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Basic drawing examples of OpenCV

Posted by Hemprasad Y. Badgujar on July 20, 2015


Drawing a line

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

Example 1: Drawing a line


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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 line
  line( image, Point( 15, 20 ), Point( 70, 50), Scalar( 110, 220, 0 ),  2, 8 );
  imshow("Image",image);
  waitKey( 0 );
  return(0);
}


Drawing a Circle

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.

Example 2: Drawing a Circle


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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);
}


Drawing an Ellipse

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 – Length of the ellipse 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.

The functions ellipse with less parameters draw an ellipse outline, a filled ellipse, an elliptic arc, or a filled ellipse sector. A piecewise-linear curve is used to approximate the elliptic arc boundary.

If you use the first variant of the function and want to draw the whole ellipse, not an arc, passstartAngle=0 and endAngle=360.

Example 3: Drawing an Ellipse


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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 ellipse
  ellipse( image, Point( 200, 200 ), Size( 100.0, 160.0 ), 45, 0, 360, Scalar( 255, 0, 0 ), 1, 8 );
  ellipse( image, Point( 200, 200 ), Size( 100.0, 160.0 ), 135, 0, 360, Scalar( 255, 0, 0 ), 10, 8 );
  ellipse( image, Point( 200, 200 ), Size( 150.0, 50.0 ), 135, 0, 360, Scalar( 0, 255, 0 ), 1, 8 );
  imshow("Image",image);
  waitKey( 0 );
  return(0);
}


Drawing a Rectangle

void rectangle(Mat& img, Point pt1, Point pt2, const Scalar& color, int thickness=1, int lineType=8, int shift=0)
Parameters:

  • img – Image.
  • pt1 – Vertex of the rectangle.
  • pt2 – Vertex of the rectangle opposite to pt1 .
  • rec – Alternative specification of the drawn rectangle.
  • colorRectangle color or brightness (grayscale image).
  • thickness – Thickness of lines that make up the rectangle. Negative values, likeCV_FILLED , mean that the function has to draw a filled rectangle.
  • lineType – Type of the line. See the line() description.
  • shift – Number of fractional bits in the point coordinates.

Example 4: Drawing a Rectangle


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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 rectangle ( 5th argument is not -ve)
  rectangle( image, Point( 15, 20 ), Point( 70, 50), Scalar( 0, 55, 255 ), +1, 4 );
  imshow("Image1",image);
  // Draw a filled rectangle ( 5th argument is -ve)
  rectangle( image, Point( 115, 120 ), Point( 170, 150), Scalar( 100, 155, 25 ), -1, 8 );
  imshow("Image2",image);
  waitKey( 0 );
  return(0);
}


Drawing a Filled Polygon

void fillPoly(Mat& img, const Point** pts, const int* npts, int ncontours, const Scalar& color, int lineType=8, int shift=0, Point offset=Point() )
Parameters:

  • img – Image.
  • pts – Array of polygons where each polygon is represented as an array of points.
  • npts – Array of polygon vertex counters.
  • ncontours – Number of contours that bind the filled region.
  • color – Polygon color.
  • lineType – Type of the polygon boundaries. See the line() description.
  • shift – Number of fractional bits in the vertex coordinates.
  • offset – Optional offset of all points of the contours.

The function fillPoly fills an area bounded by several polygonal contours. The function can fill complex areas, for example, areas with holes, contours with self-intersections (some of their parts), and so forth.

Example 4: Drawing a Filled Polygon


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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 );
  
  int w=400;
  // Draw a circle
  /** 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( image, ppt, npt, 1, Scalar( 255, 255, 255 ), 8 );
  imshow("Image",image);
  waitKey( 0 );
  return(0);
}


Putting Text in image

putText renders the specified text string in the image.

void putText(Mat& img, const string& text, Point org, int fontFace, double fontScale, Scalar color, int thickness=1, int lineType=8, bool bottomLeftOrigin=false )
Parameters:

  • img – Image.
  • text – Text string to be drawn.
  • org – Bottom-left corner of the text string in the image.
  • fontFace – Font type. One of FONT_HERSHEY_SIMPLEX, FONT_HERSHEY_PLAIN,FONT_HERSHEY_DUPLEX, FONT_HERSHEY_COMPLEX, FONT_HERSHEY_TRIPLEX,FONT_HERSHEY_COMPLEX_SMALL, FONT_HERSHEY_SCRIPT_SIMPLEX, orFONT_HERSHEY_SCRIPT_COMPLEX, where each of the font ID’s can be combined withFONT_HERSHEY_ITALIC to get the slanted letters.
  • fontScale – Font scale factor that is multiplied by the font-specific base size.
  • color – Text color.
  • thickness – Thickness of the lines used to draw a text.
  • lineType – Line type. See the line for details.
  • bottomLeftOrigin – When true, the image data origin is at the bottom-left corner. Otherwise, it is at the top-left corner.

Example 5: Putting Text in image


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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 );
  
  putText(image, "Hi all...", Point(50,100), FONT_HERSHEY_SIMPLEX, 1, Scalar(0,200,200), 4);
  imshow("Image",image);
  waitKey( 0 );
  return(0);
}

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