 );
  
          MyLine( rook_image, new Point( 0, 15*W/16 ), new Point( W, 15*W/16 ) );
          MyLine( rook_image, new Point( W/4, 7*W/8 ), new Point( W/4, W ) );
          MyLine( rook_image, new Point( W/2, 7*W/8 ), new Point( W/2, W ) );
          MyLine( rook_image, new Point( 3*W/4, 7*W/8 ), new Point( 3*W/4, W ) );
  
          HighGui.imshow( atom_window, atom_image );
          HighGui.moveWindow( atom_window, 0, 200 );
          HighGui.imshow( rook_window, rook_image );
          HighGui.moveWindow( rook_window, W, 200 );
  
          HighGui.waitKey( 0 );
          System.exit(0);
      }
  
  
      private void MyEllipse( Mat img, double angle ) {
          int thickness = 2;
          int lineType = 8;
          int shift = 0;
  
          Imgproc.ellipse( img,
                  new Point( W/2, W/2 ),
                  new Size( W/4, W/16 ),
                  angle,
                  0.0,
                  360.0,
                  new Scalar( 255, 0, 0 ),
                  thickness,
                  lineType,
                  shift );
      }
  
      private void MyFilledCircle( Mat img, Point center ) {
          int thickness = -1;
          int lineType = 8;
          int shift = 0;
  
          Imgproc.circle( img,
                  center,
                  W/32,
                  new Scalar( 0, 0, 255 ),
                  thickness,
                  lineType,
                  shift );
      }
  
      private void MyPolygon( Mat img ) {
          int lineType = 8;
          int shift = 0;
  
          Point[] rook_points = new Point[20];
          rook_points[0]  = new Point(     W/4, 7*W/8   );
          rook_points[1]  = new Point(   3*W/4, 7*W/8   );
          rook_points[2]  = new Point(   3*W/4, 13*W/16 );
          rook_points[3]  = new Point( 11*W/16, 13*W/16 );
          rook_points[4]  = new Point( 19*W/32, 3*W/8   );
          rook_points[5]  = new Point(   3*W/4, 3*W/8   );
          rook_points[6]  = new Point(   3*W/4, W/8     );
          rook_points[7]  = new Point( 26*W/40, W/8     );
          rook_points[8]  = new Point( 26*W/40, W/4     );
          rook_points[9]  = new Point( 22*W/40, W/4     );
          rook_points[10] = new Point( 22*W/40, W/8     );
          rook_points[11] = new Point( 18*W/40, W/8     );
          rook_points[12] = new Point( 18*W/40, W/4     );
          rook_points[13] = new Point( 14*W/40, W/4     );
          rook_points[14] = new Point( 14*W/40, W/8     );
          rook_points[15] = new Point(     W/4, W/8     );
          rook_points[16] = new Point(     W/4, 3*W/8   );
          rook_points[17] = new Point( 13*W/32, 3*W/8   );
          rook_points[18] = new Point(  5*W/16, 13*W/16 );
          rook_points[19] = new Point(     W/4, 13*W/16 );
  
          MatOfPoint matPt = new MatOfPoint();
          matPt.fromArray(rook_points);
  
          List<MatOfPoint> ppt = new ArrayList<MatOfPoint>();
          ppt.add(matPt);
  
          Imgproc.fillPoly(img,
                  ppt,
                  new Scalar( 255, 255, 255 ),
                  lineType,
                  shift,
                  new Point(0,0) );
      }
  
      private void MyLine( Mat img, Point start, Point end ) {
          int thickness = 2;
          int lineType = 8;
          int shift = 0;
  
          Imgproc.line( img,
                  start,
                  end,
                  new Scalar( 0, 0, 0 ),
                  thickness,
                  lineType,
                  shift );
      }
  }
  
  public class BasicGeometricDrawing {
      public static void main(String[] args) {
          // Load the native library.
          System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
          new GeometricDrawingRun().run();
      }
  }

