OpenCV  4.10.0-dev
Open Source Computer Vision
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Basic Drawing

Next Tutorial: Random generator and text with OpenCV

Original author Ana Huamán
Compatibility OpenCV >= 3.0


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()



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

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

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

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


  • 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)


  • 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.


  • 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.


  • 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


  • 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.


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