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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

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;
_Tp x
x coordinate of the point
Definition types.hpp:201

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

Code

  • This code is in your OpenCV sample folder. Otherwise you can grab it from here
    #include <opencv2/core.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 );
    }
    n-dimensional dense array class
    Definition mat.hpp:829
    Template class for specifying the size of an image or rectangle.
    Definition types.hpp:335
    #define CV_8UC3
    Definition interface.h:90
    void fillPoly(InputOutputArray img, InputArrayOfArrays pts, const Scalar &color, int lineType=LINE_8, int shift=0, Point offset=Point())
    Fills the area bounded by one or more polygons.
    void line(InputOutputArray img, Point pt1, Point pt2, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0)
    Draws a line segment connecting two points.
    @ LINE_8
    8-connected line
    Definition imgproc.hpp:894
    int main(int argc, char *argv[])
    Definition highgui_qt.cpp:3
    Definition core.hpp:107

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.

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 );

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) );

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 ) );

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 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

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 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

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: ( 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 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

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 ( 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: