OpenCV C++ Code for drawing an Ellipse

Syntax:
C++ :void ellipse(Mat& img, Point center, Size axes, double angle, double startAngle, double endAngle, const Scalar& color, int thickness=1, int lineType=8, int shift=0)

Parameters:
img – Image.
center – Center of the ellipse.
axes – Half of the size of the ellipse main axes.
angle – Ellipse rotation angle in degrees.
startAngle – Starting angle of the elliptic arc in degrees.
endAngle – Ending angle of the elliptic arc in degrees.
box – Alternative ellipse representation via RotatedRect or CvBox2D. This means that the function draws an ellipse inscribed in the rotated rectangle.
color – Ellipse color.
thickness – Thickness of the ellipse arc outline, if positive. Otherwise, this indicates that a filled ellipse sector is to be drawn.
lineType – Type of the ellipse boundary. See the line() description.
shift – Number of fractional bits in the coordinates of the center and values of axes.

Note: If you want to draw the whole ellipse and not an arc, choose startAngle 0 and endAngle 360.

Note: CIRCLE is a special case of ELLIPSE whose ECCENTRICITY is equal to ONE.
i.e whose both the axis are of equal length.

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//Code for drawing an ellipse
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
using namespace cv;
int main( )
{
Mat image = Mat::zeros( 400, 400, CV_8UC3 );
//void ellipse(Mat& img, Point center, Size axes, double angle, double startAngle, double endAngle, const Scalar& color, int thickness=1*, int lineType=8*, int shift=0);
ellipse( image, Point( 200, 200 ), Size( 100.0, 150.0 ), 45, 0, 360, Scalar( 255, 0, 0 ), 3, 8 );
ellipse( image, Point( 200, 200 ), Size( 100.0, 150.0 ), 90, 0, 360, Scalar( 0,255, 0 ), 3, 8 );
ellipse( image, Point( 200, 200 ), Size( 100.0, 150.0 ), 135, 0, 360, Scalar( 0, 0, 255 ), 3, 8 );
ellipse( image, Point( 200, 200 ), Size( 100.0, 150.0 ), 180, 0, 360, Scalar( 255,255, 0 ), 3, 8 );
imshow("Image",image);
waitKey( 0 );
return(0);
}

OpenCV C++ Code for drawing Circles

Syntax:
C++ :void circle(Mat& img, Point center, int radius, const Scalar& color, int thickness=1, int lineType=8, int shift=0)

Parameters:
img – Image where the circle is drawn.
center – Center of the circle.
radius – Radius of the circle.
color – Circle color.
thickness – Thickness of the circle outline, if positive. Negative thickness means that a filled circle is to be drawn.
lineType – Type of the circle boundary. See the line() description.
shift – Number of fractional bits in the coordinates of the center and in the radius value.
The function circle draws a simple or filled circle with a given center and radius.

Note: To draw a filled circle in Opencv ,take the value of line type as Negative (e.g -2).

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#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
using namespace cv;
  
int main( )
{    
  // Create black empty images
  Mat image = Mat::zeros( 400, 400, CV_8UC3 );
    
  // Draw a circle 
  circle( image, Point( 200, 200 ), 32.0, Scalar( 0, 0, 255 ), 1, 8 );
  imshow("Image",image);
  
  waitKey( 0 );
  return(0);
}

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Drawing a Circle by taking values from the user.

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <iostream>
using namespace cv;
using namespace std;
 
int main( )
{    
  // Create black empty images
  /*It basically defines the size of the window where circle would be drawn*/
  Mat image = Mat::zeros( 400, 400, CV_8UC3 );
  int i,x,y; 
  // Draw a circle 
  while(true)
  {
  cout<<"Enter the co-ordinates of the centre"<<endl;
  //Get value of centre from the user it should be 0<x<400 and 0<y<400
  //Bcoz size of window is (400,400)
  cin>>x>>y;
  //Display the co-ordinate of the centre in the form (x,y)
  cout<<"Co-ordinates of the centre is ("<<x<<","<<y<<")"<<endl;
  cout <<"Enter the value of radius"<<endl;
   //Take the value of centre from the user
  cin >>i;
  //Function for drawing the circle
  circle( image, Point( x, y ), i, Scalar( 0, 0, 255 ), 1, 8 );
  imshow("Image",image);
  waitKey( 1000 );
  }
  return(0);
}

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Opencv Code for drawing Concentric Circles.