Python

• This code is in your OpenCV sample folder. Otherwise you can grab it from here

  import cv2 as cv
  import numpy as np
  
  W = 400
  
  def my_ellipse(img, angle):
      thickness = 2
      line_type = 8
  
      cv.ellipse(img,
                  (W // 2, W // 2),
                  (W // 4, W // 16),
                  angle,
                  0,
                  360,
                  (255, 0, 0),
                  thickness,
                  line_type)
  
  def my_filled_circle(img, center):
      thickness = -1
      line_type = 8
  
      cv.circle(img,
                 center,
                 W // 32,
                 (0, 0, 255),
                 thickness,
                 line_type)
  
  def my_polygon(img):
      line_type = 8
  
      # Create some points
      ppt = np.array([[W / 4, 7 * W / 8], [3 * W / 4, 7 * W / 8],
                      [3 * W / 4, 13 * W / 16], [11 * W / 16, 13 * W / 16],
                      [19 * W / 32, 3 * W / 8], [3 * W / 4, 3 * W / 8],
                      [3 * W / 4, W / 8], [26 * W / 40, W / 8],
                      [26 * W / 40, W / 4], [22 * W / 40, W / 4],
                      [22 * W / 40, W / 8], [18 * W / 40, W / 8],
                      [18 * W / 40, W / 4], [14 * W / 40, W / 4],
                      [14 * W / 40, W / 8], [W / 4, W / 8],
                      [W / 4, 3 * W / 8], [13 * W / 32, 3 * W / 8],
                      [5 * W / 16, 13 * W / 16], [W / 4, 13 * W / 16]], np.int32)
      ppt = ppt.reshape((-1, 1, 2))
      cv.fillPoly(img, [ppt], (255, 255, 255), line_type)
      # Only drawind the lines would be:
      # cv.polylines(img, [ppt], True, (255, 0, 255), line_type)
  
  def my_line(img, start, end):
      thickness = 2
      line_type = 8
  
      cv.line(img,
               start,
               end,
               (0, 0, 0),
               thickness,
               line_type)
  
  atom_window = "Drawing 1: Atom"
  rook_window = "Drawing 2: Rook"
  
  # Create black empty images
  size = W, W, 3
  atom_image = np.zeros(size, dtype=np.uint8)
  rook_image = np.zeros(size, dtype=np.uint8)
  
  
  # 1.a. Creating ellipses
  my_ellipse(atom_image, 90)
  my_ellipse(atom_image, 0)
  my_ellipse(atom_image, 45)
  my_ellipse(atom_image, -45)
  
  # 1.b. Creating circles
  my_filled_circle(atom_image, (W // 2, W // 2))
  
  
  # 2. Draw a rook
  # ------------------
  # 2.a. Create a convex polygon
  my_polygon(rook_image)
  
  cv.rectangle(rook_image,
                (0, 7 * W // 8),
                (W, W),
                (0, 255, 255),
                -1,
                8)
  
  
  #  2.c. Create a few lines
  my_line(rook_image, (0, 15 * W // 16), (W, 15 * W // 16))
  my_line(rook_image, (W // 4, 7 * W // 8), (W // 4, W))
  my_line(rook_image, (W // 2, 7 * W // 8), (W // 2, W))
  my_line(rook_image, (3 * W // 4, 7 * W // 8), (3 * W // 4, W))
  
  cv.imshow(atom_window, atom_image)
  cv.moveWindow(atom_window, 0, 200)
  cv.imshow(rook_window, rook_image)
  cv.moveWindow(rook_window, W, 200)
  
  cv.waitKey(0)
  cv.destroyAllWindows()

Explanation

Since we plan to draw two examples (an atom and a rook), we have to create two images and two windows to display them.

C++

  char atom_window[] = "Drawing 1: Atom";
  char rook_window[] = "Drawing 2: Rook";

  Mat atom_image = Mat::zeros( w, w, CV_8UC3 );
  Mat rook_image = Mat::zeros( w, w, CV_8UC3 );

Java

        String atom_window = "Drawing 1: Atom";
        String rook_window = "Drawing 2: Rook";

        Mat atom_image = Mat.zeros( W, W, CvType.CV_8UC3 );
        Mat rook_image = Mat.zeros( W, W, CvType.CV_8UC3 );

Python

# Windows names
atom_window = "Drawing 1: Atom"
rook_window = "Drawing 2: Rook"

# Create black empty images
size = W, W, 3
atom_image = np.zeros(size, dtype=np.uint8)
rook_image = np.zeros(size, dtype=np.uint8)

We created functions to draw different geometric shapes. For instance, to draw the atom we used MyEllipse and MyFilledCircle:

C++

  MyEllipse( atom_image, 90 );
  MyEllipse( atom_image, 0 );
  MyEllipse( atom_image, 45 );
  MyEllipse( atom_image, -45 );

  MyFilledCircle( atom_image, Point( w/2, w/2) );

Java

        MyEllipse( atom_image, 90.0 );
        MyEllipse( atom_image, 0.0 );
        MyEllipse( atom_image, 45.0 );
        MyEllipse( atom_image, -45.0 );

        MyFilledCircle( atom_image, new Point( W/2, W/2) );

Python

# 1. Draw a simple atom:
# -----------------------

# 1.a. Creating ellipses
my_ellipse(atom_image, 90)
my_ellipse(atom_image, 0)
my_ellipse(atom_image, 45)
my_ellipse(atom_image, -45)

# 1.b. Creating circles
my_filled_circle(atom_image, (W // 2, W // 2))

And to draw the rook we employed MyLine, rectangle and a MyPolygon:

C++

  MyPolygon( rook_image );

  rectangle( rook_image,
         Point( 0, 7*w/8 ),
         Point( w, w),
         Scalar( 0, 255, 255 ),
         FILLED,
         LINE_8 );

  MyLine( rook_image, Point( 0, 15*w/16 ), Point( w, 15*w/16 ) );
  MyLine( rook_image, Point( w/4, 7*w/8 ), Point( w/4, w ) );
  MyLine( rook_image, Point( w/2, 7*w/8 ), Point( w/2, w ) );
  MyLine( rook_image, Point( 3*w/4, 7*w/8 ), Point( 3*w/4, w ) );

Java

        MyPolygon( rook_image );

        Imgproc.rectangle( rook_image,
                new Point( 0, 7*W/8 ),
                new Point( W, W),
                new Scalar( 0, 255, 255 ),
                -1,
                8,
                0 );

        MyLine( rook_image, new Point( 0, 15*W/16 ), new Point( W, 15*W/16 ) );
        MyLine( rook_image, new Point( W/4, 7*W/8 ), new Point( W/4, W ) );
        MyLine( rook_image, new Point( W/2, 7*W/8 ), new Point( W/2, W ) );
        MyLine( rook_image, new Point( 3*W/4, 7*W/8 ), new Point( 3*W/4, W ) );

Python

# 2. Draw a rook
# ------------------
# 2.a. Create a convex polygon
my_polygon(rook_image)

cv.rectangle(rook_image,
              (0, 7 * W // 8),
              (W, W),
              (0, 255, 255),
              -1,
              8)


#  2.c. Create a few lines
my_line(rook_image, (0, 15 * W // 16), (W, 15 * W // 16))
my_line(rook_image, (W // 4, 7 * W // 8), (W // 4, W))
my_line(rook_image, (W // 2, 7 * W // 8), (W // 2, W))
my_line(rook_image, (3 * W // 4, 7 * W // 8), (3 * W // 4, W))

Let's check what is inside each of these functions:

C++

MyLine

C++

void MyLine( Mat img, Point start, Point end )
{
  int thickness = 2;
  int lineType = LINE_8;

  line( img,
    start,
    end,
    Scalar( 0, 0, 0 ),
    thickness,
    lineType );
}

Java

    private void MyLine( Mat img, Point start, Point end ) {
        int thickness = 2;
        int lineType = 8;
        int shift = 0;

        Imgproc.line( img,
                start,
                end,
                new Scalar( 0, 0, 0 ),
                thickness,
                lineType,
                shift );
    }

Python

def my_line(img, start, end):
    thickness = 2
    line_type = 8

    cv.line(img,
             start,
             end,
             (0, 0, 0),
             thickness,
             line_type)

• As we can see, MyLine just call the function line() , which does the following:
  • Draw a line from Point start to Point end
  • The line is displayed in the image img
  • The line color is defined by ( 0, 0, 0 ) which is the RGB value correspondent to Black
  • The line thickness is set to thickness (in this case 2)
  • The line is a 8-connected one (lineType = 8)

MyEllipse

C++

void MyEllipse( Mat img, double angle )
{
  int thickness = 2;
  int lineType = 8;

  ellipse( img,
       Point( w/2, w/2 ),
       Size( w/4, w/16 ),
       angle,
       0,
       360,
       Scalar( 255, 0, 0 ),
       thickness,
       lineType );
}

