OpenCV C++ Code for Drawing a Semi-Circle

This Opencv Tutorial is about drawing a Semi-Circle

You might have wondered that how to draw a Semi-Circle in Opencv when we have no direct syntax available for it.
Even in the Syntax of drawing a circle in Opencv, we dont have any such parameters which can be modified for drawing a semicircle.

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

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But, we know that a circle is a special case of an ellipse whose eccentricity is 1. And in the Opencv Ellipse Syntax:

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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)

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We can find the parameters like "Start Angle" and "End Angle".
And if we want to draw an Circle From an Ellipse we just need to mention the size of both the axes as same.
Thus,Here is the Opencv Code for drawing a Semi-Circle:

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//Opencv C++ Tutorial for drawing a Semi-Circle
#include <opencv2 core.hpp="" core="">
#include <opencv2 highgui.hpp="" highgui="">
using namespace cv;
int main( )
{
 // Create black empty images
 Mat image = Mat::zeros( 500, 500, CV_8UC3 );
 // Draw a ellipse
 for(int i=10;i<=250;i=i+10)
 {
 ellipse( image, Point( 250, 250 ), Size( i, i ), 0, 0, 180, Scalar( 255, 255, 0 ), 2, 8 );
 imshow("Image",image);
 waitKey( 250 );
 }
 waitKey( 0 );
 
 return(0);
}

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Output:

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);
}