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
using namespace cv;
  
int main( )
{    
  // Create black empty images
  Mat image = Mat::zeros( 400, 400, CV_8UC3 );
    
  // Draw a circle 
  for(int i=0; i<200;i=i+30)
  {
  circle( image, Point( 200, 200 ), i, Scalar( 0, 0, 255 ), 1, 8 );
  imshow("Image",image);
  
  waitKey( 1000 );
  }
  return(0);
}

OpenCV C++ Code for drawing a Line

Syntax:
C++:void line(Mat& img, Point pt1, Point pt2, const Scalar& color, int thickness=1, int lineType=8, int shift=0)

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Parameters:
img – Image.
pt1 – First point of the line segment.
pt2 – Second point of the line segment.
color – Line color.
thickness – Line thickness.
lineType – Type of the line:
                 8 (or omitted) - 8-connected line.
                 4 - 4-connected line.
                 CV_AA - antialiased line.
shift – Number of fractional bits in the point coordinates.

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//Opencv Code for drawing a Line
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
using namespace cv;
  
int main( )
{    
  // Create black empty images
  Mat image = Mat::zeros( 400, 400, CV_8UC3 );
    
  // Draw a line 
  line( image, Point( 15, 20 ), Point( 70, 50), Scalar( 110, 220, 0 ),  2, 8 );
  imshow("Image",image);
  
  waitKey( 0 );
  return(0);
}

Zeros,Ones and Eyes in OpenCV

Matlab style Initializers

Mat::zeroes
Each element of the matrix is zero of the specified size.
Mat A;
A = Mat::zeros(3, 3, CV_32F);

Mat::ones
Each element of the matrix is one of the specified size
Mat A;
A = Mat::ones(3, 3, CV_32F);

Mat::eyes
It returns an identity matrix of the specified size.
Mat A;
A = Mat::eyes(3, 3, CV_32F);

Note:
We can also mention the scale factor of the matrix.
e.g:
A = Mat::ones(3, 3, CV_32F)* 5;
Here each element of the matrix is 5, because each element of the uniy matrix is multiplied by 5.

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#include <opencv2/core/core.hpp>
#include <iostream>
#include <opencv2/highgui/highgui.hpp> 
 
using namespace cv;
using namespace std;
 
int main()
{
    Mat imgA = Mat::eye(5, 5, CV_8UC1);
cout << "imgA = \n " << imgA << "\n\n";
 
Mat imgB = Mat::ones(4, 4, CV_8UC1);
cout << "imgB = \n " << imgB << "\n\n";
 
Mat imgC = Mat::zeros(3,3, CV_8UC1);
cout << "imgC = \n " << imgC << "\n\n";
 
return 0;
}

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Output:

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Note:
Here we have selected the single channel matrix.(CV_8UC1)
For 3 channel matrices:

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Code:

#include <opencv2/core/core.hpp>
#include <iostream>
#include <opencv2/highgui/highgui.hpp> 
 
using namespace cv;
using namespace std;
 
int main()
{
    Mat imgA = Mat::eye(5, 5, CV_8UC3);
cout << "imgA = \n " << imgA << "\n\n";
 
Mat imgB = Mat::ones(4, 4, CV_8UC3);
cout << "imgB = \n " << imgB << "\n\n";
 
Mat imgC = Mat::zeros(3,3, CV_8UC3);
cout << "imgC = \n " << imgC << "\n\n";
 
return 0;
}

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See the difference in the output.Here the zeros,ones and eyes operator is applied only to 1 channel of the matrix.Rest of the other channel elements are taken 0.Thus two columns of 0 can be seen in between.

Also we doesn't mention the no. of channels by default it takes 1.
ie. CV_8U is equivalent to CV_8UC1.