Java

    private void MyEllipse( Mat img, double angle ) {
        int thickness = 2;
        int lineType = 8;
        int shift = 0;

        Imgproc.ellipse( img,
                new Point( W/2, W/2 ),
                new Size( W/4, W/16 ),
                angle,
                0.0,
                360.0,
                new Scalar( 255, 0, 0 ),
                thickness,
                lineType,
                shift );
    }

Python

def my_ellipse(img, angle):
    thickness = 2
    line_type = 8

    cv.ellipse(img,
                (W // 2, W // 2),
                (W // 4, W // 16),
                angle,
                0,
                360,
                (255, 0, 0),
                thickness,
                line_type)

• From the code above, we can observe that the function ellipse() draws an ellipse such that:
  • The ellipse is displayed in the image img
  • The ellipse center is located in the point (w/2, w/2) and is enclosed in a box of size (w/4, w/16)
  • The ellipse is rotated angle degrees
  • The ellipse extends an arc between 0 and 360 degrees
  • The color of the figure will be ( 255, 0, 0 ) which means blue in BGR value.
  • The ellipse's thickness is 2.

MyFilledCircle

C++

void MyFilledCircle( Mat img, Point center )
{
  circle( img,
      center,
      w/32,
      Scalar( 0, 0, 255 ),
      FILLED,
      LINE_8 );
}

Java

    private void MyFilledCircle( Mat img, Point center ) {
        int thickness = -1;
        int lineType = 8;
        int shift = 0;

        Imgproc.circle( img,
                center,
                W/32,
                new Scalar( 0, 0, 255 ),
                thickness,
                lineType,
                shift );
    }

Python

def my_filled_circle(img, center):
    thickness = -1
    line_type = 8

    cv.circle(img,
               center,
               W // 32,
               (0, 0, 255),
               thickness,
               line_type)

• Similar to the ellipse function, we can observe that circle receives as arguments:
  • The image where the circle will be displayed (img)
  • The center of the circle denoted as the point center
  • The radius of the circle: w/32
  • The color of the circle: ( 0, 0, 255 ) which means Red in BGR
  • Since thickness = -1, the circle will be drawn filled.

MyPolygon

C++

void MyPolygon( Mat img )
{
  int lineType = LINE_8;

  Point rook_points[1][20];
  rook_points[0][0]  = Point(    w/4,   7*w/8 );
  rook_points[0][1]  = Point(  3*w/4,   7*w/8 );
  rook_points[0][2]  = Point(  3*w/4,  13*w/16 );
  rook_points[0][3]  = Point( 11*w/16, 13*w/16 );
  rook_points[0][4]  = Point( 19*w/32,  3*w/8 );
  rook_points[0][5]  = Point(  3*w/4,   3*w/8 );
  rook_points[0][6]  = Point(  3*w/4,     w/8 );
  rook_points[0][7]  = Point( 26*w/40,    w/8 );
  rook_points[0][8]  = Point( 26*w/40,    w/4 );
  rook_points[0][9]  = Point( 22*w/40,    w/4 );
  rook_points[0][10] = Point( 22*w/40,    w/8 );
  rook_points[0][11] = Point( 18*w/40,    w/8 );
  rook_points[0][12] = Point( 18*w/40,    w/4 );
  rook_points[0][13] = Point( 14*w/40,    w/4 );
  rook_points[0][14] = Point( 14*w/40,    w/8 );
  rook_points[0][15] = Point(    w/4,     w/8 );
  rook_points[0][16] = Point(    w/4,   3*w/8 );
  rook_points[0][17] = Point( 13*w/32,  3*w/8 );
  rook_points[0][18] = Point(  5*w/16, 13*w/16 );
  rook_points[0][19] = Point(    w/4,  13*w/16 );

  const Point* ppt[1] = { rook_points[0] };
  int npt[] = { 20 };

  fillPoly( img,
        ppt,
        npt,
        1,
        Scalar( 255, 255, 255 ),
        lineType );
}

Java

    private void MyPolygon( Mat img ) {
        int lineType = 8;
        int shift = 0;

