OpenCV  3.3.0
Open Source Computer Vision
Basic Drawing

Goals

In this tutorial you will learn how to:

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

Code

Explanation

  1. Since we plan to draw two examples (an atom and a rook), we have to create two images and two windows to display them.
    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 );
  2. We created functions to draw different geometric shapes. For instance, to draw the atom we used MyEllipse and MyFilledCircle:
    MyEllipse( atom_image, 90 );
    MyEllipse( atom_image, 0 );
    MyEllipse( atom_image, 45 );
    MyEllipse( atom_image, -45 );
    MyFilledCircle( atom_image, Point( w/2, w/2) );
  3. And to draw the rook we employed MyLine, rectangle and a MyPolygon:
    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 ) );
  4. Let's check what is inside each of these functions:
    • MyLine
      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 );
      }
      As we can see, MyLine just call the function cv::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 Scalar( 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
      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 );
      }
      From the code above, we can observe that the function cv::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 Scalar( 255, 0, 0) which means blue in BGR value.
      • The ellipse's thickness is 2.
    • MyFilledCircle
      void MyFilledCircle( Mat img, Point center )
      {
      circle( img,
      center,
      w/32,
      Scalar( 0, 0, 255 ),
      FILLED,
      LINE_8 );
      }
      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: Scalar(0, 0, 255) which means Red in BGR
      • Since thickness = -1, the circle will be drawn filled.
    • MyPolygon
      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 );
      }
      To draw a filled polygon we use the function cv::fillPoly . We note that:
      • The polygon will be drawn on img
      • The vertices of the polygon are the set of points in ppt
      • The total number of vertices to be drawn are npt
      • The number of polygons to be drawn is only 1
      • The color of the polygon is defined by Scalar( 255, 255, 255), which is the BGR value for white
    • rectangle
      rectangle( rook_image,
      Point( 0, 7*w/8 ),
      Point( w, w),
      Scalar( 0, 255, 255 ),
      FILLED,
      LINE_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 ** Point( 0, 7*w/8 )** andPoint( w, w)**
      • The color of the rectangle is given by Scalar(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:

Drawing_1_Tutorial_Result_0.png
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