In this tutorial you will learn how to:

- Use Point to define 2D points in an image.
- Use Scalar and why it is useful
- 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

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

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

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

or

```
Point pt = Point(10, 8);
```

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 RGB 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 RGB color such as:

*Red = c*,*Green = b*and*Blue = a*

Since we plan to draw two examples (an atom and a rook), we have to create 02 images and two windows to display them.

/// Windows names char atom_window[] = "Drawing 1: Atom"; char rook_window[] = "Drawing 2: Rook"; /// Create black empty images Mat atom_image = Mat::zeros( w, w, CV_8UC3 ); Mat rook_image = Mat::zeros( w, w, CV_8UC3 );

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

*MyEllipse*and*MyFilledCircle*:/// 1. Draw a simple atom: /// 1.a. Creating ellipses MyEllipse( atom_image, 90 ); MyEllipse( atom_image, 0 ); MyEllipse( atom_image, 45 ); MyEllipse( atom_image, -45 ); /// 1.b. Creating circles MyFilledCircle( atom_image, Point( w/2.0, w/2.0) );

And to draw the rook we employed

*MyLine*,*rectangle*and a*MyPolygon*:/// 2. Draw a rook /// 2.a. Create a convex polygon MyPolygon( rook_image ); /// 2.b. Creating rectangles rectangle( rook_image, Point( 0, 7*w/8.0 ), Point( w, w), Scalar( 0, 255, 255 ), -1, 8 ); /// 2.c. Create a few lines 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 ) );

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

*MyLine*void MyLine( Mat img, Point start, Point end ) { int thickness = 2; int lineType = 8; line( img, start, end, Scalar( 0, 0, 0 ), thickness, lineType ); }

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
**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)

- Draw a line from Point
*MyEllipse*void MyEllipse( Mat img, double angle ) { int thickness = 2; int lineType = 8; ellipse( img, Point( w/2.0, w/2.0 ), Size( w/4.0, w/16.0 ), angle, 0, 360, Scalar( 255, 0, 0 ), thickness, lineType ); }

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/2.0, w/2.0)**and is enclosed in a box of size**(w/4.0, w/16.0)** - 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, 255, 0)**which means blue in RGB value. - The ellipse’s
**thickness**is 2.

- The ellipse is displayed in the image
*MyFilledCircle*void MyFilledCircle( Mat img, Point center ) { int thickness = -1; int lineType = 8; circle( img, center, w/32.0, Scalar( 0, 0, 255 ), thickness, lineType ); }

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/32.0** - The color of the circle:
**Scalar(0, 0, 255)**which means*Red*in BGR - Since
**thickness**= -1, the circle will be drawn filled.

- The image where the circle will be displayed (
*MyPolygon*void MyPolygon( Mat img ) { int lineType = 8; /** Create some points */ Point rook_points[1][20]; rook_points[0][0] = Point( w/4.0, 7*w/8.0 ); rook_points[0][1] = Point( 3*w/4.0, 7*w/8.0 ); rook_points[0][2] = Point( 3*w/4.0, 13*w/16.0 ); rook_points[0][3] = Point( 11*w/16.0, 13*w/16.0 ); rook_points[0][4] = Point( 19*w/32.0, 3*w/8.0 ); rook_points[0][5] = Point( 3*w/4.0, 3*w/8.0 ); rook_points[0][6] = Point( 3*w/4.0, w/8.0 ); rook_points[0][7] = Point( 26*w/40.0, w/8.0 ); rook_points[0][8] = Point( 26*w/40.0, w/4.0 ); rook_points[0][9] = Point( 22*w/40.0, w/4.0 ); rook_points[0][10] = Point( 22*w/40.0, w/8.0 ); rook_points[0][11] = Point( 18*w/40.0, w/8.0 ); rook_points[0][12] = Point( 18*w/40.0, w/4.0 ); rook_points[0][13] = Point( 14*w/40.0, w/4.0 ); rook_points[0][14] = Point( 14*w/40.0, w/8.0 ); rook_points[0][15] = Point( w/4.0, w/8.0 ); rook_points[0][16] = Point( w/4.0, 3*w/8.0 ); rook_points[0][17] = Point( 13*w/32.0, 3*w/8.0 ); rook_points[0][18] = Point( 5*w/16.0, 13*w/16.0 ); rook_points[0][19] = Point( w/4.0, 13*w/16.0) ; 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 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*

- The polygon will be drawn on
*rectangle*rectangle( rook_image, Point( 0, 7*w/8.0 ), Point( w, w), Scalar( 0, 255, 255 ), -1, 8 );

Finally we have the 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/8.0 )** and
**Point( 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
**-1**, the rectangle will be filled.

- The rectangle will be drawn on

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

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