        Point[] rook_points = new Point[20];
        rook_points[0]  = new Point(     W/4, 7*W/8   );
        rook_points[1]  = new Point(   3*W/4, 7*W/8   );
        rook_points[2]  = new Point(   3*W/4, 13*W/16 );
        rook_points[3]  = new Point( 11*W/16, 13*W/16 );
        rook_points[4]  = new Point( 19*W/32, 3*W/8   );
        rook_points[5]  = new Point(   3*W/4, 3*W/8   );
        rook_points[6]  = new Point(   3*W/4, W/8     );
        rook_points[7]  = new Point( 26*W/40, W/8     );
        rook_points[8]  = new Point( 26*W/40, W/4     );
        rook_points[9]  = new Point( 22*W/40, W/4     );
        rook_points[10] = new Point( 22*W/40, W/8     );
        rook_points[11] = new Point( 18*W/40, W/8     );
        rook_points[12] = new Point( 18*W/40, W/4     );
        rook_points[13] = new Point( 14*W/40, W/4     );
        rook_points[14] = new Point( 14*W/40, W/8     );
        rook_points[15] = new Point(     W/4, W/8     );
        rook_points[16] = new Point(     W/4, 3*W/8   );
        rook_points[17] = new Point( 13*W/32, 3*W/8   );
        rook_points[18] = new Point(  5*W/16, 13*W/16 );
        rook_points[19] = new Point(     W/4, 13*W/16 );

        MatOfPoint matPt = new MatOfPoint();
        matPt.fromArray(rook_points);

        List<MatOfPoint> ppt = new ArrayList<MatOfPoint>();
        ppt.add(matPt);

        Imgproc.fillPoly(img,
                ppt,
                new Scalar( 255, 255, 255 ),
                lineType,
                shift,
                new Point(0,0) );
    }

Python

def my_polygon(img):
    line_type = 8

    # Create some points
    ppt = np.array([[W / 4, 7 * W / 8], [3 * W / 4, 7 * W / 8],
                    [3 * W / 4, 13 * W / 16], [11 * W / 16, 13 * W / 16],
                    [19 * W / 32, 3 * W / 8], [3 * W / 4, 3 * W / 8],
                    [3 * W / 4, W / 8], [26 * W / 40, W / 8],
                    [26 * W / 40, W / 4], [22 * W / 40, W / 4],
                    [22 * W / 40, W / 8], [18 * W / 40, W / 8],
                    [18 * W / 40, W / 4], [14 * W / 40, W / 4],
                    [14 * W / 40, W / 8], [W / 4, W / 8],
                    [W / 4, 3 * W / 8], [13 * W / 32, 3 * W / 8],
                    [5 * W / 16, 13 * W / 16], [W / 4, 13 * W / 16]], np.int32)
    ppt = ppt.reshape((-1, 1, 2))
    cv.fillPoly(img, [ppt], (255, 255, 255), line_type)
    # Only drawind the lines would be:
    # cv.polylines(img, [ppt], True, (255, 0, 255), line_type)

• To draw a filled polygon we use the function fillPoly() . We note that:
  • The polygon will be drawn on img
  • The vertices of the polygon are the set of points in ppt
  • The color of the polygon is defined by ( 255, 255, 255 ), which is the BGR value for white

rectangle

C++

  rectangle( rook_image,
         Point( 0, 7*w/8 ),
         Point( w, w),
         Scalar( 0, 255, 255 ),
         FILLED,
         LINE_8 );

Java

        Imgproc.rectangle( rook_image,
                new Point( 0, 7*W/8 ),
                new Point( W, W),
                new Scalar( 0, 255, 255 ),
                -1,
                8,
                0 );

Python

# 2.b. Creating rectangles
cv.rectangle(rook_image,
              (0, 7 * W // 8),
              (W, W),
              (0, 255, 255),
              -1,
              8)

• Finally we have the cv::rectangle function (we did not create a special function for this guy). We note that:
  • The rectangle will be drawn on rook_image
  • Two opposite vertices of the rectangle are defined by ( 0, 7*w/8 ) and ( w, w )
  • The color of the rectangle is given by ( 0, 255, 255 ) which is the BGR value for yellow
  • Since the thickness value is given by FILLED (-1), the rectangle will be filled.

Result

Compiling and running your program should give you a result like this: