OpenCV  5.0.0-pre
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

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()

OpenCV Theory

Point

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

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

Code

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.

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:

MyLine

  • 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

  • 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

  • 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

  • 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

  • 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: